2022
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Goldberga, I., Patris, N., Chen, C. H., Thomassot, E., Trébosc, J., Hung, I., Gan, Z., Berthomieu, D., Métro, T. X., Bonhomme, C., Gervais, C., Laurencin, D. First direct insight into the local environment and dynamics of water molecules in the whewellite mineral phase: mechanochemical isotopic enrichment and high-resolution 17O and 2H NMR analyses (Article de journal) Dans: Materials Chemistry, 2022. @article{Goldberga_etal2022,
title = {First direct insight into the local environment and dynamics of water molecules in the whewellite mineral phase: mechanochemical isotopic enrichment and high-resolution 17O and 2H NMR analyses},
author = {I. Goldberga and N. Patris and C. H. Chen and E. Thomassot and J. Tr\'{e}bosc and I. Hung and Z. Gan and D. Berthomieu and T. X. M\'{e}tro and C. Bonhomme and C. Gervais and D. Laurencin},
doi = {10.26434/chemrxiv-2022-dghph},
year = {2022},
date = {2022-01-01},
journal = {Materials Chemistry},
abstract = {Calcium oxalate minerals of general formula CaC2O4.xH2O are widely present in nature and usually associated with pathological calcifications, constituting up to 70 -- 80% of the mineral component of renal calculi. The monohydrate phase (CaC2O4.H2O, COM) is the most stable form, accounting for the majority of the hydrated calcium oxalates found. These mineral phases have been studied extensively via X-ray diffraction, IR spectroscopy and, to a lesser extent, using 1H, 13C and 43Ca solid-state NMR spectroscopy. However, several aspects of their structure and reactivity are still unclear, such as the evolution from low- to high-temperature COM structures (LT-COM and HT-COM, respectively), and the involvement of water molecules in this phase transition. Here, we report for the first time a 17O and 2H solid-state NMR investigation of the local structure and dynamics of water in the COM phase. A new procedure for the selective 17O- and 2H-isotopic enrichment of water molecules within the COM mineral is presented using mechanochemistry, which employs only microliter quantities of enriched water, and leads to exchange yields up to textasciitilde30%. 17O NMR allows both crystallographically inequivalent water molecules in the LT-COM structure to be resolved, while 2H NMR studies provide unambiguous evidence that these water molecules are undergoing different types of motions at high temperatures without exchanging with one another. Dynamics appear to be essential for water molecules in these structures, which have not been accounted for in previous structural studies on the HT-COM structure due to lack of available tools --- highlighting the importance of such NMR investigations for refining the crystallographic data of biologically relevant minerals like calcium oxalates.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Calcium oxalate minerals of general formula CaC2O4.xH2O are widely present in nature and usually associated with pathological calcifications, constituting up to 70 -- 80% of the mineral component of renal calculi. The monohydrate phase (CaC2O4.H2O, COM) is the most stable form, accounting for the majority of the hydrated calcium oxalates found. These mineral phases have been studied extensively via X-ray diffraction, IR spectroscopy and, to a lesser extent, using 1H, 13C and 43Ca solid-state NMR spectroscopy. However, several aspects of their structure and reactivity are still unclear, such as the evolution from low- to high-temperature COM structures (LT-COM and HT-COM, respectively), and the involvement of water molecules in this phase transition. Here, we report for the first time a 17O and 2H solid-state NMR investigation of the local structure and dynamics of water in the COM phase. A new procedure for the selective 17O- and 2H-isotopic enrichment of water molecules within the COM mineral is presented using mechanochemistry, which employs only microliter quantities of enriched water, and leads to exchange yields up to textasciitilde30%. 17O NMR allows both crystallographically inequivalent water molecules in the LT-COM structure to be resolved, while 2H NMR studies provide unambiguous evidence that these water molecules are undergoing different types of motions at high temperatures without exchanging with one another. Dynamics appear to be essential for water molecules in these structures, which have not been accounted for in previous structural studies on the HT-COM structure due to lack of available tools --- highlighting the importance of such NMR investigations for refining the crystallographic data of biologically relevant minerals like calcium oxalates. |
Lorenzon, S., Novella, D., Nimis, P., Jacobsen, S. D., Thomassot, E., Pamato, M. G., Prosperi, L., Lorenzetti, A., Alvaro, M., Brenker, F., Salvadego, F., Nestola, F. Ringwoodite and zirconia inclusions indicate downward travel of super-deep diamonds (Article de journal) Dans: Geology, 2022. @article{Lorenzon_etal2022,
title = {Ringwoodite and zirconia inclusions indicate downward travel of super-deep diamonds},
author = {S. Lorenzon and D. Novella and P. Nimis and S. D. Jacobsen and E. Thomassot and M. G. Pamato and L. Prosperi and A. Lorenzetti and M. Alvaro and F. Brenker and F. Salvadego and F. Nestola},
doi = {10.1130/G50111.1},
year = {2022},
date = {2022-01-01},
journal = {Geology},
abstract = {Natural diamonds and their inclusions provide unique glimpses of mantle processes from as deep as \^{a}`u800 km and dating back to 3.5 G.y. Once formed, diamonds are commonly interpreted to travel upward, either slowly within mantle upwellings or rapidly within explosive, carbonate-rich magmas erupting at the surface. Although global tectonics induce subduction of material from shallow depths into the deep mantle, mineralogical evidence for downward movements of diamonds has never been reported. We report the finding of an unusual composite inclusion consisting of ringwoodite (the second finding to date), tetragonal zirconia, and coesite within an alluvial super-deep diamond from the Central African Republic. We interpret zirconia + coesite and ringwoodite as prograde transformation products after zircon or reidite (ZrSiO4) and olivine or wadsleyite, respectively. This inclusion assemblage can be explained if the diamond traveled downward after entrapping olivine/wadsleyite + zircon/reidite, dragged down by a subducting slab, before being delivered to the surface. This indicates that the commonly assumed view that diamonds form at, and capture material from, a specific mantle level and then travel upward is probably too simplistic.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Natural diamonds and their inclusions provide unique glimpses of mantle processes from as deep as â`u800 km and dating back to 3.5 G.y. Once formed, diamonds are commonly interpreted to travel upward, either slowly within mantle upwellings or rapidly within explosive, carbonate-rich magmas erupting at the surface. Although global tectonics induce subduction of material from shallow depths into the deep mantle, mineralogical evidence for downward movements of diamonds has never been reported. We report the finding of an unusual composite inclusion consisting of ringwoodite (the second finding to date), tetragonal zirconia, and coesite within an alluvial super-deep diamond from the Central African Republic. We interpret zirconia + coesite and ringwoodite as prograde transformation products after zircon or reidite (ZrSiO4) and olivine or wadsleyite, respectively. This inclusion assemblage can be explained if the diamond traveled downward after entrapping olivine/wadsleyite + zircon/reidite, dragged down by a subducting slab, before being delivered to the surface. This indicates that the commonly assumed view that diamonds form at, and capture material from, a specific mantle level and then travel upward is probably too simplistic. |
Vezinet, A., Thomassot, E., Luo, Y., Sarkar, C., Pearson, D. G. Diachronous redistribution of Hf and Nd isotopes at the crystal scale---consequences for the isotopic evolution of a poly-metamorphic crustal terrane (Article de journal) Dans: Geosciences, vol. 12, no. 1, p. 36, 2022. @article{Vezinet_etal2022,
title = {Diachronous redistribution of Hf and Nd isotopes at the crystal scale---consequences for the isotopic evolution of a poly-metamorphic crustal terrane},
author = {A. Vezinet and E. Thomassot and Y. Luo and C. Sarkar and D. G. Pearson},
doi = {10.3390/geosciences12010036},
year = {2022},
date = {2022-01-01},
journal = {Geosciences},
volume = {12},
number = {1},
pages = {36},
abstract = {In metamorphic rocks, mineral species react over a range of pressure--temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure--temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history.Here, we present in-situ U--Pb/Sm--Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U--Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived fromthe oldest zircon cores and apatite U--Pb/Sm--Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the handspecimenscale (\^{a}¤textasciitilde0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In metamorphic rocks, mineral species react over a range of pressure--temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure--temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history.Here, we present in-situ U--Pb/Sm--Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U--Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived fromthe oldest zircon cores and apatite U--Pb/Sm--Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the handspecimenscale (â¤textasciitilde0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event. |
2021
|
Barré, G., Thomassot, E., Michels, R., Cartigny, P., Strzerzynski, P., Truche, L. Multiple sulfur isotopes signature of Thermochemical Sulfate Reduction (TSR) : Insights from Alpine Triassic evaporites (Article de journal) Dans: Earth and Planetary Science Letters, vol. 576, p. 117231, 2021. @article{Barr_etal2021,
title = {Multiple sulfur isotopes signature of Thermochemical Sulfate Reduction (TSR) : Insights from Alpine Triassic evaporites},
author = {G. Barr\'{e} and E. Thomassot and R. Michels and P. Cartigny and P. Strzerzynski and L. Truche},
doi = {10.1016/j.epsl.2021.117231},
year = {2021},
date = {2021-01-01},
journal = {Earth and Planetary Science Letters},
volume = {576},
pages = {117231},
abstract = {The sulfur cycle is driven by redox processes, among which sulfate reduction is of primary importance. Sulfate is reduced to sulfide either abiotically by Thermochemical Sulfate Reduction (TSR) or biotically by Microbial Sulfate Reduction (MSR). Although these two processes occur at different temperature regimes (\>100◦C and \<80◦C, respectively), they generate similar by-products (e.g., sulfides, elemental sulfur). The 34S/32S ratio is often used as the sole criterion to identify the origin of reduced sulfur compounds, but overlaps prevent unambiguous conclusions. Contrary to MSR, the multiple sulfur isotopic signatures ($delta$33S, $delta$34S, $delta$36S) of natural TSR remains uncharacterized. Here, we performed multiple sulfur isotopes analyses of sulfates, sulfides, and elemental sulfur from six sites in the Alpine Triassic evaporites formation to better constrain the isotopic signatures of TSR. Unlike MSR, TSR can induce slight negative deviations (�'u33S down to −0.08textdegree/textdegreetextdegree) relative to the initial sulfate -33S value, which significantly discriminates between these two processes. Isotopic equilibria between anhydrite and either elemental sulfur or sulfides (pyrite or chalcopyrite) were verified according to their mass-fractionation exponents (33/34$theta$=0.5140and 0.5170, respectively). Using sulfate-elemental sulfur (�'u34SSO42−-S8) or sulfate-sulfide (�'u34SSO42−-S2−) fractionation pairs and respective fractionation factors (34$alpha$) for samples that fulfilled the criteria of isotopic equilibrium, we determined the precipitation temperatures of elemental sulfur and sulfides (pyrite or chalcopyrite) to be 194 textpm14◦C and 293--488◦C, respectively. Interestingly, the obtained temperature of elemental sulfur precipitation corresponds exactly to the solid-liquid phase transition of native sulfur. Using �'u33S vs. $delta$34S and �'u33S vs. �'u36S diagrams, we are able to fully explain the isotopic signatures of disequilibrium sulfides by the mixing of sulfate with either elemental or organic sulfur in the aqueous fluid. Mixing curves allow the determination of the relative proportions of sulfate and organic and elemental sulfur, the latter being formed by the recombination of polysulfides during cooling. It appears that the sulfidestextquoteright signatures are best explained by a 33% contribution of polysulfides (i.e., elemental sulfur signatures), consistent with the relative proportion of dissolved polysulfides previously measured in fluid inclusions from this formation at \>200◦C. Finally, no sulfur mass independent fractionation (S-MIF) is observed in this evaporitic formation, consistent with the TSR signature generated both at equilibrium and by mixing. This implies that TSR does not generate S-MIFs. Our results thus provide multiple sulfur isotopes signatures of TSR, which may be used to reliably identify this process in variable geological settings.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The sulfur cycle is driven by redox processes, among which sulfate reduction is of primary importance. Sulfate is reduced to sulfide either abiotically by Thermochemical Sulfate Reduction (TSR) or biotically by Microbial Sulfate Reduction (MSR). Although these two processes occur at different temperature regimes (>100◦C and <80◦C, respectively), they generate similar by-products (e.g., sulfides, elemental sulfur). The 34S/32S ratio is often used as the sole criterion to identify the origin of reduced sulfur compounds, but overlaps prevent unambiguous conclusions. Contrary to MSR, the multiple sulfur isotopic signatures ($delta$33S, $delta$34S, $delta$36S) of natural TSR remains uncharacterized. Here, we performed multiple sulfur isotopes analyses of sulfates, sulfides, and elemental sulfur from six sites in the Alpine Triassic evaporites formation to better constrain the isotopic signatures of TSR. Unlike MSR, TSR can induce slight negative deviations (�'u33S down to −0.08textdegree/textdegreetextdegree) relative to the initial sulfate -33S value, which significantly discriminates between these two processes. Isotopic equilibria between anhydrite and either elemental sulfur or sulfides (pyrite or chalcopyrite) were verified according to their mass-fractionation exponents (33/34$theta$=0.5140and 0.5170, respectively). Using sulfate-elemental sulfur (�'u34SSO42−-S8) or sulfate-sulfide (�'u34SSO42−-S2−) fractionation pairs and respective fractionation factors (34$alpha$) for samples that fulfilled the criteria of isotopic equilibrium, we determined the precipitation temperatures of elemental sulfur and sulfides (pyrite or chalcopyrite) to be 194 textpm14◦C and 293--488◦C, respectively. Interestingly, the obtained temperature of elemental sulfur precipitation corresponds exactly to the solid-liquid phase transition of native sulfur. Using �'u33S vs. $delta$34S and �'u33S vs. �'u36S diagrams, we are able to fully explain the isotopic signatures of disequilibrium sulfides by the mixing of sulfate with either elemental or organic sulfur in the aqueous fluid. Mixing curves allow the determination of the relative proportions of sulfate and organic and elemental sulfur, the latter being formed by the recombination of polysulfides during cooling. It appears that the sulfidestextquoteright signatures are best explained by a 33% contribution of polysulfides (i.e., elemental sulfur signatures), consistent with the relative proportion of dissolved polysulfides previously measured in fluid inclusions from this formation at >200◦C. Finally, no sulfur mass independent fractionation (S-MIF) is observed in this evaporitic formation, consistent with the TSR signature generated both at equilibrium and by mixing. This implies that TSR does not generate S-MIFs. Our results thus provide multiple sulfur isotopes signatures of TSR, which may be used to reliably identify this process in variable geological settings. |
Bouden, N., Villeneuve, J., Marrocchi, Y., Deloule, E., Füri, E., Gurenko, A., Piani, L., Thomassot, E., Peres, P., Fernandes, F. Triple oxygen isotope measurements by multi-collector secondary ion mass spectrometry (Article de journal) Dans: Frontiers in Earth Science, p. 8:601169, 2021. @article{Bouden_etal2021,
title = {Triple oxygen isotope measurements by multi-collector secondary ion mass spectrometry},
author = {N. Bouden and J. Villeneuve and Y. Marrocchi and E. Deloule and E. F\"{u}ri and A. Gurenko and L. Piani and E. Thomassot and P. Peres and F. Fernandes},
doi = {doi: 10.3389/feart.2020.601169},
year = {2021},
date = {2021-01-01},
journal = {Frontiers in Earth Science},
pages = {8:601169},
abstract = {Secondary ion mass spectrometry (SIMS) is a powerful technique for in situ triple oxygen isotope measurements that has been used for more than 30 years. Since pioneering works performed on small-radius ion microprobes in the mid-80s, tremendous progress has been made in terms of analytical precision, spatial resolution and analysis duration. In this respect, the emergence in the mid-90s of the large-radius ion microprobe equipped with a multi-collector system (MC-SIMS) was a game changer. Further developments achieved on CAMECA MC-SIMS since then (e.g., stability of the electronics, enhanced transmission of secondary ions, automatic centering of the secondary ion beam, enhanced control of the magnetic field, 1012$\Omega$ resistor for the Faraday cup amplifiers) allow nowadays to routinely measure oxygen isotopic ratios (18O/16O and 17O/16O) in various matrices with a precision (internal error and reproducibility) better than 0.5texttenthousand (2$sigma$), a spatial resolution smaller than 10 $mu$m and in a few minutes per analysis. This paper focuses on the application of the MC-SIMS technique to the in situ monitoring of mass-independent triple oxygen isotope variations.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Secondary ion mass spectrometry (SIMS) is a powerful technique for in situ triple oxygen isotope measurements that has been used for more than 30 years. Since pioneering works performed on small-radius ion microprobes in the mid-80s, tremendous progress has been made in terms of analytical precision, spatial resolution and analysis duration. In this respect, the emergence in the mid-90s of the large-radius ion microprobe equipped with a multi-collector system (MC-SIMS) was a game changer. Further developments achieved on CAMECA MC-SIMS since then (e.g., stability of the electronics, enhanced transmission of secondary ions, automatic centering of the secondary ion beam, enhanced control of the magnetic field, 1012$Ømega$ resistor for the Faraday cup amplifiers) allow nowadays to routinely measure oxygen isotopic ratios (18O/16O and 17O/16O) in various matrices with a precision (internal error and reproducibility) better than 0.5texttenthousand (2$sigma$), a spatial resolution smaller than 10 $mu$m and in a few minutes per analysis. This paper focuses on the application of the MC-SIMS technique to the in situ monitoring of mass-independent triple oxygen isotope variations. |
Burness, S., Thomassot, E., Smart, K. A., Tappe, S. Sulphur isotopes ($delta$34S and �'u33S) in sulphides from cratonic mantle eclogites : A glimpse of volatile cycling in ancient subduction zones (Article de journal) Dans: Earth and Planetary Science Letters, vol. 572, p. 117118, 2021. @article{Burness_etal2021,
title = {Sulphur isotopes ($delta$34S and �'u33S) in sulphides from cratonic mantle eclogites : A glimpse of volatile cycling in ancient subduction zones},
author = {S. Burness and E. Thomassot and K. A. Smart and S. Tappe},
doi = {10.1016/j.epsl.2021.117118},
year = {2021},
date = {2021-01-01},
journal = {Earth and Planetary Science Letters},
volume = {572},
pages = {117118},
abstract = {Multiple sulphur isotopic compositions of sulphides from Kaapvaal craton mantle eclogites allow to elucidate the recycling of sulphur into the deep Earth and to differentiate between recycled crust and mantle origins of eclogite-hosted sulphides, including the precious metals that they capture. We present multiple sulphur isotope ratio measurements by secondary ion mass spectrometry for sulphides from a collection of mantle-derived eclogite xenoliths from Proterozoic and Mesozoic kimberlite occurrences in South Africa (Premier, Roberts Victor, Jagersfontein). Previous work established that the host eclogites have elemental and oxygen isotopic compositions in support of seawater-altered oceanic lithosphere protoliths, and for many of these xenolith suites Archean ages have been suggested.The eclogite-hosted sulphides have $delta$34S values from −5.7 to +29textdegree/textdegreetextdegree, with the upper end of this wide range representing the highest-ever recorded $delta$34S composition of material derived from the Earthtextquoterights mantle. The �'u33S values range from −0.29to +0.18textdegree/textdegreetextdegree and do not record significant mass-independent sulphur isotope fractionation, i.e., there is no compelling S-MIF signature. Most of the sulphide grains have $delta$34S values that fall within a range between −6 and +4textdegree/textdegreetextdegree and they probably retain an isotopic record of sulphides that formed originally within altered oceanic crust. In contrast, the highly positive $delta$34S values from +13 to +29textdegree/textdegreetextdegree detected in sulphide grains from a single eclogite xenolith are similar to those of marine sulphates, which were probably a minor sulphur component of the oceanic crustal protolith. The lack of a significant S-MIF signature in the eclogitic sulphides that show $delta$34S evidence for a recycled crust origin implies that this sulphur component stems from a \<2.4Ga post-Archean surficial reservoir. This finding suggests that the cratonic mantle eclogites may have formed from post-Archean oceanic crust (e.g., Paleoproterozoic eclogite protoliths), or-- as is preferred here-- the textquoteleftsurficialtextquoteright sulphur was introduced into the cratonic root during relatively young metasomatic events and is thus unrelated to eclogite petrogenesis and Archean continent formation.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Multiple sulphur isotopic compositions of sulphides from Kaapvaal craton mantle eclogites allow to elucidate the recycling of sulphur into the deep Earth and to differentiate between recycled crust and mantle origins of eclogite-hosted sulphides, including the precious metals that they capture. We present multiple sulphur isotope ratio measurements by secondary ion mass spectrometry for sulphides from a collection of mantle-derived eclogite xenoliths from Proterozoic and Mesozoic kimberlite occurrences in South Africa (Premier, Roberts Victor, Jagersfontein). Previous work established that the host eclogites have elemental and oxygen isotopic compositions in support of seawater-altered oceanic lithosphere protoliths, and for many of these xenolith suites Archean ages have been suggested.The eclogite-hosted sulphides have $delta$34S values from −5.7 to +29textdegree/textdegreetextdegree, with the upper end of this wide range representing the highest-ever recorded $delta$34S composition of material derived from the Earthtextquoterights mantle. The �'u33S values range from −0.29to +0.18textdegree/textdegreetextdegree and do not record significant mass-independent sulphur isotope fractionation, i.e., there is no compelling S-MIF signature. Most of the sulphide grains have $delta$34S values that fall within a range between −6 and +4textdegree/textdegreetextdegree and they probably retain an isotopic record of sulphides that formed originally within altered oceanic crust. In contrast, the highly positive $delta$34S values from +13 to +29textdegree/textdegreetextdegree detected in sulphide grains from a single eclogite xenolith are similar to those of marine sulphates, which were probably a minor sulphur component of the oceanic crustal protolith. The lack of a significant S-MIF signature in the eclogitic sulphides that show $delta$34S evidence for a recycled crust origin implies that this sulphur component stems from a <2.4Ga post-Archean surficial reservoir. This finding suggests that the cratonic mantle eclogites may have formed from post-Archean oceanic crust (e.g., Paleoproterozoic eclogite protoliths), or-- as is preferred here-- the textquoteleftsurficialtextquoteright sulphur was introduced into the cratonic root during relatively young metasomatic events and is thus unrelated to eclogite petrogenesis and Archean continent formation. |
Cavalazzi, B., Lemelle, L., Simionovici, A., Cady, S. L., Russell, M. J., Bailo, E., Canteri, R., Enrico, E., Manceau, A., Maris, A., Salomé, M., Thomassot, E., Bouden, N., Tucoulou, R., Hofmann, A. Cellular remains in a 3.42-billion-year-old subseafloor hydrothermal environment (Article de journal) Dans: Science Advances, vol. 7, no. 29, p. eabf3963, 2021. @article{Cavalazzi_etal2021,
title = {Cellular remains in a 3.42-billion-year-old subseafloor hydrothermal environment},
author = {B. Cavalazzi and L. Lemelle and A. Simionovici and S. L. Cady and M. J. Russell and E. Bailo and R. Canteri and E. Enrico and A. Manceau and A. Maris and M. Salom\'{e} and E. Thomassot and N. Bouden and R. Tucoulou and A. Hofmann},
doi = {10.1126/sciadv.abf3963},
year = {2021},
date = {2021-01-01},
journal = {Science Advances},
volume = {7},
number = {29},
pages = {eabf3963},
abstract = {Subsurface habitats on Earth host an extensive extant biosphere and likely provided one of Earthtextquoterights earliest microbial habitats. Although the site of lifetextquoterights emergence continues to be debated, evidence of early life provides insights into its early evolution and metabolic affinity. Here, we present the discovery of exceptionally well-preserved, 3.42-billion-year-old putative filamentous microfossils that inhabited a paleo-subseafloor hydrothermal vein system of the Barberton greenstone belt in South Africa. The filaments colonized the walls of conduits created by low-temperature hydrothermal fluid. Combined with their morphological and chemical characteristics as investigated over a range of scales, they can be considered the oldest methanogens and/or methanotrophs that thrived in an ultramafic volcanic substrate.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Subsurface habitats on Earth host an extensive extant biosphere and likely provided one of Earthtextquoterights earliest microbial habitats. Although the site of lifetextquoterights emergence continues to be debated, evidence of early life provides insights into its early evolution and metabolic affinity. Here, we present the discovery of exceptionally well-preserved, 3.42-billion-year-old putative filamentous microfossils that inhabited a paleo-subseafloor hydrothermal vein system of the Barberton greenstone belt in South Africa. The filaments colonized the walls of conduits created by low-temperature hydrothermal fluid. Combined with their morphological and chemical characteristics as investigated over a range of scales, they can be considered the oldest methanogens and/or methanotrophs that thrived in an ultramafic volcanic substrate. |
Gress, M. U., Koornneef, J. M., Thomassot, E., Chinn, I. L., Zuilen, K., Davies, G. R. Sm-Nd isochron ages coupled with C-N isotope data of eclogitic diamonds from Jwaneng, Botswana (Article de journal) Dans: Geochimica et Cosmochimica Acta, vol. 293, p. 1–17, 2021. @article{Gress_etal2021,
title = {Sm-Nd isochron ages coupled with C-N isotope data of eclogitic diamonds from Jwaneng, Botswana},
author = {M. U. Gress and J. M. Koornneef and E. Thomassot and I. L. Chinn and K. Zuilen and G. R. Davies},
doi = {10.1016/j.gca.2020.10.010},
year = {2021},
date = {2021-01-01},
journal = {Geochimica et Cosmochimica Acta},
volume = {293},
pages = {1--17},
abstract = {Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamondstextquoteright geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different textquoteleftmechanismstextquoteright of diamond formation and define two isochrons, one Paleoproterozoic (1.8 Ga) and one Neoproterozoic (0.85 Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (\>1000s km), volatile-rich metasomatism related to formation and re-working of the Proto Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (\<10 km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (d13C \< -21.1texttenthousand) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earthtextquoterights long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamondstextquoteright geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different textquoteleftmechanismstextquoteright of diamond formation and define two isochrons, one Paleoproterozoic (1.8 Ga) and one Neoproterozoic (0.85 Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (>1000s km), volatile-rich metasomatism related to formation and re-working of the Proto Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (<10 km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (d13C < -21.1texttenthousand) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earthtextquoterights long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events. |
Gress, M. U., Pearson, D. G., Chinn, I. L., Thomassot, E., Davies, G. R. Mesozoic to Paleoproterozoic diamond growth beneath Botswana recorded by Re-Os ages from individual eclogitic and websteritic inclusions (Article de journal) Dans: Lithos, vol. 388-389, p. 106058, 2021. @article{Gress_etal2021_2,
title = {Mesozoic to Paleoproterozoic diamond growth beneath Botswana recorded by Re-Os ages from individual eclogitic and websteritic inclusions},
author = {M. U. Gress and D. G. Pearson and I. L. Chinn and E. Thomassot and G. R. Davies},
doi = {10.1016/j.lithos.2021.106058},
year = {2021},
date = {2021-01-01},
journal = {Lithos},
volume = {388-389},
pages = {106058},
abstract = {Res-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Resingle bondOs ages (786 textpm 250 Ma) are within uncertainty of previously determined Smsingle bondNd ages (853 textpm 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Resingle bondOs and Smsingle bondNd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (textasciitilde 2.0 to 1.7 Ga), Meso- (textasciitilde 1.2 to 1.1 Ga) and Neoproterozoic (textasciitilde 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100textquoterights of km illustrating the significance of diamond inclusions for monitoring continental tectonics.Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich textquotelefttextquoteleftfibroustextquoterighttextquoteright and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Res-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Resingle bondOs ages (786 textpm 250 Ma) are within uncertainty of previously determined Smsingle bondNd ages (853 textpm 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Resingle bondOs and Smsingle bondNd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (textasciitilde 2.0 to 1.7 Ga), Meso- (textasciitilde 1.2 to 1.1 Ga) and Neoproterozoic (textasciitilde 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100textquoterights of km illustrating the significance of diamond inclusions for monitoring continental tectonics.Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich textquotelefttextquoteleftfibroustextquoterighttextquoteright and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle. |
Gress, M. U., Timmerman, S., Chinn, I. L., Koornneef, J. M., Thomassot, E., Valk, E. A. S., Zuilen, K., Bouden, N., Davies, G. R Two billion years of episodic and simultaneous websteritic and eclogitic diamond formation beneath the Orapa kimberlite cluster, Botswana (Article de journal) Dans: Contributions to Mineralogy and Petrology, vol. 176, no. 54, 2021. @article{Gress_etal2021_3,
title = {Two billion years of episodic and simultaneous websteritic and eclogitic diamond formation beneath the Orapa kimberlite cluster, Botswana},
author = {M. U. Gress and S. Timmerman and I. L. Chinn and J. M. Koornneef and E. Thomassot and E. A. S. Valk and K. Zuilen and N. Bouden and G. R Davies},
doi = {10.1007/s00410-021-01802-8},
year = {2021},
date = {2021-01-01},
journal = {Contributions to Mineralogy and Petrology},
volume = {176},
number = {54},
abstract = {The Sm--Nd isotope systematics and geochemistry of eclogitic, websteritic and peridotitic garnet and clinopyroxene inclusions together with characteristics of their corresponding diamond hosts are presented for the Letlhakane mine, Botswana. These data are supplemented with new inclusion data from the nearby (20--30 km) Orapa and Damtshaa mines to evaluate the nature and scale of diamond-forming processes beneath the NW part of the Kalahari Craton and to provide insight into the evolution of the deep carbon cycle. The Sm--Nd isotope compositions of the diamond inclusions indicate five well-defined, discrete eclogitic and websteritic diamond-forming events in the Orapa kimberlite cluster at 220 textpm 80 Ma, 746 textpm 100 Ma, 1110 textpm 64 Ma, 1698 textpm 280 Ma and 2341 textpm 21 Ma. In addition, two poorly constrained events suggest ancient eclogitic (\> 2700 Ma) and recent eclogitic and websteritic diamond formation (\< 140 Ma). Together with sub-calcic garnets from two harzburgitic diamonds that have Archaean Nd mantle model ages (TCHUR) between 2.86 and 3.38 Ga, the diamonds studied here span almost the entire temporal evolution of the SCLM of the Kalahari Craton. The new data demonstrate, for the first time, that diamond formation occurs simultaneously and episodically in different parageneses, reflecting metasomatism of the compositionally heterogeneous SCLM beneath the area ( 200 km2). Diamond formation can be directly related to major tectono-magmatic events that impacted the Kalahari Craton such as crustal accretion, continental breakup and large igneous provinces. Compositions of dated inclusions, in combination with marked variations in the carbon and nitrogen isotope compositions of the host diamonds, record mixing arrays between a minimum of three components (A : peridotitic mantle ; B : eclogites dominated by mafic material ; C : eclogites that include recycled sedimentary material). Diamond formation appears dominated by local fluid--rock interactions involving different protoliths in the SCLM. Redistribution of carbon during fluid--rock interactions generally masks any potential temporal changes of the deep carbon cycle.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The Sm--Nd isotope systematics and geochemistry of eclogitic, websteritic and peridotitic garnet and clinopyroxene inclusions together with characteristics of their corresponding diamond hosts are presented for the Letlhakane mine, Botswana. These data are supplemented with new inclusion data from the nearby (20--30 km) Orapa and Damtshaa mines to evaluate the nature and scale of diamond-forming processes beneath the NW part of the Kalahari Craton and to provide insight into the evolution of the deep carbon cycle. The Sm--Nd isotope compositions of the diamond inclusions indicate five well-defined, discrete eclogitic and websteritic diamond-forming events in the Orapa kimberlite cluster at 220 textpm 80 Ma, 746 textpm 100 Ma, 1110 textpm 64 Ma, 1698 textpm 280 Ma and 2341 textpm 21 Ma. In addition, two poorly constrained events suggest ancient eclogitic (> 2700 Ma) and recent eclogitic and websteritic diamond formation (< 140 Ma). Together with sub-calcic garnets from two harzburgitic diamonds that have Archaean Nd mantle model ages (TCHUR) between 2.86 and 3.38 Ga, the diamonds studied here span almost the entire temporal evolution of the SCLM of the Kalahari Craton. The new data demonstrate, for the first time, that diamond formation occurs simultaneously and episodically in different parageneses, reflecting metasomatism of the compositionally heterogeneous SCLM beneath the area ( 200 km2). Diamond formation can be directly related to major tectono-magmatic events that impacted the Kalahari Craton such as crustal accretion, continental breakup and large igneous provinces. Compositions of dated inclusions, in combination with marked variations in the carbon and nitrogen isotope compositions of the host diamonds, record mixing arrays between a minimum of three components (A : peridotitic mantle ; B : eclogites dominated by mafic material ; C : eclogites that include recycled sedimentary material). Diamond formation appears dominated by local fluid--rock interactions involving different protoliths in the SCLM. Redistribution of carbon during fluid--rock interactions generally masks any potential temporal changes of the deep carbon cycle. |
Kitayama, Y., Thomassot, E., Galy, A., Kordakov, A., Golovin, A., DtextquoterightEyrames, E. Geochemical evidence for carbon and chlorine enrichments in the mantle source of kimberlites (Udachnaya pipe, Siberian craton) (Article de journal) Dans: Geochimica et Cosmochimica Acta, vol. 315, p. 295–316, 2021. @article{Kitayama_etal2021,
title = {Geochemical evidence for carbon and chlorine enrichments in the mantle source of kimberlites (Udachnaya pipe, Siberian craton)},
author = {Y. Kitayama and E. Thomassot and A. Galy and A. Kordakov and A. Golovin and E. DtextquoterightEyrames},
doi = {10.1016/j.gca.2021.09.021},
year = {2021},
date = {2021-01-01},
journal = {Geochimica et Cosmochimica Acta},
volume = {315},
pages = {295--316},
abstract = {Deep, carbonate-rich melts are key constituents of kimberlites and are crucial for understanding the cycle of volatile elements in the mantle. On the Siberian craton, the Udachnaya-East kimberlite hosts extremely well-preserved nodules composed of chlorides + carbonates + sulfates, that do not present any relict sedimentary textures. These salty nodules display textures that are commonly observed in quenched liquids and may thus represent the very last stage liquid of the kimberlite. Alternatively, they could represent assimilated sedimentary material, or even post-magmatic hydrothermal alteration, because kimberlites are known to ascend through the lithosphere while assimilating material from their wall rocks.Here we focus specifically on those chloride-carbonate nodules, which are composed of 70% chloride + 30% alkali-carbonate and sulfate, and used two radiogenic systems (Rb-Sr, Sm-Nd) and the isotopic composition of sulfur, in addition to their major and trace element compositions (n = 3). We then compared the results with the same geochemical data on host kimberlites (n = 4), sedimentary cover (n = 3) and hydrothermal veins (n = 3).Taken together, our results show that the nodules are not the product of a contamination by the Cambrian sedimentary cover. Trace element patterns of the nodules display extreme enrichments in the same elements that are relatively depleted in the host kimberlite but also in kimberlites worldwide (K, Rb, Sr, Pb), suggesting that chloride-carbonate nodules are snapshots of the latest stage liquid present in the kimberlite system. Their isotopic compositions (Rb-Sr, Sm-Nd and $delta$34S) are consistent with a common magmatic source with their host kimberlite. We propose that chloride-carbonate nodules record a missing compositional endmember, which could explain the trend towards more radiogenic Sr isotope ratios at nearly constant Nd signatures observed in their host kimberlite, as well as in other kimberlites worldwide. This observed trend suggests the presence of a recycled component with high Rb/Sr (such as salts or terrigenous sediments) in the mantle sampled by some kimberlites, either in the lithosphere or the asthenosphere. This study highlights that the role of alkalies and halogens may have been underestimated in the genesis of kimberlites at depths where diamonds are stable, as well as in more evolved magmatic stages. Segregations of chlorides and carbonates occur specifically in sulfate-bearing kimberlites, which may thus sample a mantle domain in which sulfates with $delta$34S \> 0texttenthousand are dominant. The existence of such a reservoir could explain the apparent imbalance observed between the chondritic value ($delta$34S of 0texttenthousand) and the negative S isotopic compositions of mantle sulfides (MORB and peridotites).},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Deep, carbonate-rich melts are key constituents of kimberlites and are crucial for understanding the cycle of volatile elements in the mantle. On the Siberian craton, the Udachnaya-East kimberlite hosts extremely well-preserved nodules composed of chlorides + carbonates + sulfates, that do not present any relict sedimentary textures. These salty nodules display textures that are commonly observed in quenched liquids and may thus represent the very last stage liquid of the kimberlite. Alternatively, they could represent assimilated sedimentary material, or even post-magmatic hydrothermal alteration, because kimberlites are known to ascend through the lithosphere while assimilating material from their wall rocks.Here we focus specifically on those chloride-carbonate nodules, which are composed of 70% chloride + 30% alkali-carbonate and sulfate, and used two radiogenic systems (Rb-Sr, Sm-Nd) and the isotopic composition of sulfur, in addition to their major and trace element compositions (n = 3). We then compared the results with the same geochemical data on host kimberlites (n = 4), sedimentary cover (n = 3) and hydrothermal veins (n = 3).Taken together, our results show that the nodules are not the product of a contamination by the Cambrian sedimentary cover. Trace element patterns of the nodules display extreme enrichments in the same elements that are relatively depleted in the host kimberlite but also in kimberlites worldwide (K, Rb, Sr, Pb), suggesting that chloride-carbonate nodules are snapshots of the latest stage liquid present in the kimberlite system. Their isotopic compositions (Rb-Sr, Sm-Nd and $delta$34S) are consistent with a common magmatic source with their host kimberlite. We propose that chloride-carbonate nodules record a missing compositional endmember, which could explain the trend towards more radiogenic Sr isotope ratios at nearly constant Nd signatures observed in their host kimberlite, as well as in other kimberlites worldwide. This observed trend suggests the presence of a recycled component with high Rb/Sr (such as salts or terrigenous sediments) in the mantle sampled by some kimberlites, either in the lithosphere or the asthenosphere. This study highlights that the role of alkalies and halogens may have been underestimated in the genesis of kimberlites at depths where diamonds are stable, as well as in more evolved magmatic stages. Segregations of chlorides and carbonates occur specifically in sulfate-bearing kimberlites, which may thus sample a mantle domain in which sulfates with $delta$34S > 0texttenthousand are dominant. The existence of such a reservoir could explain the apparent imbalance observed between the chondritic value ($delta$34S of 0texttenthousand) and the negative S isotopic compositions of mantle sulfides (MORB and peridotites). |
Vezinet, A., Pearson, D. G., Thomassot, E. Effects of contamination on whole-rock isochrons in ancient rocks: A numerical modelling approach (Article de journal) Dans: Lithos, vol. 386-387, p. 106040, 2021. @article{Vezinet_etal2021,
title = {Effects of contamination on whole-rock isochrons in ancient rocks: A numerical modelling approach},
author = {A. Vezinet and D. G. Pearson and E. Thomassot},
doi = {10.1016/j.lithos.2021.106040},
year = {2021},
date = {2021-01-01},
journal = {Lithos},
volume = {386-387},
pages = {106040},
abstract = {Radiometric decay systems have played a crucial role in developing our understanding of the evolution of the early Earth. There are two main types of protocols for isotope measurements in geological materials: (i) bulk dissolution of rocks, or whole-grains and (ii) spatially resolved techniques (laser-ablation or ion-beam). These two approaches have sometimes led to results that are not easily reconciled for early Earth crustal rocks (�W 3.6 Ga). While initial radiogenic isotope signatures (e.g. initial 176Hf/177Hf or initial 143Nd/144Nd) obtained from whole-rock protocols are significantly above chondritic values, indicative of extensive chemical differentiation of the mantle before 3.6 Ga, data from spatially resolved analysis of individual mineral growth domains point toward much less dramatic differentiation. This is indicated by the majority of data falling close to models of Earthtextquoterights mantle that had not experienced major silicate melt removal into the crust. These data show chondritic or sub-chondritic signatures. Interpretations of whole rock isochrons are built on assumptions about the history and relationship of a number of different samples to each other. At the heart of these assumptions, the effects of secondary process--such as metasomatism--on isotopic compositions and consequently on the age and initial ratio of isochrons, are often considered negligible. In order to evaluate the possible effects of metasomatism and metamorphism on co-genetic igneous suites we modelled the impact of contamination by an external component on both the isochron slope (the apparent age) and the isochron intercept (the initial radiogenic isotope signature). A significant outcome is that the age significance of some of the modified isochron arrays remains to a large extent within uncertainty of the original crystallisation age of the igneous suite. In other words, the original age signature is preserved, but with lower precision. The intercept of the isochron, from which the initial isotope ratio is calculated, however is often significantly modified, which has consequences for the interpretation of these signatures. Our results provide an explanation for the discrepancy between whole-rock and spatially-resolved results observed in early Earth material. Lastly, our results, applied to studies of ancient crustal rocks, are interpreted as indicative of no significantly depleted mantle domains before 3.6 Ga, and no Hfsingle bondNd isotopes decoupling at that time.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Radiometric decay systems have played a crucial role in developing our understanding of the evolution of the early Earth. There are two main types of protocols for isotope measurements in geological materials: (i) bulk dissolution of rocks, or whole-grains and (ii) spatially resolved techniques (laser-ablation or ion-beam). These two approaches have sometimes led to results that are not easily reconciled for early Earth crustal rocks (�W 3.6 Ga). While initial radiogenic isotope signatures (e.g. initial 176Hf/177Hf or initial 143Nd/144Nd) obtained from whole-rock protocols are significantly above chondritic values, indicative of extensive chemical differentiation of the mantle before 3.6 Ga, data from spatially resolved analysis of individual mineral growth domains point toward much less dramatic differentiation. This is indicated by the majority of data falling close to models of Earthtextquoterights mantle that had not experienced major silicate melt removal into the crust. These data show chondritic or sub-chondritic signatures. Interpretations of whole rock isochrons are built on assumptions about the history and relationship of a number of different samples to each other. At the heart of these assumptions, the effects of secondary process--such as metasomatism--on isotopic compositions and consequently on the age and initial ratio of isochrons, are often considered negligible. In order to evaluate the possible effects of metasomatism and metamorphism on co-genetic igneous suites we modelled the impact of contamination by an external component on both the isochron slope (the apparent age) and the isochron intercept (the initial radiogenic isotope signature). A significant outcome is that the age significance of some of the modified isochron arrays remains to a large extent within uncertainty of the original crystallisation age of the igneous suite. In other words, the original age signature is preserved, but with lower precision. The intercept of the isochron, from which the initial isotope ratio is calculated, however is often significantly modified, which has consequences for the interpretation of these signatures. Our results provide an explanation for the discrepancy between whole-rock and spatially-resolved results observed in early Earth material. Lastly, our results, applied to studies of ancient crustal rocks, are interpreted as indicative of no significantly depleted mantle domains before 3.6 Ga, and no Hfsingle bondNd isotopes decoupling at that time. |
Chen, C. H., Mentink-Vigier, F., Trébosc, J., Goldberga, I., Gaveau, P., E.,, Thomassot, Labeling and probing the silica surface using mechanochemistry and 17O NMR spectroscopy (Article de journal) Dans: Chemistry A European Journal, 2021. @article{Chen_etal2021,
title = {Labeling and probing the silica surface using mechanochemistry and 17O NMR spectroscopy},
author = {C. H. Chen and F. Mentink-Vigier and J. Tr\'{e}bosc and I. Goldberga and P. Gaveau and E. and Thomassot},
doi = {10.1002/chem.202101421},
year = {2021},
date = {2021-01-01},
journal = {Chemistry A European Journal},
abstract = {In recent years, there has been increasing interest in developing cost-efficient, fast, and user-friendly 17 O enrichment protocols to help understand the structure and reactivity of materials using 17 O NMR. Here, we show for the first time how ball milling (BM) can be used to selectively and efficiently enrich the surface of fumed silica, which is widely used at the industrial scale. Short milling times (up to 15 min) allowed modulation of the enrichment level (up to ca. 5%) without significantly changing the nature of the material. High-precision 17 O-compositions were measured at different milling times using large-geometry secondary-ion mass spectrometry LG-SIMS. High-resolution 17 O NMR analyses (including at 35.2 T) allowed clear identification of the signals from siloxane (Si-O-Si) and silanols (Si-OH), while DNP analyses, performed using direct 17 O polarization and indirect 17 O 1 H CP excitation, agreed with selective labeling of the surface. Information on the distribution of Si-OH environments at the surface was obtained from 2D 1 H- 17 O D-HMQC correlations. Finally, the surface-labeled silica was reacted with titania and using 17 O DNP, their common interface was probed and Si-O-Ti bonds identified.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In recent years, there has been increasing interest in developing cost-efficient, fast, and user-friendly 17 O enrichment protocols to help understand the structure and reactivity of materials using 17 O NMR. Here, we show for the first time how ball milling (BM) can be used to selectively and efficiently enrich the surface of fumed silica, which is widely used at the industrial scale. Short milling times (up to 15 min) allowed modulation of the enrichment level (up to ca. 5%) without significantly changing the nature of the material. High-precision 17 O-compositions were measured at different milling times using large-geometry secondary-ion mass spectrometry LG-SIMS. High-resolution 17 O NMR analyses (including at 35.2 T) allowed clear identification of the signals from siloxane (Si-O-Si) and silanols (Si-OH), while DNP analyses, performed using direct 17 O polarization and indirect 17 O 1 H CP excitation, agreed with selective labeling of the surface. Information on the distribution of Si-OH environments at the surface was obtained from 2D 1 H- 17 O D-HMQC correlations. Finally, the surface-labeled silica was reacted with titania and using 17 O DNP, their common interface was probed and Si-O-Ti bonds identified. |
2020
|
Barré, G., Strzerzynski, P., Michels, R., Guillot, S., Cartigny, P., Thomassot, E., Lorgeoux, C., Assayag, N., Truche, L. Tectono-metamorphic evolution of an evaporitic décollement as recorded by mineral and fluid geochemistry : The textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright (Western Alps) case study (Article de journal) Dans: Lithos, vol. 358-359, p. 105419, 2020. @article{Barr_etal2020,
title = {Tectono-metamorphic evolution of an evaporitic d\'{e}collement as recorded by mineral and fluid geochemistry : The textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright (Western Alps) case study},
author = {G. Barr\'{e} and P. Strzerzynski and R. Michels and S. Guillot and P. Cartigny and E. Thomassot and C. Lorgeoux and N. Assayag and L. Truche},
doi = {10.1016/j.lithos.2020.105419},
year = {2020},
date = {2020-01-01},
journal = {Lithos},
volume = {358-359},
pages = {105419},
abstract = {Evaporites play a major role on the structuration of collisional orogens especially when they act as d\'{e}collement units. However, their exact pressure-temperature-deformation (P-T-d) paths are poorly documented. In this study, the first direct P-T-d constraints of the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright formation (western French Alps) have been established. An innovative association of structural geology, petrography, crystallochemistry, and detailed study of both fluid inclusions and stable isotopes (C, O) analysis has been applied to this evaporitic facies. Geochemical analysis shows that the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright formation has recorded the three typical metamorphic and deformational events of the Alps (namely D1, D2 and D3). These different constraints allow the determination of the first P-T path determination for this unit. Metamorphic peak conditions of the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright are at 16.6 textpm 2.3 kbar and 431 textdegreeC textpm 28 textdegreeC. This formation was buried at similar conditions than the oceanic units. During the exhumation path, the D1-D2 transition is reached at 350 textdegreeC textpm 20 textdegreeC and 6.5 textpm 1.8 kbar and the D2-D3 transition is assumed to be at 259 textdegreeC textpm 24 textdegreeC and 2.0 textpm 1.0 kbar (Strzerzynski et al., 2012). Peak P-T conditions overlap those of the median Liguro-Piemontese units but are different from those of the Brian\c{c}onnais units. It implies 1) an active and crucial role of the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright during the exhumation of the Alpine oceanic complex, and 2) confirms the allochthonous and more distal origin of the European Tethyan passive margin of the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright formation. Consideration of sulfates dehydration probably between 15.0 and 16.6 kbar and 200 and 300 textdegreeC, allows to discuss pore pressure excess and its mechanical consequences on the exhumation process. This process is very likely to amplify the textquotelefttextquoteleftd\'{e}collementtextquoterighttextquoteright effect of the evaporites and allow the nappe stack formation.This illustrates the role of this formation as a d\'{e}collement surface. This difference of evolution highlights the major role of the evaporitic formations on the exhumation and structuration of a collisional chain. Such methodology could contribute to decipher the role of evaporites in the structural context of other collisional chains such as Himalaya, Pyrenees or Zagros.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Evaporites play a major role on the structuration of collisional orogens especially when they act as décollement units. However, their exact pressure-temperature-deformation (P-T-d) paths are poorly documented. In this study, the first direct P-T-d constraints of the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright formation (western French Alps) have been established. An innovative association of structural geology, petrography, crystallochemistry, and detailed study of both fluid inclusions and stable isotopes (C, O) analysis has been applied to this evaporitic facies. Geochemical analysis shows that the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright formation has recorded the three typical metamorphic and deformational events of the Alps (namely D1, D2 and D3). These different constraints allow the determination of the first P-T path determination for this unit. Metamorphic peak conditions of the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright are at 16.6 textpm 2.3 kbar and 431 textdegreeC textpm 28 textdegreeC. This formation was buried at similar conditions than the oceanic units. During the exhumation path, the D1-D2 transition is reached at 350 textdegreeC textpm 20 textdegreeC and 6.5 textpm 1.8 kbar and the D2-D3 transition is assumed to be at 259 textdegreeC textpm 24 textdegreeC and 2.0 textpm 1.0 kbar (Strzerzynski et al., 2012). Peak P-T conditions overlap those of the median Liguro-Piemontese units but are different from those of the Briançonnais units. It implies 1) an active and crucial role of the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright during the exhumation of the Alpine oceanic complex, and 2) confirms the allochthonous and more distal origin of the European Tethyan passive margin of the textquotelefttextquoteleftNappe des Gypsestextquoterighttextquoteright formation. Consideration of sulfates dehydration probably between 15.0 and 16.6 kbar and 200 and 300 textdegreeC, allows to discuss pore pressure excess and its mechanical consequences on the exhumation process. This process is very likely to amplify the textquotelefttextquoteleftdécollementtextquoterighttextquoteright effect of the evaporites and allow the nappe stack formation.This illustrates the role of this formation as a décollement surface. This difference of evolution highlights the major role of the evaporitic formations on the exhumation and structuration of a collisional chain. Such methodology could contribute to decipher the role of evaporites in the structural context of other collisional chains such as Himalaya, Pyrenees or Zagros. |
Castillo-Olivier, M., Giuliani, A., Griffin, W. L., OtextquoterightReilly, S. Y., Drysdale, R. N., Abersteiner, A., Thomassot, E., Li, X. H. New constraints on the source, composition, and post‑emplacement modification of kimberlites from in situ C--O--Sr‑isotope analyses of carbonates from the Benfontein sills (South Africa) (Article de journal) Dans: Contributions to Mineralogy and Petrology, vol. 175, no. 33, 2020. @article{Castillo-Olivier_etal2020,
title = {New constraints on the source, composition, and post‑emplacement modification of kimberlites from in situ C--O--Sr‑isotope analyses of carbonates from the Benfontein sills (South Africa)},
author = {M. Castillo-Olivier and A. Giuliani and W. L. Griffin and S. Y. OtextquoterightReilly and R. N. Drysdale and A. Abersteiner and E. Thomassot and X. H. Li},
doi = {10.1007/s00410-020-1662-7},
year = {2020},
date = {2020-01-01},
journal = {Contributions to Mineralogy and Petrology},
volume = {175},
number = {33},
abstract = {Primary carbonates in kimberlites are the main CO2 carriers in kimberlites and thus can be used to constrain the original carbon and oxygen-isotope composition of kimberlite melts and their deep mantle sources. However, the contribution of syn- and post-emplacement processes to the modification of the C--O-isotope composition of kimberlites is yet to be fully constrained. This study aims to shed new light on this topic through a detailed textural, compositional (major and trace elements), and in situ C--O--Sr isotopic characterisation of carbonates in the Benfontein kimberlite sills (Kimberley, South Africa). Our multi-technique approach not only reveals the petrographic and geochemical complexity of carbonates in kimberlites in unprecedented detail, but also allows identification of the processes that led to their formation, including: (1) magmatic crystallisation of Sr-rich calcite laths and groundmass; (2) crystallisation of late groundmass calcite from hydrothermal fluids; and (3) variable degrees of crustal contamination in carbonate-rich diapirs and secondary veins. These diapirs most likely resulted from a residual C--O--H fluid or carbonate melt with contributions from methane-rich fluids from the Dwyka shale wall rock, leading to higher 87Sr/86Sr and $delta$18O, but lower $delta$13C values than in pristine magmatic calcite. Before coalescing into the diapiric segregations, these fluids/melts also variably entrained early formed calcite laths and groundmass phases. Comparison between in situ and bulk-carbonate analyses confirms that O isotopic analyses of bulk carbonates from kimberlite rocks are not representative of the original isotopic signature of the kimberlite magma, whereas bulk C-isotope compositions are similar to those of the pristine magmatic carbonates. Calcite laths and most groundmass grains at Benfontein preserve isotopic values ($delta$18O = 6--8texttenthousand and $delta$13C = − 4 to − 6texttenthousand), similar to those of unaltered carbonatites worldwide, which, therefore, probably correspond to those of their parental melts. This narrow range suggests kimberlite derivation from a mantle source with little contribution from recycled crustal material unless the recycled material had isotopic composition indistinguishable from typical mantle values.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Primary carbonates in kimberlites are the main CO2 carriers in kimberlites and thus can be used to constrain the original carbon and oxygen-isotope composition of kimberlite melts and their deep mantle sources. However, the contribution of syn- and post-emplacement processes to the modification of the C--O-isotope composition of kimberlites is yet to be fully constrained. This study aims to shed new light on this topic through a detailed textural, compositional (major and trace elements), and in situ C--O--Sr isotopic characterisation of carbonates in the Benfontein kimberlite sills (Kimberley, South Africa). Our multi-technique approach not only reveals the petrographic and geochemical complexity of carbonates in kimberlites in unprecedented detail, but also allows identification of the processes that led to their formation, including: (1) magmatic crystallisation of Sr-rich calcite laths and groundmass; (2) crystallisation of late groundmass calcite from hydrothermal fluids; and (3) variable degrees of crustal contamination in carbonate-rich diapirs and secondary veins. These diapirs most likely resulted from a residual C--O--H fluid or carbonate melt with contributions from methane-rich fluids from the Dwyka shale wall rock, leading to higher 87Sr/86Sr and $delta$18O, but lower $delta$13C values than in pristine magmatic calcite. Before coalescing into the diapiric segregations, these fluids/melts also variably entrained early formed calcite laths and groundmass phases. Comparison between in situ and bulk-carbonate analyses confirms that O isotopic analyses of bulk carbonates from kimberlite rocks are not representative of the original isotopic signature of the kimberlite magma, whereas bulk C-isotope compositions are similar to those of the pristine magmatic carbonates. Calcite laths and most groundmass grains at Benfontein preserve isotopic values ($delta$18O = 6--8texttenthousand and $delta$13C = − 4 to − 6texttenthousand), similar to those of unaltered carbonatites worldwide, which, therefore, probably correspond to those of their parental melts. This narrow range suggests kimberlite derivation from a mantle source with little contribution from recycled crustal material unless the recycled material had isotopic composition indistinguishable from typical mantle values. |
Howell, D., Stachel, T., Stern, R. A., Pearson, D. G., Nestola, F., Hardman, M. F., Harris, J. W., Jaques, A. L., Shirey, S. B., Cartigny, P., Smit, K. W., Ailbach, S., Brenker, F. E., Jacob, D. E., Thomassot, E., Walter, M. J., Navon, O. Deep carbon through time : Earthtextquoterights diamond record and its implications for carbon cycling and fluid speciation in the mantle (Article de journal) Dans: Geochimica et Cosmochimica Acta, vol. 275, p. 99–122, 2020. @article{Howell_etal2020,
title = {Deep carbon through time : Earthtextquoterights diamond record and its implications for carbon cycling and fluid speciation in the mantle},
author = {D. Howell and T. Stachel and R. A. Stern and D. G. Pearson and F. Nestola and M. F. Hardman and J. W. Harris and A. L. Jaques and S. B. Shirey and P. Cartigny and K. W. Smit and S. Ailbach and F. E. Brenker and D. E. Jacob and E. Thomassot and M. J. Walter and O. Navon},
doi = {10.1016/j.gca.2020.02.011},
year = {2020},
date = {2020-01-01},
journal = {Geochimica et Cosmochimica Acta},
volume = {275},
pages = {99--122},
abstract = {Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earthtextquoterights history. Diamondstextquoteright inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as �`u3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from �`u3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total $delta$13C data range from −17.1 to −1.9 texttenthousand (P = −8.4 to −1.9 texttenthousand ; E = −17.1 to −2.1texttenthousand) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm ; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since \> 3 Ga. The mode in $delta$13C of peridotitic diamonds has been at −5 (textpm2) texttenthousand since the earliest diamond growth �`u3.5 Ga, and this mode is also observed in the eclogitic diamond record since �`u3 Ga. The skewness of eclogitic diamondstextquoteright $delta$13C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent.No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of \<1texttenthousand, but also that the principal $delta$13C mode of −5 textpm 1texttenthousand in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earthtextquoterights history. Diamondstextquoteright inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as �`u3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from �`u3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total $delta$13C data range from −17.1 to −1.9 texttenthousand (P = −8.4 to −1.9 texttenthousand ; E = −17.1 to −2.1texttenthousand) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm ; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in $delta$13C of peridotitic diamonds has been at −5 (textpm2) texttenthousand since the earliest diamond growth �`u3.5 Ga, and this mode is also observed in the eclogitic diamond record since �`u3 Ga. The skewness of eclogitic diamondstextquoteright $delta$13C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent.No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1texttenthousand, but also that the principal $delta$13C mode of −5 textpm 1texttenthousand in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga. |
Liu, J., Pearson, D. G., Shu, Q., Sigurdsson, H., Thomassot, E., Alard, O. Dating post-Archean lithospheric mantle: Insights fromRe-Os and Lu-Hf isotopic systematics of the CameroonVolcanic Line peridotites (Article de journal) Dans: Geochimica et Cosmochimica Acta, vol. 278, p. 177–198, 2020. @article{Liu_etal2020,
title = {Dating post-Archean lithospheric mantle: Insights fromRe-Os and Lu-Hf isotopic systematics of the CameroonVolcanic Line peridotites},
author = {J. Liu and D. G. Pearson and Q. Shu and H. Sigurdsson and E. Thomassot and O. Alard},
doi = {10.1016/j.gca.2019.07.010},
year = {2020},
date = {2020-01-01},
journal = {Geochimica et Cosmochimica Acta},
volume = {278},
pages = {177--198},
abstract = {Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to theincreasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in datingpost-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is impor-tant to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peri-dotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes,highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyosin the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized bylow to moderate olivine Fo contents (88.9--91.2) and low spinel Cr# (8.4--19.3), together with moderate to high whole-rockAl2O3contents (2.0--3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to \<20%partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of meltdepletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyrox-ene with143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration,most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenescannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recentmetasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget ofthese rocks and has176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rockLu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron fromortho- and clinopyroxenes gives an age of 2.01 textpm 0.18 Ga (1r), and when olivine and spinel are considered, is 1.82textpm 0.14 Ga (1r). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2--2.4 Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0 Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at2.0 Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that iscomplementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archeanperidotites. The recognition of2.0 Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyosregion was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleopro-terozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin ofthe CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source ofthe CVL basalts.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to theincreasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in datingpost-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is impor-tant to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peri-dotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes,highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyosin the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized bylow to moderate olivine Fo contents (88.9--91.2) and low spinel Cr# (8.4--19.3), together with moderate to high whole-rockAl2O3contents (2.0--3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20%partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of meltdepletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyrox-ene with143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration,most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenescannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recentmetasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget ofthese rocks and has176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rockLu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron fromortho- and clinopyroxenes gives an age of 2.01 textpm 0.18 Ga (1r), and when olivine and spinel are considered, is 1.82textpm 0.14 Ga (1r). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2--2.4 Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0 Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at2.0 Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that iscomplementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archeanperidotites. The recognition of2.0 Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyosregion was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleopro-terozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin ofthe CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source ofthe CVL basalts. |
Lorand, J. P., Labidi, J., Rollion-Bard, C., Thomassot, E., Bellucci, J. J., Whitehouse, M., Nemchin, A., Humayun, M., Farquhar, J., Hewins, R. H., Zanda, B., Pont, S. The sulfur budget and sulfur isotopic composition of Martian regolith breccia NWA 7533 (Article de journal) Dans: Meteoritics & Planetary Science, vol. 55, no. 9, p. 2097–2116, 2020. @article{Lorand_etal2020,
title = {The sulfur budget and sulfur isotopic composition of Martian regolith breccia NWA 7533},
author = {J. P. Lorand and J. Labidi and C. Rollion-Bard and E. Thomassot and J. J. Bellucci and M. Whitehouse and A. Nemchin and M. Humayun and J. Farquhar and R. H. Hewins and B. Zanda and S. Pont},
doi = {10.1111/maps.13564},
year = {2020},
date = {2020-01-01},
journal = {Meteoritics \& Planetary Science},
volume = {55},
number = {9},
pages = {2097--2116},
abstract = {The sulfur isotope budget of Martian regolith breccia (NWA 7533) has been addressed from conventional fluorination bulk rock analyses and ion microprobe in situ analyses. The bulk rock analyses yield 865 textpm 50 ppm S in agreement with LA‐ICP‐MS analyses. These new data support previous estimates of 80% S loss resulting from terrestrial weathering of NWA 7533 pyrite. Pyrite is by far the major S host. Apatite and Fe oxyhydroxides are negligible S carriers, as are the few tiny igneous pyrrhotite--pentlandite sulfide grains included in lithic clasts so far identified. A small nonzero $Delta$33S (−0.029 textpm 0.010texttenthousand) signal is clearly resolved at the 2$sigma$ level in the bulk rock analyses, coupled with negative CDT‐normalized $delta$34S (−2.54 textpm 0.10texttenthousand), and near‐zero $Delta$36S (0.002 textpm 0.09texttenthousand). In situ analyses also yield negative $Delta$33S values (−0.05 to −0.30texttenthousand) with only a few positive $Delta$33S up to +0.38texttenthousand. The slight discrepancy compared to the bulk rock results is attributed to a possible sampling bias. The occurrence of mass‐independent fractionation (MIF) supports a model of NWA 7533 pyrite formation from surface sulfur that experienced photochemical reaction(s). The driving force that recycled crustal S in NWA 7533 lithologies---magmatic intrusions or impact‐induced heat---is presently unclear. However, in situ analyses also gave negative $delta$34S values (+1 to −5.8texttenthousand). Such negative values in the hydrothermal setting of NWA 7533 are reflective of hydrothermal sulfides precipitated from H2S/HS‐ aqueous fluid produced via open‐system thermochemical reduction of sulfates at high temperatures (\>300 textdegreeC).},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The sulfur isotope budget of Martian regolith breccia (NWA 7533) has been addressed from conventional fluorination bulk rock analyses and ion microprobe in situ analyses. The bulk rock analyses yield 865 textpm 50 ppm S in agreement with LA‐ICP‐MS analyses. These new data support previous estimates of 80% S loss resulting from terrestrial weathering of NWA 7533 pyrite. Pyrite is by far the major S host. Apatite and Fe oxyhydroxides are negligible S carriers, as are the few tiny igneous pyrrhotite--pentlandite sulfide grains included in lithic clasts so far identified. A small nonzero $Delta$33S (−0.029 textpm 0.010texttenthousand) signal is clearly resolved at the 2$sigma$ level in the bulk rock analyses, coupled with negative CDT‐normalized $delta$34S (−2.54 textpm 0.10texttenthousand), and near‐zero $Delta$36S (0.002 textpm 0.09texttenthousand). In situ analyses also yield negative $Delta$33S values (−0.05 to −0.30texttenthousand) with only a few positive $Delta$33S up to +0.38texttenthousand. The slight discrepancy compared to the bulk rock results is attributed to a possible sampling bias. The occurrence of mass‐independent fractionation (MIF) supports a model of NWA 7533 pyrite formation from surface sulfur that experienced photochemical reaction(s). The driving force that recycled crustal S in NWA 7533 lithologies---magmatic intrusions or impact‐induced heat---is presently unclear. However, in situ analyses also gave negative $delta$34S values (+1 to −5.8texttenthousand). Such negative values in the hydrothermal setting of NWA 7533 are reflective of hydrothermal sulfides precipitated from H2S/HS‐ aqueous fluid produced via open‐system thermochemical reduction of sulfates at high temperatures (>300 textdegreeC). |
2019
|
Fitzpayne, A., Giuliani, A., Harris, C., Thomassot, E., Cheng, C., Hergt, J. Evidence for subduction-related signatures in the southern African lithosphere from the N-O isotopic composition of metasomatic mantle minerals (Article de journal) Dans: Geochimica et Cosmochimica Acta, vol. 266, p. 237–257, 2019. @article{Fitzpayne_etal2019,
title = {Evidence for subduction-related signatures in the southern African lithosphere from the N-O isotopic composition of metasomatic mantle minerals},
author = {A. Fitzpayne and A. Giuliani and C. Harris and E. Thomassot and C. Cheng and J. Hergt},
doi = {10.1016/j.gca.2019.02.037},
year = {2019},
date = {2019-01-01},
journal = {Geochimica et Cosmochimica Acta},
volume = {266},
pages = {237--257},
abstract = {Current understanding of the fate of subducted material (and related fluids) in the deep Earth can be improved by combining major and trace element geochemistry with stable isotopic compositions of mantle rocks or minerals. Limited isotopic fractionation during high temperature processes means that significant deviations from mantle-like isotope ratios in mantle rocks probably result from recycling of surficial material. To determine the effects and origins of mantle metasomatic fluids/melts, new d15N and d18O data have been collected for thirteen mantle xenoliths -- harzburgites, wehrlites, lherzolites, and MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) rocks -- from the Bultfontein kimberlite (Kimberley, South Africa), which show varying degrees of metasomatism. The d18O values of olivine and orthopyroxene in phlogopite-free harzburgites match the mantle composition (d18Oolivine = +5.2 textpm 0.3texttenthousand ; d18Oorthopyroxene = +5.7 textpm 0.3texttenthousand ; 2 s.d.), consistent with previous inferences that harzburgites were formed by interaction with ancient silica-rich melts unrelated to subduction processes. Wehrlite samples display mineral compositional characteristics (e.g., low La/Zr in clinopyroxene) resembling those of other products of kimberlite melt metasomatism, such as PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks. The inferred interaction with kimberlite melts may be responsible for O isotopic disequilibrium between clinopyroxene and olivine (D18O = +0.2texttenthousand) in the wehrlites of this study. In contrast with broadly mantle-like d18O values, the d15N value of phlogopite in a wehrlite sample (+5.9texttenthousand) differs from the mantle composition (d15N=-5 textpm 2texttenthousand). This unusual N isotopic composition in kimberlite-related mantle products might indicate that a recycled crustal component occurred in the source of the Kimberley kimberlites, or was assimilated during interaction with the lithospheric mantle. Similar major and trace element characteristics in clinopyroxene from phlogopite-lherzolite and MARID samples suggest metasomatism by fluids of similar composition. Lherzolite and MARID clinopyroxene d18O values (as low as +4.4texttenthousand) extend below those reported in mantle peridotites (i.e. d18Oclinopyroxene = +5.6 textpm 0.3texttenthousand ; 2 s.d.), and strong negative correlations are found between mineral d18O values and major element compositions (e.g., Na2O contents in clinopyroxene). Furthermore, phlogopite d15N values (+4 to +7texttenthousand) in the studied lherzolite and MARID samples are higher than mantle values. Combined, the low d18O-high d15N isotopic signatures of MARID and lherzolite samples suggest progressive mantle metasomatism by a melt containing a recycled oceanic crust (eclogitic) component. This study demonstrates that progressive enrichment of the subcontinental lithospheric mantle may be inextricably linked to plate tectonics via recycling of subducted crustal material into the deep mantle.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Current understanding of the fate of subducted material (and related fluids) in the deep Earth can be improved by combining major and trace element geochemistry with stable isotopic compositions of mantle rocks or minerals. Limited isotopic fractionation during high temperature processes means that significant deviations from mantle-like isotope ratios in mantle rocks probably result from recycling of surficial material. To determine the effects and origins of mantle metasomatic fluids/melts, new d15N and d18O data have been collected for thirteen mantle xenoliths -- harzburgites, wehrlites, lherzolites, and MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) rocks -- from the Bultfontein kimberlite (Kimberley, South Africa), which show varying degrees of metasomatism. The d18O values of olivine and orthopyroxene in phlogopite-free harzburgites match the mantle composition (d18Oolivine = +5.2 textpm 0.3texttenthousand ; d18Oorthopyroxene = +5.7 textpm 0.3texttenthousand ; 2 s.d.), consistent with previous inferences that harzburgites were formed by interaction with ancient silica-rich melts unrelated to subduction processes. Wehrlite samples display mineral compositional characteristics (e.g., low La/Zr in clinopyroxene) resembling those of other products of kimberlite melt metasomatism, such as PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks. The inferred interaction with kimberlite melts may be responsible for O isotopic disequilibrium between clinopyroxene and olivine (D18O = +0.2texttenthousand) in the wehrlites of this study. In contrast with broadly mantle-like d18O values, the d15N value of phlogopite in a wehrlite sample (+5.9texttenthousand) differs from the mantle composition (d15N=-5 textpm 2texttenthousand). This unusual N isotopic composition in kimberlite-related mantle products might indicate that a recycled crustal component occurred in the source of the Kimberley kimberlites, or was assimilated during interaction with the lithospheric mantle. Similar major and trace element characteristics in clinopyroxene from phlogopite-lherzolite and MARID samples suggest metasomatism by fluids of similar composition. Lherzolite and MARID clinopyroxene d18O values (as low as +4.4texttenthousand) extend below those reported in mantle peridotites (i.e. d18Oclinopyroxene = +5.6 textpm 0.3texttenthousand ; 2 s.d.), and strong negative correlations are found between mineral d18O values and major element compositions (e.g., Na2O contents in clinopyroxene). Furthermore, phlogopite d15N values (+4 to +7texttenthousand) in the studied lherzolite and MARID samples are higher than mantle values. Combined, the low d18O-high d15N isotopic signatures of MARID and lherzolite samples suggest progressive mantle metasomatism by a melt containing a recycled oceanic crust (eclogitic) component. This study demonstrates that progressive enrichment of the subcontinental lithospheric mantle may be inextricably linked to plate tectonics via recycling of subducted crustal material into the deep mantle. |
Vézinet, A., Thomassot, E., Pearson, D. G., Stern, R. A., Luo, Y., Sarkar, C. Extreme $delta$18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga (Article de journal) Dans: Geology, vol. 47, no. 7, p. 605–608, 2019. @article{Vzinet_etal2019,
title = {Extreme $delta$18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga},
author = {A. V\'{e}zinet and E. Thomassot and D. G. Pearson and R. A. Stern and Y. Luo and C. Sarkar},
year = {2019},
date = {2019-01-01},
journal = {Geology},
volume = {47},
number = {7},
pages = {605--608},
abstract = {The most ancient rocks in the geological record provide insights into the processes that shaped the evolution and composition of the first continental masses. To better constrain these processes, we made a detailed study of a ca. 3.86 Ga felsic meta-igneous rock from the Eoarchean Saglek Block (North Atlantic Craton) that experienced high-grade metamorphism at ca. 3.5 Ga. Our robust zircon-isotope plus trace-element analyses reveal metamorphic zircon domains with $delta$18O values up to +9texttenthousand at ca. 3.5 Ga, which are the highest values so far measured in any pre--3.0 Ga zircons, metamorphic or igneous, extracted from unambiguous (meta)igneous host rocks. Such elevated zircon $delta$18O signatures clearly document the involvement of mature supracrustal precursors (mafic volcanics textpm clastic/chemical sediments) during the reworking of 3.86 Ga crust at ca. 3.5 Ga. This study provides unequivocal evidence for hydrosphere--crust interactions and reworking processes resulting in metamorphic zircon growth at ca. 3.5 Ga, namely 1 Ga before the Archean-Proterozoic transition.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The most ancient rocks in the geological record provide insights into the processes that shaped the evolution and composition of the first continental masses. To better constrain these processes, we made a detailed study of a ca. 3.86 Ga felsic meta-igneous rock from the Eoarchean Saglek Block (North Atlantic Craton) that experienced high-grade metamorphism at ca. 3.5 Ga. Our robust zircon-isotope plus trace-element analyses reveal metamorphic zircon domains with $delta$18O values up to +9texttenthousand at ca. 3.5 Ga, which are the highest values so far measured in any pre--3.0 Ga zircons, metamorphic or igneous, extracted from unambiguous (meta)igneous host rocks. Such elevated zircon $delta$18O signatures clearly document the involvement of mature supracrustal precursors (mafic volcanics textpm clastic/chemical sediments) during the reworking of 3.86 Ga crust at ca. 3.5 Ga. This study provides unequivocal evidence for hydrosphere--crust interactions and reworking processes resulting in metamorphic zircon growth at ca. 3.5 Ga, namely 1 Ga before the Archean-Proterozoic transition. |
2018
|
Griffin, W. L., Huang, J. X., Thomassot, E., Gain, S. E. M., Toledo, V., OtextquoterightReilly, S. Y. Super-reducing conditions in ancient and modern volcanic systems: sources and behaviour of carbon-rich fluids in the lithospheric mantle (Article de journal) Dans: Mineralogy and Petrology, vol. 112, no. 1, p. 101–114, 2018. @article{Griffin_etal2018,
title = {Super-reducing conditions in ancient and modern volcanic systems: sources and behaviour of carbon-rich fluids in the lithospheric mantle},
author = {W. L. Griffin and J. X. Huang and E. Thomassot and S. E. M. Gain and V. Toledo and S. Y. OtextquoterightReilly},
doi = {10.1007/s00710-018-0575-x},
year = {2018},
date = {2018-01-01},
journal = {Mineralogy and Petrology},
volume = {112},
number = {1},
pages = {101--114},
abstract = {Oxygen fugacity (textflorinO2) is a key parameter of Earthtextquoterights mantle, because it controls the speciation of the fluids migrating at depth; a major question is whether the sublithospheric mantle is metal-saturated, keeping textflorinO2 near the Iron-Wustite (IW) buffer reaction. Cretaceous basaltic pyroclastic rocks on Mt. Carmel, Israel erupted in an intraplate environment with a thin, hot lithosphere. They contain abundant aggregates of hopper-shaped crystals of Ti-rich corundum, which have trapped melts with phenocryst assemblages (Ti2O3, SiC, TiC, silicides, native V) requiring extremely low textflorinO2. These assemblages are interpreted to reflect interaction between basaltic melts and mantle-derived fluids dominated by CH4+H2. Similar highly reduced assemblages are found associated with volcanism in a range of tectonic situations including subduction zones, major continental collisions, intraplate settings, craton margins and the cratons sampled by kimberlites. This distribution, and the worldwide similarity of $delta$13C in mantle-derived SiC and associated diamonds, suggest a widespread process, involving similar sources and independent of tectonic setting. We suggest that the common factor is the ascent of abiotic (CH4+H2) fluids from the sublithospheric mantle; this would imply that much of themantle ismetal-saturated, consistent with observations ofmetallic inclusions in sublithospheric diamonds (e.g. Smith et al. 2016). Such fluids, perhaps carried in rapidly ascending deep-seated magmas, could penetrate high up into a depleted cratonic root, establishing the observed trend of decreasing textflorinO2 with depth (e.g. Yaxley et al. in Lithos 140:142--151, 2012). However, repeated metasomatism (associated with the intrusion of silicate melts) will raise the FeO content near the base of the craton over time, developing a carapace of oxidizing material that would prevent the rise of CH4-rich fluids into higher levels of the subcontinental lithospheric mantle (SCLM). Oxidation of these fluids would release CO2 and H2O to drive metasomatism and low-degree melting both in the carapace and higher in the SCLM. This model can explain the genesis of cratonic diamonds from both reduced and oxidized fluids, the existence of SiC as inclusions in diamonds, and the abundance of SiC in some kimberlites. It should encourage further study of the fine fractions of heavy-mineral concentrates from all types of explosive volcanism.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Oxygen fugacity (textflorinO2) is a key parameter of Earthtextquoterights mantle, because it controls the speciation of the fluids migrating at depth; a major question is whether the sublithospheric mantle is metal-saturated, keeping textflorinO2 near the Iron-Wustite (IW) buffer reaction. Cretaceous basaltic pyroclastic rocks on Mt. Carmel, Israel erupted in an intraplate environment with a thin, hot lithosphere. They contain abundant aggregates of hopper-shaped crystals of Ti-rich corundum, which have trapped melts with phenocryst assemblages (Ti2O3, SiC, TiC, silicides, native V) requiring extremely low textflorinO2. These assemblages are interpreted to reflect interaction between basaltic melts and mantle-derived fluids dominated by CH4+H2. Similar highly reduced assemblages are found associated with volcanism in a range of tectonic situations including subduction zones, major continental collisions, intraplate settings, craton margins and the cratons sampled by kimberlites. This distribution, and the worldwide similarity of $delta$13C in mantle-derived SiC and associated diamonds, suggest a widespread process, involving similar sources and independent of tectonic setting. We suggest that the common factor is the ascent of abiotic (CH4+H2) fluids from the sublithospheric mantle; this would imply that much of themantle ismetal-saturated, consistent with observations ofmetallic inclusions in sublithospheric diamonds (e.g. Smith et al. 2016). Such fluids, perhaps carried in rapidly ascending deep-seated magmas, could penetrate high up into a depleted cratonic root, establishing the observed trend of decreasing textflorinO2 with depth (e.g. Yaxley et al. in Lithos 140:142--151, 2012). However, repeated metasomatism (associated with the intrusion of silicate melts) will raise the FeO content near the base of the craton over time, developing a carapace of oxidizing material that would prevent the rise of CH4-rich fluids into higher levels of the subcontinental lithospheric mantle (SCLM). Oxidation of these fluids would release CO2 and H2O to drive metasomatism and low-degree melting both in the carapace and higher in the SCLM. This model can explain the genesis of cratonic diamonds from both reduced and oxidized fluids, the existence of SiC as inclusions in diamonds, and the abundance of SiC in some kimberlites. It should encourage further study of the fine fractions of heavy-mineral concentrates from all types of explosive volcanism. |
Nicoli, G., Thomassot, E., Schannor, M., Vezinet, A., Jovovic, I. Constraining a Precambrian Wilson Cycle lifespan: An example from the ca. 1.8 Ga Nagssugtoqidian Orogen, Southeastern Greenland (Article de journal) Dans: Lithos, vol. 296-299, p. 1–16, 2018. @article{Nicoli_etal2018,
title = {Constraining a Precambrian Wilson Cycle lifespan: An example from the ca. 1.8 Ga Nagssugtoqidian Orogen, Southeastern Greenland},
author = {G. Nicoli and E. Thomassot and M. Schannor and A. Vezinet and I. Jovovic},
year = {2018},
date = {2018-01-01},
journal = {Lithos},
volume = {296-299},
pages = {1--16},
abstract = {In the Phanerozoic, plate tectonic processes involve the fragmentation of the continental mass, extension and spreading of oceanic domains, subduction of the oceanic lithosphere and lateral shortening that culminate with continental collision (i.e. Wilson cycle). Unlike modern orogenic settings and despite the collection of evidence in the geological record, we lack information to identify such a sequence of events in the Precambrian. This is why it is particularly difficult to track plate tectonics back to 2.0 Ga and beyond. In this study, we aim to show that a multidisciplinary approach on a selected set of samples from a given orogeny can be used to place constraints on crustal evolution within a P-T-t-d-X space. We combine field geology, petrological observations, thermodynamic modelling (Theriak-Domino) and radiogenic (U-Pb, Lu-Hf) and stable isotopes ($delta$18O) to quantify the duration of the different steps of a Wilson cycle. For the purpose of this study, we focus on the Proterozoic Nagssugtoqidian Orogenic Belt (NOB), in the Tasiilaq area, South-East Greenland. Our study reveals that the Nagssugtoqidian Orogen was the result of a complete three stages juvenile crust production (Xjuv)--recycling/reworking sequence: (I) During the 2.60--2.95 Ga period, the Neoarchean Skjoldungen Orogen remobilised basement lithologies formed at TDM 2.91 Ga with progressive increase of the discharge of reworked material (X juv from 75% to 50%;$delta$ 18O: 4 -- 8.5texttenthousand). (II) After a period of crustal stabilization(2.35-- 2.60 Ga), discrete juvenile material inputs ($delta$18 O: 5 -- 6 texttenthousand ) at T DM 2.35 Ga argue for the formation of an oceanic lithosphere and sea floor spreading over a period of textasciitilde0.2 Ga (Xjuv from b 25% to 70%). Lateral shortening is set to have started at ca. 2.05 Ga with the accretion of volcanic/magmatic arcs (i.e Ammassalik Intrusive Complex) and by subduction of small oceanic domains (M1: 520 textpm 60 textdegreeC at 6.6 textpm 1.4 kbar). (III) Continental collision between the North Atlantic Craton and the Rae Craton occurred at 1.84 -- 1.89 Ga. Crustal thickening of textasciitilde25 km was accompanied by regional metamorphism M2 (690 textpm 20 textdegreeC at 6.25 textpm 0.25 kbar) and remobilization of pre-existing supracrustal lithologies (X juv textasciitilde40%; $delta$ 18 O: 5 -- 10.5 texttenthousand).Rates and durations obtained for sea floor spreading (175 textpm 25 Ma), subduction (125 textpm 75 Ma) and continental collision (ca. 60 Ma) are similar to those observed in Phanerozoic Wilson Cycle but differ from what was estimated for Archean terrains. Therefore, timespans of the different steps of a Wilson cycle might have progressively changed over time as a response to the progressive cratonization of the lithosphere.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In the Phanerozoic, plate tectonic processes involve the fragmentation of the continental mass, extension and spreading of oceanic domains, subduction of the oceanic lithosphere and lateral shortening that culminate with continental collision (i.e. Wilson cycle). Unlike modern orogenic settings and despite the collection of evidence in the geological record, we lack information to identify such a sequence of events in the Precambrian. This is why it is particularly difficult to track plate tectonics back to 2.0 Ga and beyond. In this study, we aim to show that a multidisciplinary approach on a selected set of samples from a given orogeny can be used to place constraints on crustal evolution within a P-T-t-d-X space. We combine field geology, petrological observations, thermodynamic modelling (Theriak-Domino) and radiogenic (U-Pb, Lu-Hf) and stable isotopes ($delta$18O) to quantify the duration of the different steps of a Wilson cycle. For the purpose of this study, we focus on the Proterozoic Nagssugtoqidian Orogenic Belt (NOB), in the Tasiilaq area, South-East Greenland. Our study reveals that the Nagssugtoqidian Orogen was the result of a complete three stages juvenile crust production (Xjuv)--recycling/reworking sequence: (I) During the 2.60--2.95 Ga period, the Neoarchean Skjoldungen Orogen remobilised basement lithologies formed at TDM 2.91 Ga with progressive increase of the discharge of reworked material (X juv from 75% to 50%;$delta$ 18O: 4 -- 8.5texttenthousand). (II) After a period of crustal stabilization(2.35-- 2.60 Ga), discrete juvenile material inputs ($delta$18 O: 5 -- 6 texttenthousand ) at T DM 2.35 Ga argue for the formation of an oceanic lithosphere and sea floor spreading over a period of textasciitilde0.2 Ga (Xjuv from b 25% to 70%). Lateral shortening is set to have started at ca. 2.05 Ga with the accretion of volcanic/magmatic arcs (i.e Ammassalik Intrusive Complex) and by subduction of small oceanic domains (M1: 520 textpm 60 textdegreeC at 6.6 textpm 1.4 kbar). (III) Continental collision between the North Atlantic Craton and the Rae Craton occurred at 1.84 -- 1.89 Ga. Crustal thickening of textasciitilde25 km was accompanied by regional metamorphism M2 (690 textpm 20 textdegreeC at 6.25 textpm 0.25 kbar) and remobilization of pre-existing supracrustal lithologies (X juv textasciitilde40%; $delta$ 18 O: 5 -- 10.5 texttenthousand).Rates and durations obtained for sea floor spreading (175 textpm 25 Ma), subduction (125 textpm 75 Ma) and continental collision (ca. 60 Ma) are similar to those observed in Phanerozoic Wilson Cycle but differ from what was estimated for Archean terrains. Therefore, timespans of the different steps of a Wilson cycle might have progressively changed over time as a response to the progressive cratonization of the lithosphere. |
Vezinet, A., Pearson, D. G., Thomassot, E., Stern, R. A., Sarkar, C., Luo, Y., Fisher, C. M. Hydrothermally-altered mafic crust as source for early Earth TTG: Pb/Hf/O isotope and trace element evidence in zircon from TTG of the Eoarchean Saglek Block, N. Labrador (Article de journal) Dans: Earth and Planetary Science Letters, vol. 503, p. 95–107, 2018. @article{Vezinet_etal2018,
title = {Hydrothermally-altered mafic crust as source for early Earth TTG: Pb/Hf/O isotope and trace element evidence in zircon from TTG of the Eoarchean Saglek Block, N. Labrador},
author = {A. Vezinet and D. G. Pearson and E. Thomassot and R. A. Stern and C. Sarkar and Y. Luo and C. M. Fisher},
doi = {10.1016/j.epsl.2018.09.015},
year = {2018},
date = {2018-01-01},
journal = {Earth and Planetary Science Letters},
volume = {503},
pages = {95--107},
abstract = {The North Atlantic craton hosts extensive exposures of Eoarchean crust, spread through areas of Western Greenland and Northern Labrador (Canada). Of these two areas, the crust of the Saglek Block of Northern Labrador has received far less attention from the scrutiny of modern analytical methods than its better documented Western Greenland equivalent, the Itsaq Gneiss Complex. Here, we present the first coupled trace element and U--Pb/Hf/O isotope dataset for zircon from an early TTG component of the Saglek Block. The combination of textural, elemental and isotopic in-situanalyses enables selection of the least disturbed zircon domains. From these it is demonstrated that the oldest felsic remnants exposed in the Saglek Block were emplaced 3.86 textpm0.01 billionyr (Ga) ago through partial melting of basaltic protoliths. The Hf isotope signature of the oldest zircon domains from the Saglek Block TTG indicates derivation from sources that did not undergo substantial Lu/Hf fractionation, resulting in initial Hf isotope compositions that are chondritic within uncertainty. The oxygen isotope ratios of the least disturbed zircon portions vary from 5.38 textpm0.16textdegree/textdegreetextdegreeto 6.64 textpm0.19textdegree/textdegreetextdegreeand document the interaction of the TTG protoliths with Earthtextquoterights early hydrosphere at low temperature (≤150--200◦C) prior partial melting in the Eoarchean. The results support TTG production in the Eoarchean from variably hydrated basaltic protoliths.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The North Atlantic craton hosts extensive exposures of Eoarchean crust, spread through areas of Western Greenland and Northern Labrador (Canada). Of these two areas, the crust of the Saglek Block of Northern Labrador has received far less attention from the scrutiny of modern analytical methods than its better documented Western Greenland equivalent, the Itsaq Gneiss Complex. Here, we present the first coupled trace element and U--Pb/Hf/O isotope dataset for zircon from an early TTG component of the Saglek Block. The combination of textural, elemental and isotopic in-situanalyses enables selection of the least disturbed zircon domains. From these it is demonstrated that the oldest felsic remnants exposed in the Saglek Block were emplaced 3.86 textpm0.01 billionyr (Ga) ago through partial melting of basaltic protoliths. The Hf isotope signature of the oldest zircon domains from the Saglek Block TTG indicates derivation from sources that did not undergo substantial Lu/Hf fractionation, resulting in initial Hf isotope compositions that are chondritic within uncertainty. The oxygen isotope ratios of the least disturbed zircon portions vary from 5.38 textpm0.16textdegree/textdegreetextdegreeto 6.64 textpm0.19textdegree/textdegreetextdegreeand document the interaction of the TTG protoliths with Earthtextquoterights early hydrosphere at low temperature (≤150--200◦C) prior partial melting in the Eoarchean. The results support TTG production in the Eoarchean from variably hydrated basaltic protoliths. |
2017
|
DtextquoterightEyrames, E., Thomassot, E., Kitayama, Y., Golovin, A., Korsakov, A., Ionov, D. A mantle origin for sulfates in the unusual textquotelefttextquoteleftsaltytextquoterighttextquoteright Udachnaya-East kimberlite from sulfur abundances, speciation and their relationship with groundmass carbonates (Article de journal) Dans: Bulletin de la Société Géologique de France, vol. 188, p. 67–74, 2017. @article{DEyrames_etal2017,
title = {A mantle origin for sulfates in the unusual textquotelefttextquoteleftsaltytextquoterighttextquoteright Udachnaya-East kimberlite from sulfur abundances, speciation and their relationship with groundmass carbonates},
author = {E. DtextquoterightEyrames and E. Thomassot and Y. Kitayama and A. Golovin and A. Korsakov and D. Ionov},
year = {2017},
date = {2017-01-01},
journal = {Bulletin de la Soci\'{e}t\'{e} G\'{e}ologique de France},
volume = {188},
pages = {67--74},
abstract = {The Udachnaya-East pipe in Yakutia in Siberia hosts a unique dry (serpentine-free) body of hypabyssal kimberlite (\<0.64wt% H2O), associated with a less dry type of kimberlite and a serpentinized kimberlitic breccia. The dry kimberlite is anomalously rich in salts (Na2O and Cl both up to 6wt%) whereas the slightly less dry and the breccia kimberlite are salt free. Yet the Udachnaya kimberlite is a group-I kimberlite, as is the archetypical kimberlite from Kimberley, South Africa. Samples were studied from the three different types of kimberlite (dry-salt},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The Udachnaya-East pipe in Yakutia in Siberia hosts a unique dry (serpentine-free) body of hypabyssal kimberlite (<0.64wt% H2O), associated with a less dry type of kimberlite and a serpentinized kimberlitic breccia. The dry kimberlite is anomalously rich in salts (Na2O and Cl both up to 6wt%) whereas the slightly less dry and the breccia kimberlite are salt free. Yet the Udachnaya kimberlite is a group-I kimberlite, as is the archetypical kimberlite from Kimberley, South Africa. Samples were studied from the three different types of kimberlite (dry-salt |
Kitayama, Y., Thomassot, E., Galy, A., Golovin, A., Korsakov, A., DtextquoterightEyrames, E., Assayag, N., Bouden, N., Ionov, D. Co-magmatic sulfides and sulfates in the Udachnaya-East pipe (Siberia): A record of the redox state and isotopic composition of sulfur in kimberlites and their mantle sources (Article de journal) Dans: Chemical Geology, vol. 455, p. 315–330, 2017. @article{Kitayama_etal2017,
title = {Co-magmatic sulfides and sulfates in the Udachnaya-East pipe (Siberia): A record of the redox state and isotopic composition of sulfur in kimberlites and their mantle sources},
author = {Y. Kitayama and E. Thomassot and A. Galy and A. Golovin and A. Korsakov and E. DtextquoterightEyrames and N. Assayag and N. Bouden and D. Ionov},
doi = {10.1016/j.chemgeo.2016.10.037},
year = {2017},
date = {2017-01-01},
journal = {Chemical Geology},
volume = {455},
pages = {315--330},
abstract = {Kimberlites of the Udachnaya-East pipe (Siberia) include a uniquely dry and serpentine-free rock type with anomalously high contents of chlorine (Cl ≤ 6.1 wt%), alkalies (Na2O + K2O ≤ 10 wt%) and sulfur (S ≤ 0.50 wt%), referred to as a textquotelefttextquoteleftsaltytextquoterighttextquoteright kimberlite. The straightforward interpretation is that the Na-, K-, Cl- and S-rich components originate directly from a carbonate-chloride kimberlitic magma that is anhydrous and alkali-rich. However, because brines and evaporites are present on the Siberian craton, previous studies proposed that the kimberlitic magma was contaminated by the assimilation of salt-rich crustal rocks. To clarify the origin of high Cl, alkalies and S in this unusual kimberlite, here we determine its sulfur speciation and isotopic composition and compare it to that of non-salty kimberlites and kimberlitic breccia from the same pipe, as well as potential contamination sources (hydrothermal sulfides and sulfates, country-rock sediment and brine collected in the area). The average $delta$34S of sulfides is−1.4 textpm 2.2texttenthousand in the salty kimberlite, 2.1 textpm 2.7texttenthousand in the non-salty kimberlites and 14.2 textpm 5.8texttenthousand in the breccia. The average $delta$34S of sulfates in the salty kimberlites is 11.1 textpm 1.8texttenthousand and 27.3textpm 1.6texttenthousand in the breccia. In contrast, the $delta$34S of potential contaminants range from 20 to 42texttenthousand for hydrothermal sulfides, from 16 to 34texttenthousand for hydrothermal sulfates, 34texttenthousand for a country-rock sediment (Chukuck suite) and the regional brine aquifer. Our isotope analyses show that (1) in the salty kimberlites, neither sulfates nor sulfides can be simply explained by brine infiltration, hydrothermal alteration or the assimilation of known salt-rich country rocks and instead, we propose that they are late magmatic phases ; (2) in the non-salty kimberlite and breccia, brine infiltration lead to sulfate reduction and the formation of secondary sulfides -- this explains the removal of salts, alkali-carbonates and sulfates, as well as theminor olivine serpentinization ; (3) hydrothermal sulfur was added to the kimberlitic breccia, but not to the massive kimberlites. In situ measurements of sulfides confirm this scenario, clearly showing the addition of two sulfide populations in the breccia (pyrite-pyrrhotites with average $delta$34S of 7.9 textpm 3.4texttenthousand and chalcopyrites with average $delta$34S of 38.0 textpm 0.4texttenthousand) whereas the salty and non-salty kimberlites preserve a unique population of djerfisherites (Cl- and K-rich sulfides) with $delta$34S values within themantle range. This study provides the first direct evidence of alkaline igneous rocks in which magmatic sulfate ismore abundant than sulfide. Although sulfates have been rarely reported in mantle materials, sulfate rich melts may be more common in the mantle than previously thought and could balance the sulfur isotope budget of Earthtextquoterights mantle.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Kimberlites of the Udachnaya-East pipe (Siberia) include a uniquely dry and serpentine-free rock type with anomalously high contents of chlorine (Cl ≤ 6.1 wt%), alkalies (Na2O + K2O ≤ 10 wt%) and sulfur (S ≤ 0.50 wt%), referred to as a textquotelefttextquoteleftsaltytextquoterighttextquoteright kimberlite. The straightforward interpretation is that the Na-, K-, Cl- and S-rich components originate directly from a carbonate-chloride kimberlitic magma that is anhydrous and alkali-rich. However, because brines and evaporites are present on the Siberian craton, previous studies proposed that the kimberlitic magma was contaminated by the assimilation of salt-rich crustal rocks. To clarify the origin of high Cl, alkalies and S in this unusual kimberlite, here we determine its sulfur speciation and isotopic composition and compare it to that of non-salty kimberlites and kimberlitic breccia from the same pipe, as well as potential contamination sources (hydrothermal sulfides and sulfates, country-rock sediment and brine collected in the area). The average $delta$34S of sulfides is−1.4 textpm 2.2texttenthousand in the salty kimberlite, 2.1 textpm 2.7texttenthousand in the non-salty kimberlites and 14.2 textpm 5.8texttenthousand in the breccia. The average $delta$34S of sulfates in the salty kimberlites is 11.1 textpm 1.8texttenthousand and 27.3textpm 1.6texttenthousand in the breccia. In contrast, the $delta$34S of potential contaminants range from 20 to 42texttenthousand for hydrothermal sulfides, from 16 to 34texttenthousand for hydrothermal sulfates, 34texttenthousand for a country-rock sediment (Chukuck suite) and the regional brine aquifer. Our isotope analyses show that (1) in the salty kimberlites, neither sulfates nor sulfides can be simply explained by brine infiltration, hydrothermal alteration or the assimilation of known salt-rich country rocks and instead, we propose that they are late magmatic phases ; (2) in the non-salty kimberlite and breccia, brine infiltration lead to sulfate reduction and the formation of secondary sulfides -- this explains the removal of salts, alkali-carbonates and sulfates, as well as theminor olivine serpentinization ; (3) hydrothermal sulfur was added to the kimberlitic breccia, but not to the massive kimberlites. In situ measurements of sulfides confirm this scenario, clearly showing the addition of two sulfide populations in the breccia (pyrite-pyrrhotites with average $delta$34S of 7.9 textpm 3.4texttenthousand and chalcopyrites with average $delta$34S of 38.0 textpm 0.4texttenthousand) whereas the salty and non-salty kimberlites preserve a unique population of djerfisherites (Cl- and K-rich sulfides) with $delta$34S values within themantle range. This study provides the first direct evidence of alkaline igneous rocks in which magmatic sulfate ismore abundant than sulfide. Although sulfates have been rarely reported in mantle materials, sulfate rich melts may be more common in the mantle than previously thought and could balance the sulfur isotope budget of Earthtextquoterights mantle. |
2016
|
Delavault, H., Chauvel, C., Thomassot, E., Devey, C., Dazas, B. Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume (Article de journal) Dans: Proceedings of the National Academy of Sciences of the United States of America, 2016. @article{Delavault_etal2016,
title = {Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume},
author = {H. Delavault and C. Chauvel and E. Thomassot and C. Devey and B. Dazas},
doi = {10.1073/pnas.1523805113},
year = {2016},
date = {2016-01-01},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
abstract = {The isotopic diversity of oceanic island basalts (OIB) is usually attributed to the influence, in their sources, of ancient material recycled into the mantle, although the nature, age, and quantities of this material remain controversial. The unradiogenic Pb isotope signature of the enriched mantle I (EM I) source of basalts from, for example, Pitcairn or Walvis Ridge has been variously attributed to recycled pelagic sediments, lower continental crust, or recycled subcontinental lithosphere. Our study helps resolve this debate by showing that Pitcairn lavas contain sulfides whose sulfur isotopic compositions are affected by mass-independent fractionation (S-MIF down to $Delta$33S = −0.8), something which is thought to have occurred on Earth only before 2.45 Ga, constraining the youngest possible age of the EM I source component. With this independent age constraint and a Monte Carlo refinement modeling of lead isotopes, we place the likely Pitcairn source age at 2.5 Ga to 2.6 Ga. The Pb, Sr, Nd, and Hf isotopic mixing arrays show that the Archean EM I material was poor in trace elements, resembling Archean sediment. After subduction, this Archean sediment apparently remained stored in the deep Earth for billions of years before returning to the surface as Pitcairntextasciiacutes characteristic EM I signature. The presence of negative S-MIF in the deep mantle may also help resolve the problem of an apparent deficit of negative $Delta$33S anomalies so far found in surface reservoirs.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The isotopic diversity of oceanic island basalts (OIB) is usually attributed to the influence, in their sources, of ancient material recycled into the mantle, although the nature, age, and quantities of this material remain controversial. The unradiogenic Pb isotope signature of the enriched mantle I (EM I) source of basalts from, for example, Pitcairn or Walvis Ridge has been variously attributed to recycled pelagic sediments, lower continental crust, or recycled subcontinental lithosphere. Our study helps resolve this debate by showing that Pitcairn lavas contain sulfides whose sulfur isotopic compositions are affected by mass-independent fractionation (S-MIF down to $Delta$33S = −0.8), something which is thought to have occurred on Earth only before 2.45 Ga, constraining the youngest possible age of the EM I source component. With this independent age constraint and a Monte Carlo refinement modeling of lead isotopes, we place the likely Pitcairn source age at 2.5 Ga to 2.6 Ga. The Pb, Sr, Nd, and Hf isotopic mixing arrays show that the Archean EM I material was poor in trace elements, resembling Archean sediment. After subduction, this Archean sediment apparently remained stored in the deep Earth for billions of years before returning to the surface as Pitcairntextasciiacutes characteristic EM I signature. The presence of negative S-MIF in the deep mantle may also help resolve the problem of an apparent deficit of negative $Delta$33S anomalies so far found in surface reservoirs. |
2015
|
Thomassot, E. Mass spectrometry finds traces of the earliest atmosphere on Earth (Article de journal) Dans: Trends in Analytical Chemistry, vol. 66, p. iv-ix, 2015. @article{Thomassot2015,
title = {Mass spectrometry finds traces of the earliest atmosphere on Earth},
author = {E. Thomassot},
year = {2015},
date = {2015-01-01},
journal = {Trends in Analytical Chemistry},
volume = {66},
pages = {iv-ix},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
|
Thomassot, E., OtextquoterightNeil, J., Francis, D., Cartigny, P., Wing, B. A. Atmospheric record in the Hadean Eon from multiple sulfur isotope measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec) (Article de journal) Dans: Proceedings of the National Academy of Sciences, no. 1, 2015. @article{Thomassot_etal2015,
title = {Atmospheric record in the Hadean Eon from multiple sulfur isotope measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec)},
author = {E. Thomassot and J. OtextquoterightNeil and D. Francis and P. Cartigny and B. A. Wing},
doi = {10.1073/pnas.1419681112},
year = {2015},
date = {2015-01-01},
journal = {Proceedings of the National Academy of Sciences},
number = {1},
abstract = {Mass-independent fractionation of sulfur isotopes (S-MIF) results from photochemical reactions involving short-wavelength UV light. The presence of these anomalies in Archean sediments [(4--2.5 billion years ago, (Ga)] implies that the early atmosphere was free of the appropriate UV absorbers, of which ozone is the most important in the modern atmosphere. Consequently, S-MIF is considered someof the strongest evidence for the lack of free atmospheric oxygen before 2.4 Ga. Although temporal variations in the S-MIF record are thought to depend on changes in the abundances of gas and aerosol species, our limited understanding of photochemical mechanisms complicates interpretation of the S-MIF record in terms of atmospheric composition. Multiple sulfur isotope compositions ($delta$33S, $delta$34S, and $delta$36S) of the \>3.8 billion-year-old Nuvvuagittuq Greenstone Belt (Ungava peninsula) have been investigated to track the early origins of S-MIF. Anomalous S-isotope compositions ($Delta$33S up to +2.2texttenthousand) confirm a sedimentary origin of sulfide-bearing banded iron and silica-rich formations. Sharp isotopic transitions across sedimentary/igneous lithological boundaries indicate that primary surficial S-isotope compositions have been preserved despite a complicated metamorphic history. Furthermore, Nuvvuagittuq metasediments recorded coupled variations in 33S/32S, 34S/32S, and 36S/32S that are statistically indistinguishable from those identified several times later in the Archean. The recurrence of the same S-isotope pattern at both ends of the Archean Eon is unexpected, given the complex atmospheric, geological, and biological pathways involved in producing and preserving this fractionation. It implies that, within 0.8 billion years of Earthtextquoterights formation, a common mechanism for S-MIF production was established in the atmosphere.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Mass-independent fractionation of sulfur isotopes (S-MIF) results from photochemical reactions involving short-wavelength UV light. The presence of these anomalies in Archean sediments [(4--2.5 billion years ago, (Ga)] implies that the early atmosphere was free of the appropriate UV absorbers, of which ozone is the most important in the modern atmosphere. Consequently, S-MIF is considered someof the strongest evidence for the lack of free atmospheric oxygen before 2.4 Ga. Although temporal variations in the S-MIF record are thought to depend on changes in the abundances of gas and aerosol species, our limited understanding of photochemical mechanisms complicates interpretation of the S-MIF record in terms of atmospheric composition. Multiple sulfur isotope compositions ($delta$33S, $delta$34S, and $delta$36S) of the >3.8 billion-year-old Nuvvuagittuq Greenstone Belt (Ungava peninsula) have been investigated to track the early origins of S-MIF. Anomalous S-isotope compositions ($Delta$33S up to +2.2texttenthousand) confirm a sedimentary origin of sulfide-bearing banded iron and silica-rich formations. Sharp isotopic transitions across sedimentary/igneous lithological boundaries indicate that primary surficial S-isotope compositions have been preserved despite a complicated metamorphic history. Furthermore, Nuvvuagittuq metasediments recorded coupled variations in 33S/32S, 34S/32S, and 36S/32S that are statistically indistinguishable from those identified several times later in the Archean. The recurrence of the same S-isotope pattern at both ends of the Archean Eon is unexpected, given the complex atmospheric, geological, and biological pathways involved in producing and preserving this fractionation. It implies that, within 0.8 billion years of Earthtextquoterights formation, a common mechanism for S-MIF production was established in the atmosphere. |
2014
|
Cartigny, P., Palot, M., Thomassot, E., Harris, J. W. Diamond formation: A stable isotope perspective (Article de journal) Dans: Annual Review of Earth and Planetary Science, vol. 42, p. 699–732, 2014. @article{Cartigny_etal2014,
title = {Diamond formation: A stable isotope perspective},
author = {P. Cartigny and M. Palot and E. Thomassot and J. W. Harris},
doi = {10.1146/annurev-earth-042711-105259},
year = {2014},
date = {2014-01-01},
journal = {Annual Review of Earth and Planetary Science},
volume = {42},
pages = {699--732},
abstract = {Primarily on the basis of C, N, S, and O stable isotope systematics, this article reviews recent achievements in understanding diamond formation and growth in Earthtextquoterights mantle. Diamond is a metasomatic mineral that results from either the reduction or oxidation of mobile C-bearing liquids (fluids or melts) that intrude preexisting lithologies (eclogites, peridotites, and metamorphic rocks). This process seems ubiquitous, as it occurs over a large range of depths and extends through time. Diamond-forming carbon derives mainly from the convective asthenosphere. Most of its isotopic anomalies reflect fractionation processes in the lithospheric mantle, which are attributed to diamond precipitation itself and/or a mineralogical control occurring prior to diamond precipitation. Evidence for a mineralogical control would be the decoupling of the 15N/14N ratios in eclogitic diamond from other tracers of subduction in inclusions in the same diamond. C isotope anomalies related to subduction are rare and are best seen in diamonds from the transition zone.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Primarily on the basis of C, N, S, and O stable isotope systematics, this article reviews recent achievements in understanding diamond formation and growth in Earthtextquoterights mantle. Diamond is a metasomatic mineral that results from either the reduction or oxidation of mobile C-bearing liquids (fluids or melts) that intrude preexisting lithologies (eclogites, peridotites, and metamorphic rocks). This process seems ubiquitous, as it occurs over a large range of depths and extends through time. Diamond-forming carbon derives mainly from the convective asthenosphere. Most of its isotopic anomalies reflect fractionation processes in the lithospheric mantle, which are attributed to diamond precipitation itself and/or a mineralogical control occurring prior to diamond precipitation. Evidence for a mineralogical control would be the decoupling of the 15N/14N ratios in eclogitic diamond from other tracers of subduction in inclusions in the same diamond. C isotope anomalies related to subduction are rare and are best seen in diamonds from the transition zone. |
Truche, L., Bazarkina, E. F., Barré, G., Thomassot, E., Berger, G., Dubessy, J., Robert, P. The role of S−3 ion in thermochemical sulphate reduction: Geological and geochemical implications (Article de journal) Dans: Earth and Planetary Science Letters, vol. 396, p. 190–200, 2014. @article{Truche_etal2014,
title = {The role of S−3 ion in thermochemical sulphate reduction: Geological and geochemical implications},
author = {L. Truche and E. F. Bazarkina and G. Barr\'{e} and E. Thomassot and G. Berger and J. Dubessy and P. Robert},
doi = {10.1016/j.epsl.2014.04.018},
year = {2014},
date = {2014-01-01},
journal = {Earth and Planetary Science Letters},
volume = {396},
pages = {190--200},
abstract = {Thermochemical sulphate reduction (TSR) plays a crucial role in the global sulphur cycle in the Earthtextquoterights crust, and may affect current and past sulphur isotopic records. However, the extrapolation of experimental reaction rates measured at high temperature (above 200 textdegreeC) towards lower temperatures, as well as the interpretation of the sulphur isotopic fractionation recorded in natural samples, require an accurate description of the elementary steps controlling these reactions. We addressed this question through dedicated experiments. Based on in situ Raman spectroscopy measurements, we show that the trisulphur ion S−3 is the dominant intermediate sulphur valence species involved in abiogenic sulphate reduction processes initiated by H2S, over a wide range of temperature (100-350 textdegreeC) and solution compositions, whatever the electron donor considered. The in situ spectroscopic data reported here unambiguously demonstrate the presence of S−3 at temperatures as low as 100 textdegreeC. The presence of S−3 is critical to achieve rapid sulphate reduction, especially at low temperature. We propose that any dissolved constituent which decreases the dielectric constant of water, or which yields favourable S−3 coordination, will stabilise the trisulphur ion (thus promoting TSR) at T and pH conditions that are less extreme than previously thought. The importance of S−3in these processes should also be taken into account when discussing the mass-independent sulphur isotopic compositions recorded in natural and/or experimental TSR-related samples.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Thermochemical sulphate reduction (TSR) plays a crucial role in the global sulphur cycle in the Earthtextquoterights crust, and may affect current and past sulphur isotopic records. However, the extrapolation of experimental reaction rates measured at high temperature (above 200 textdegreeC) towards lower temperatures, as well as the interpretation of the sulphur isotopic fractionation recorded in natural samples, require an accurate description of the elementary steps controlling these reactions. We addressed this question through dedicated experiments. Based on in situ Raman spectroscopy measurements, we show that the trisulphur ion S−3 is the dominant intermediate sulphur valence species involved in abiogenic sulphate reduction processes initiated by H2S, over a wide range of temperature (100-350 textdegreeC) and solution compositions, whatever the electron donor considered. The in situ spectroscopic data reported here unambiguously demonstrate the presence of S−3 at temperatures as low as 100 textdegreeC. The presence of S−3 is critical to achieve rapid sulphate reduction, especially at low temperature. We propose that any dissolved constituent which decreases the dielectric constant of water, or which yields favourable S−3 coordination, will stabilise the trisulphur ion (thus promoting TSR) at T and pH conditions that are less extreme than previously thought. The importance of S−3in these processes should also be taken into account when discussing the mass-independent sulphur isotopic compositions recorded in natural and/or experimental TSR-related samples. |
2012
|
Kitayama, Y., Thomassot, E., OtextquoterightNeil, J., Wing, B. A. Sulfur- and oxygen-isotope constraints on the sedimentary history of apparent conglomerates from the Nuvvuagittuq Greenstone Belt (Nunavik, Québec) (Article de journal) Dans: Earth and Planetary Science Letters, vol. 355-356, p. 271–282, 2012. @article{Kitayama_etal2012,
title = {Sulfur- and oxygen-isotope constraints on the sedimentary history of apparent conglomerates from the Nuvvuagittuq Greenstone Belt (Nunavik, Qu\'{e}bec)},
author = {Y. Kitayama and E. Thomassot and J. OtextquoterightNeil and B. A. Wing},
doi = {10.1016/j.epsl.2012.08.026},
year = {2012},
date = {2012-01-01},
journal = {Earth and Planetary Science Letters},
volume = {355-356},
pages = {271--282},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
|
2009
|
Thomassot, E., Cartigny, P., Harris, J. W., Lorand, J. P., Rollion-Bard, C., Chaussidon, M. Metasomatic diamond growth: A multi-isotope study (13C, 15N, 33S, 34S) of sulphide inclusions and their host diamonds from Jwaneng (Botswana) (Article de journal) Dans: Earth and Planetary Science Letters, vol. 282, no. 1-4, p. 79–90, 2009. @article{Thomassot_etal2009,
title = {Metasomatic diamond growth: A multi-isotope study (13C, 15N, 33S, 34S) of sulphide inclusions and their host diamonds from Jwaneng (Botswana)},
author = {E. Thomassot and P. Cartigny and J. W. Harris and J. P. Lorand and C. Rollion-Bard and M. Chaussidon},
doi = {10.1016/j.epsl.2009.03.001},
year = {2009},
date = {2009-01-01},
journal = {Earth and Planetary Science Letters},
volume = {282},
number = {1-4},
pages = {79--90},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
|
