2023
|
Caurant, C., Debret, B., Ménez, B., Nicollet, C., Bouilhol, P. Redox heterogeneities in a subducting slab: Example from the Monviso meta-ophiolite (Western Alps, Italy) (Article de journal) Dans: Lithos, vol. 446-447, p. 107136, 2023. @article{Caurant_etal2023,
title = {Redox heterogeneities in a subducting slab: Example from the Monviso meta-ophiolite (Western Alps, Italy)},
author = {C. Caurant and B. Debret and B. M\'{e}nez and C. Nicollet and P. Bouilhol},
doi = {10.1016/j.lithos.2023.107136},
year = {2023},
date = {2023-01-01},
journal = {Lithos},
volume = {446-447},
pages = {107136},
abstract = {Variations of redox conditions (i.e., oxygen fugacity, fO2) accompanying slab dehydration in subduction zones are subject to ongoing controversies, especially since the interplay between redox sensitive elements during prograde metamorphism remains complex and, likely, variable at the slab scale. Here we investigate fO2 variations during serpentinite dehydration and their feedback on the stability of sulfur and carbon compounds by studying the eclogitic Monviso meta-ophiolite (Western Alps, Italy). Despite a complex metamorphic history, the Monviso massif has preserved a complete section of oceanic lithosphere, from seafloor metasediments, meta-ophicarbonates and metabasites to deep-seated metagabbros and metaserpentinites. By bringing new estimates in the northern massif, we show that these lithologies have recorded a homogeneous pressure and temperature (P-T) climax, at 520--570 textdegreeC and 2.6--2.7 GPa, on the whole meta-ophiolite. Despite this homogeneous P-T record, serpentinite forming minerals imply strong variations in fO2 according to their position in the slab, from high fO2 conditions (textasciitilde FMQ +2) in the deep-seated lithologies made of heazlewoodite-magnetite-olivine assemblages to low fO2 (textasciitilde FMQ −4) in the paleoseafloor lithologies made of pentlandite-awaruite-olivine. This redox gradient is opposed to what is expected at mid-ocean ridges and is therefore likely set up during serpentinite dehydration. Such variations of fO2 conditions also influenced carbon distribution and redox state within the lithosphere. In particular, reducing conditions associated with brucite breakdown in paleoseafloor serpentinites promote the formation of disordered carbonaceous matter over inorganic carbonates. Newly-formed disordered carbonaceous matter could subsequently be recycled in the deep mantle, with the potential to play a major role on the deep carbon cycle.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Variations of redox conditions (i.e., oxygen fugacity, fO2) accompanying slab dehydration in subduction zones are subject to ongoing controversies, especially since the interplay between redox sensitive elements during prograde metamorphism remains complex and, likely, variable at the slab scale. Here we investigate fO2 variations during serpentinite dehydration and their feedback on the stability of sulfur and carbon compounds by studying the eclogitic Monviso meta-ophiolite (Western Alps, Italy). Despite a complex metamorphic history, the Monviso massif has preserved a complete section of oceanic lithosphere, from seafloor metasediments, meta-ophicarbonates and metabasites to deep-seated metagabbros and metaserpentinites. By bringing new estimates in the northern massif, we show that these lithologies have recorded a homogeneous pressure and temperature (P-T) climax, at 520--570 textdegreeC and 2.6--2.7 GPa, on the whole meta-ophiolite. Despite this homogeneous P-T record, serpentinite forming minerals imply strong variations in fO2 according to their position in the slab, from high fO2 conditions (textasciitilde FMQ +2) in the deep-seated lithologies made of heazlewoodite-magnetite-olivine assemblages to low fO2 (textasciitilde FMQ −4) in the paleoseafloor lithologies made of pentlandite-awaruite-olivine. This redox gradient is opposed to what is expected at mid-ocean ridges and is therefore likely set up during serpentinite dehydration. Such variations of fO2 conditions also influenced carbon distribution and redox state within the lithosphere. In particular, reducing conditions associated with brucite breakdown in paleoseafloor serpentinites promote the formation of disordered carbonaceous matter over inorganic carbonates. Newly-formed disordered carbonaceous matter could subsequently be recycled in the deep mantle, with the potential to play a major role on the deep carbon cycle. |
2022
|
Bouilhol, P., Debret, B., Inglis, E., Warembourg, M., Grocolas, T., Rigaudier, T., Villeneuve, J., Burton, K. W. Decoupling of inorganic and organic carbon during slab mantle devolatilisation (Article de journal) Dans: Nature Communications, vol. 13, no. 308, 2022. @article{Bouilhol_etal2022,
title = {Decoupling of inorganic and organic carbon during slab mantle devolatilisation},
author = {P. Bouilhol and B. Debret and E. Inglis and M. Warembourg and T. Grocolas and T. Rigaudier and J. Villeneuve and K. W. Burton},
doi = {10.1038/s41467-022-27970-0},
year = {2022},
date = {2022-01-01},
journal = {Nature Communications},
volume = {13},
number = {308},
abstract = {Serpentinites are an important sink for both inorganic and organic carbon, and their behavior during subduction is thought to play a fundamental role in the global cycling of carbon. Here we show that fluid-derived veins are preserved within the Zermatt-Saas ultra-high pressure serpentinites providing key evidence for carbonate mobility during serpentinite devolatilisation. We show through the O, C, and Sr isotope analyses of vein minerals and the host serpentinites that about 90% of the meta-serpentinite inorganic carbon is remobilized during slab devolatilisation. In contrast, graphite-like carbonaceous compounds remain trapped within the host rock as inclusions within metamorphic olivine while the bulk elemental and isotope composition of organic carbon remains relatively unchanged during the subduction process. This shows a decoupling behavior of carbon during serpentinite dehydration in subduction zones. This process will therefore facilitate the transfer of inorganic carbon to the mantle wedge and the preferential slab sequestration of organic carbon en route to the deep mantle.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Serpentinites are an important sink for both inorganic and organic carbon, and their behavior during subduction is thought to play a fundamental role in the global cycling of carbon. Here we show that fluid-derived veins are preserved within the Zermatt-Saas ultra-high pressure serpentinites providing key evidence for carbonate mobility during serpentinite devolatilisation. We show through the O, C, and Sr isotope analyses of vein minerals and the host serpentinites that about 90% of the meta-serpentinite inorganic carbon is remobilized during slab devolatilisation. In contrast, graphite-like carbonaceous compounds remain trapped within the host rock as inclusions within metamorphic olivine while the bulk elemental and isotope composition of organic carbon remains relatively unchanged during the subduction process. This shows a decoupling behavior of carbon during serpentinite dehydration in subduction zones. This process will therefore facilitate the transfer of inorganic carbon to the mantle wedge and the preferential slab sequestration of organic carbon en route to the deep mantle. |
Grocolas, T., Bouilhol, P., Caro, G., Mojzsis, S. Eoarchean subduction-like magmatism recorded in 3750 Ma mafic--ultramafic rocks of the Ukaliq supracrustal belt (Québec) (Article de journal) Dans: Contributions to Mineralogy and Petrology, vol. 177, no. 39, 2022. @article{Grocolas_etal2022,
title = {Eoarchean subduction-like magmatism recorded in 3750 Ma mafic--ultramafic rocks of the Ukaliq supracrustal belt (Qu\'{e}bec)},
author = {T. Grocolas and P. Bouilhol and G. Caro and S. Mojzsis},
doi = {10.1007/s00410-022-01904-x},
year = {2022},
date = {2022-01-01},
journal = {Contributions to Mineralogy and Petrology},
volume = {177},
number = {39},
abstract = {Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Qu\'{e}bec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 textdegreeC. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative $mu$142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred : (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression ; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 textdegreeC. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative $mu$142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred : (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression ; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction. |
Tielke, J. A., Peslier, A. H., Christoffersen, R., Erickson, T. M., Cine, II., Jakubek, R. S., Cintala, M. J., Rahman, Z., Fries, M. D., Bouilhol, P. Dislocation generation in experimentally shocked olivine crystals (Article de journal) Dans: Journal of Geophysical Research (Planets), vol. 127, p. e2021JE007042, 2022. @article{Tielke_etal2022,
title = {Dislocation generation in experimentally shocked olivine crystals},
author = {J. A. Tielke and A. H. Peslier and R. Christoffersen and T. M. Erickson and II. Cine and R. S. Jakubek and M. J. Cintala and Z. Rahman and M. D. Fries and P. Bouilhol},
doi = {10.1029/2021JE007042},
year = {2022},
date = {2022-01-01},
journal = {Journal of Geophysical Research (Planets)},
volume = {127},
pages = {e2021JE007042},
abstract = {During shock metamorphism, crystals in planetary bodies are exposed to extreme conditions of stress, pressure, and temperature, resulting in the development of a range of shock-induced defects, including dislocations. To investigate the shock-induced development of dislocations in olivine, one of the most common minerals found in meteorites and other planetary samples, a series of shock experiments were carried out on olivine single crystals. Olivine crystals were shocked to peak reverberation pressures ranging from 21.3 to 58.7 GPa in a flat-plate accelerator. The crystals were characterized using electron backscatter diffraction (EBSD), Raman spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, and electron microprobe. EBSD analyses reveal that geometrically necessary dislocations form on all common slip systems. Raman spectroscopy reveals that the full width at half height of characteristic peaks increases linearly as a function of shock pressure. Dislocation loops with the Burgers vector b = [001] have higher densities near fractures as revealed by TEM analyses, whereas dislocations away from fractures are more dependent on the crystal orientation relative to impact direction. The type and density of dislocations that form during shock are largely independent of the starting hydrogen content of crystals. Calculations that incorporate post-shock cooling rates of meteorites ejected from planetary surfaces, dislocation annihilation rates in olivine, and dislocation densities measured in shocked olivine suggest that shock-induced dislocations are preserved in olivine in meteorites ejected from planetary bodies and therefore give insight into the conditions of ancient impact events in the solar system.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
During shock metamorphism, crystals in planetary bodies are exposed to extreme conditions of stress, pressure, and temperature, resulting in the development of a range of shock-induced defects, including dislocations. To investigate the shock-induced development of dislocations in olivine, one of the most common minerals found in meteorites and other planetary samples, a series of shock experiments were carried out on olivine single crystals. Olivine crystals were shocked to peak reverberation pressures ranging from 21.3 to 58.7 GPa in a flat-plate accelerator. The crystals were characterized using electron backscatter diffraction (EBSD), Raman spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, and electron microprobe. EBSD analyses reveal that geometrically necessary dislocations form on all common slip systems. Raman spectroscopy reveals that the full width at half height of characteristic peaks increases linearly as a function of shock pressure. Dislocation loops with the Burgers vector b = [001] have higher densities near fractures as revealed by TEM analyses, whereas dislocations away from fractures are more dependent on the crystal orientation relative to impact direction. The type and density of dislocations that form during shock are largely independent of the starting hydrogen content of crystals. Calculations that incorporate post-shock cooling rates of meteorites ejected from planetary surfaces, dislocation annihilation rates in olivine, and dislocation densities measured in shocked olivine suggest that shock-induced dislocations are preserved in olivine in meteorites ejected from planetary bodies and therefore give insight into the conditions of ancient impact events in the solar system. |
Couzinié, S., Bouilhol, P., Laurent, O., Grocolas, T., Montel, J. M. Cambro--Ordovician ferrosilicic magmatism along the northern Gondwana margin: constraints from the Cézarenque--Joyeuse gneiss complex (French Massif Central) (Article de journal) Dans: BSGF- Earth Sciences Bulletin, vol. 193, no. 15, 2022. @article{Couzini_etal2022,
title = {Cambro--Ordovician ferrosilicic magmatism along the northern Gondwana margin: constraints from the C\'{e}zarenque--Joyeuse gneiss complex (French Massif Central)},
author = {S. Couzini\'{e} and P. Bouilhol and O. Laurent and T. Grocolas and J. M. Montel},
doi = {10.1051/bsgf/2022010},
year = {2022},
date = {2022-01-01},
journal = {BSGF- Earth Sciences Bulletin},
volume = {193},
number = {15},
abstract = {It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian--Floian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian--Lower Ordovician igneous bodies. In this study, we document the case of the ‘‘C\'{e}zarenque--Joyeuse’’ gneiss complex in the C\'{e}vennes parautochthon domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to greywackes. Collectively, it is inferred that the bulk of the C\'{e}zarenque--Joyeuse gneisses represents former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA--ICP--MS U--Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1thinspacetextpmthinspace5.5thinspaceMa and 483.0thinspacetextpmthinspace5.5thinspaceMa which unambiguously ties the C\'{e}zarenque--Joyeuse gneisses to the Furongian--Lower Ordovician volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750--820thinspacetextdegreeC of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron--w\"{u}stite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian--Lower Ordovician igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an intracontinental rift setting and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian--Floian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian--Lower Ordovician igneous bodies. In this study, we document the case of the ‘‘Cézarenque--Joyeuse’’ gneiss complex in the Cévennes parautochthon domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to greywackes. Collectively, it is inferred that the bulk of the Cézarenque--Joyeuse gneisses represents former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA--ICP--MS U--Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1thinspacetextpmthinspace5.5thinspaceMa and 483.0thinspacetextpmthinspace5.5thinspaceMa which unambiguously ties the Cézarenque--Joyeuse gneisses to the Furongian--Lower Ordovician volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750--820thinspacetextdegreeC of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron--wüstite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian--Lower Ordovician igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an intracontinental rift setting and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences. |
Debret, B., Ménez, B., Bouquerel, H., Bouilhol, P., Mattielli, N., Pisapia, C., Rigaudier, T., Williams, H. M. High-pressure synthesis and storage of solid organic compounds in active subduction zones (Article de journal) Dans: Science Advances, vol. 8, p. eabo2397, 2022. @article{Debret_etal2022,
title = {High-pressure synthesis and storage of solid organic compounds in active subduction zones},
author = {B. Debret and B. M\'{e}nez and H. Bouquerel and P. Bouilhol and N. Mattielli and C. Pisapia and T. Rigaudier and H. M. Williams},
doi = {10.1126/sciadv.abo2397},
year = {2022},
date = {2022-01-01},
journal = {Science Advances},
volume = {8},
pages = {eabo2397},
abstract = {Recent thermodynamic and experimental studies have suggested that volatile organic compounds (e.g., methane, formate, and acetate) can be produced and stabilized in subduction zones, potentially playing an important role in the deep carbon cycle. However, field evidence for the high-pressure production and storage of solid organic compounds is missing. Here, we examine forearc serpentinite clasts recovered by drilling mud volcanoes above the Mariana subduction zone. Notable correlations between carbon and iron stable-isotope signatures and fluid-mobile element (B, As and Sb) concentrations provide evidence for the percolation of slab-derived CO2-rich aqueous fluids through the forearc mantle. The presence of carbonaceous matter rich in aliphatic moieties within high-temperature clasts (\>350textdegreeC) demonstrates that molecular hydrogen production associated with forearc serpentinization is an efficient mechanism for the reduction and conversion of slab-derived CO2-rich fluids into solid organic compounds. These findings emphasize the need to consider the forearc mantle as an important reservoir of organic carbon on Earth.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Recent thermodynamic and experimental studies have suggested that volatile organic compounds (e.g., methane, formate, and acetate) can be produced and stabilized in subduction zones, potentially playing an important role in the deep carbon cycle. However, field evidence for the high-pressure production and storage of solid organic compounds is missing. Here, we examine forearc serpentinite clasts recovered by drilling mud volcanoes above the Mariana subduction zone. Notable correlations between carbon and iron stable-isotope signatures and fluid-mobile element (B, As and Sb) concentrations provide evidence for the percolation of slab-derived CO2-rich aqueous fluids through the forearc mantle. The presence of carbonaceous matter rich in aliphatic moieties within high-temperature clasts (>350textdegreeC) demonstrates that molecular hydrogen production associated with forearc serpentinization is an efficient mechanism for the reduction and conversion of slab-derived CO2-rich fluids into solid organic compounds. These findings emphasize the need to consider the forearc mantle as an important reservoir of organic carbon on Earth. |
2021
|
Couzinié, S., Bouilhol, P., Laurent, O., Marko, L., Moyen, J. F. When zircon drowns: Elusive geochronological record of water-fluxed orthogneiss melting in the Velay dome (Massif Central, France) (Article de journal) Dans: Lithos, vol. 384-385, p. 106938, 2021. @article{Couzini_etal2021,
title = {When zircon drowns: Elusive geochronological record of water-fluxed orthogneiss melting in the Velay dome (Massif Central, France)},
author = {S. Couzini\'{e} and P. Bouilhol and O. Laurent and L. Marko and J. F. Moyen},
doi = {10.1016/j.lithos.2020.105938},
year = {2021},
date = {2021-01-01},
journal = {Lithos},
volume = {384-385},
pages = {106938},
abstract = {Zircon U-Pb geochronology is routinely performed to unravel the timing and duration of melting events within the continental crust. A comprehensive understanding of the zircon behavior during anatexis is therefore paramount to accurately assess the impact of crustalmelting on orogenic processes.Wereport on an anatectic systemfrom the southern part of the Velay dome (Variscan French Massif Central) encompassing late Carboniferous migmatites developed at the expense of peraluminous orthogneisses and associated with (leuco)granite bodies intrusive at higher structural levels. Our zircon U-Pb geochronological survey (494 analyzed grains) evidences a very scant record of the Variscan anatectic event. Indeed, oscillatory-zoned melt-precipitated zircon grains extracted from metatexites, leucosomes and (leuco)granites almost systematically yielded the late Neoproterozoic crystallization age of the felsic igneous protolith. Field and petrographic observations, whole-rock geochemical signatures, phase equilibriumand thermodynamically-constrained trace elementmodelling collectively indicatethat partialmelting of the orthogneisses took place at T textasciitilde 700 textdegreeC andwas fluxed by ingress of externalwater. The lack of new zircon growth is ascribed to a range of processes ultimately related to the low melting temperature including limited zircon solubility in the melt phase, occlusion of zircon in non-reacting source biotite and sluggish zircon dissolution kinetics. Peraluminous orthogneisses, yet highly fusible due to their textquotelefttextquotelefteutectictextquoterighttextquoteright subsolidus mineral assemblage, exhibit a very limited zircon record of low temperature water-fluxed melting. Thus, zirconhas little potential to provide reliable chronological constraints on water-fluxed melting episodes affecting such lithologies},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Zircon U-Pb geochronology is routinely performed to unravel the timing and duration of melting events within the continental crust. A comprehensive understanding of the zircon behavior during anatexis is therefore paramount to accurately assess the impact of crustalmelting on orogenic processes.Wereport on an anatectic systemfrom the southern part of the Velay dome (Variscan French Massif Central) encompassing late Carboniferous migmatites developed at the expense of peraluminous orthogneisses and associated with (leuco)granite bodies intrusive at higher structural levels. Our zircon U-Pb geochronological survey (494 analyzed grains) evidences a very scant record of the Variscan anatectic event. Indeed, oscillatory-zoned melt-precipitated zircon grains extracted from metatexites, leucosomes and (leuco)granites almost systematically yielded the late Neoproterozoic crystallization age of the felsic igneous protolith. Field and petrographic observations, whole-rock geochemical signatures, phase equilibriumand thermodynamically-constrained trace elementmodelling collectively indicatethat partialmelting of the orthogneisses took place at T textasciitilde 700 textdegreeC andwas fluxed by ingress of externalwater. The lack of new zircon growth is ascribed to a range of processes ultimately related to the low melting temperature including limited zircon solubility in the melt phase, occlusion of zircon in non-reacting source biotite and sluggish zircon dissolution kinetics. Peraluminous orthogneisses, yet highly fusible due to their textquotelefttextquotelefteutectictextquoterighttextquoteright subsolidus mineral assemblage, exhibit a very limited zircon record of low temperature water-fluxed melting. Thus, zirconhas little potential to provide reliable chronological constraints on water-fluxed melting episodes affecting such lithologies |
Debret, B., Garrido, C. J., Pons, J., Bouilhol, P., Inglis, E., Sanchez-Vizcaino, V. Lopez, Williams, H. Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones (Article de journal) Dans: Geochimica et Cosmochimica Acta, vol. 296, p. 210–225, 2021. @article{Debret_etal2021,
title = {Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones},
author = {B. Debret and C. J. Garrido and J. Pons and P. Bouilhol and E. Inglis and V. Lopez Sanchez-Vizcaino and H. Williams},
doi = {10.1016/j.gca.2020.12.001},
year = {2021},
date = {2021-01-01},
journal = {Geochimica et Cosmochimica Acta},
volume = {296},
pages = {210--225},
abstract = {Subducted serpentinites have the potential to control the exchange of volatile and redox sensitive elements (e.g., Fe, S, C, N) between the slab, the mantle wedge and the deep mantle. Here we examine the mobility of iron and zinc in serpentinite-derived fluids by using their stable isotopes ($delta$56Fe and $delta$66Zn) in high-pressure subducted meta-serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-serpentinite) and antigorite-olivine-orthopyroxene (transitional lithologies) -bearing serpentinites, and chlorite-bearing harzburgite (Chl-harzburgite), displaying granofels, spinifex and fine-grained recrystallized textures. Those rocks were formed at eclogite facies conditions (1.6--1.9 GPa and 680--710 textdegreeC). The mean $delta$56Fe of all the Cerro del Almirez meta-serpentinites (+0.05 textpm 0.01texttenthousand) is identical within an error to that of primitive mantle (+0.03 textpm 0.03texttenthousand). A positive correlation between $delta$56Fe and indices of peridotite protolith fertility (e.g., Al2O3/SiO2) suggests that the $delta$56Fe values of Cerro del Almirez samples predominantly reflect protolith compositional variations, likely produced by prior episodes of melt extraction. In contrast, the Zn concentrations ([Zn] = 34--67 ppm) and isotope signatures ($delta$66Zn = +0.18 -- +0.55texttenthousand) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle ([Zn] = 54 ppm ; $delta$66Zn = +0.16 textpm 0.06texttenthousand). The Atg-serpentinites ([Zn] = 34--46 ppm ; $delta$66Zn = +0.23 textpm 0.06texttenthousand) display similar [Zn] and $delta$66Zn values to those of slab serpentinites from other high-pressure meta-ophiolites. Both [Zn] and $delta$66Zn increase in transitional lithologies ([Zn] = 45--67 ppm ; $delta$66Zn = +0.30 textpm 0.06texttenthousand) and Chl-harzburgites with granofels ([Zn] = 38--59 ppm ; $delta$66Zn = +0.33 textpm 0.04texttenthousand) or spinifex ([Zn] = 48--66 ppm ; $delta$66Zn = +0.43 textpm 0.09texttenthousand) textures. Importantly, Cerro del Almirez transitional lithologies and Chl-harzburgites display abnormally high [Zn] relative to abyssal peridotites and serpentinites (29--45 ppm) and a positive correlation exists between [Zn] and $delta$66Zn. This correlation is interpreted to reflect the mobilization of Zn by subduction zone fluids at high pressures and temperatures coupled with significant Zn stable isotope fractionation. An increase in [Zn] and $delta$66Zn from Atg-serpentinite to Chl-harzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both [Zn] and $delta$66Zn record the interaction of the transitional lithologies and the Chl-harzburgites with fluids that had equilibrated with metasedimentary rocks. Quantitative models show that metasediment derived fluids can have isotopically heavy Zn as a consequence of sediment carbonate dissolution and subsequent Zn complexation with carbonate species in the released fluids (e.g., [ZnHCO3(H2O)5+] or [ZnCO3(H2O)3]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy $delta$66Zn signature and hence explain the $delta$66Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy $delta$66Zn of the Cerro del Almirez samples is thus serpentinite dehydration accompanied by the open system infiltration of the massif by oxidized, carbonate-rich sediment-derived fluids released during prograde subduction-related metamorphism.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Subducted serpentinites have the potential to control the exchange of volatile and redox sensitive elements (e.g., Fe, S, C, N) between the slab, the mantle wedge and the deep mantle. Here we examine the mobility of iron and zinc in serpentinite-derived fluids by using their stable isotopes ($delta$56Fe and $delta$66Zn) in high-pressure subducted meta-serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-serpentinite) and antigorite-olivine-orthopyroxene (transitional lithologies) -bearing serpentinites, and chlorite-bearing harzburgite (Chl-harzburgite), displaying granofels, spinifex and fine-grained recrystallized textures. Those rocks were formed at eclogite facies conditions (1.6--1.9 GPa and 680--710 textdegreeC). The mean $delta$56Fe of all the Cerro del Almirez meta-serpentinites (+0.05 textpm 0.01texttenthousand) is identical within an error to that of primitive mantle (+0.03 textpm 0.03texttenthousand). A positive correlation between $delta$56Fe and indices of peridotite protolith fertility (e.g., Al2O3/SiO2) suggests that the $delta$56Fe values of Cerro del Almirez samples predominantly reflect protolith compositional variations, likely produced by prior episodes of melt extraction. In contrast, the Zn concentrations ([Zn] = 34--67 ppm) and isotope signatures ($delta$66Zn = +0.18 -- +0.55texttenthousand) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle ([Zn] = 54 ppm ; $delta$66Zn = +0.16 textpm 0.06texttenthousand). The Atg-serpentinites ([Zn] = 34--46 ppm ; $delta$66Zn = +0.23 textpm 0.06texttenthousand) display similar [Zn] and $delta$66Zn values to those of slab serpentinites from other high-pressure meta-ophiolites. Both [Zn] and $delta$66Zn increase in transitional lithologies ([Zn] = 45--67 ppm ; $delta$66Zn = +0.30 textpm 0.06texttenthousand) and Chl-harzburgites with granofels ([Zn] = 38--59 ppm ; $delta$66Zn = +0.33 textpm 0.04texttenthousand) or spinifex ([Zn] = 48--66 ppm ; $delta$66Zn = +0.43 textpm 0.09texttenthousand) textures. Importantly, Cerro del Almirez transitional lithologies and Chl-harzburgites display abnormally high [Zn] relative to abyssal peridotites and serpentinites (29--45 ppm) and a positive correlation exists between [Zn] and $delta$66Zn. This correlation is interpreted to reflect the mobilization of Zn by subduction zone fluids at high pressures and temperatures coupled with significant Zn stable isotope fractionation. An increase in [Zn] and $delta$66Zn from Atg-serpentinite to Chl-harzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both [Zn] and $delta$66Zn record the interaction of the transitional lithologies and the Chl-harzburgites with fluids that had equilibrated with metasedimentary rocks. Quantitative models show that metasediment derived fluids can have isotopically heavy Zn as a consequence of sediment carbonate dissolution and subsequent Zn complexation with carbonate species in the released fluids (e.g., [ZnHCO3(H2O)5+] or [ZnCO3(H2O)3]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy $delta$66Zn signature and hence explain the $delta$66Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy $delta$66Zn of the Cerro del Almirez samples is thus serpentinite dehydration accompanied by the open system infiltration of the massif by oxidized, carbonate-rich sediment-derived fluids released during prograde subduction-related metamorphism. |
2020
|
Horn, C., Bouilhol, P., Skemer, P. Serpentinization, deformation, and seismic anisotropy in the subduction mantle wedge (Article de journal) Dans: Geochemistry Geophysics Geosystems G3, vol. 21, no. 4, 2020. @article{Horn_etal2020,
title = {Serpentinization, deformation, and seismic anisotropy in the subduction mantle wedge},
author = {C. Horn and P. Bouilhol and P. Skemer},
doi = {10.1029/2020GC008950},
year = {2020},
date = {2020-01-01},
journal = {Geochemistry Geophysics Geosystems G3},
volume = {21},
number = {4},
abstract = {Antigorite is a hydrous sheet silicate with strongly anisotropic seismic and rheological properties. Hydrous minerals such as antigorite have been invoked to explain numerous geologic observations within subduction zones including intermediate‐depth earthquakes, arc volcanism, the persistent weakness of the subduction interface, trench‐parallel S wave splitting, and episodic tremor and slip. To understand how the presence of antigorite‐bearing rocks affects observations of seismic anisotropy, three mylonites from the Kohistan palaeo‐island arc in Pakistan were analysed using electron backscatter diffraction. A fourth sample, which displayed optical evidence for crystallographically controlled replacements of olivine, was also investigated using electron backscatter diffraction to identify potential topotactic relationships. The resulting data were used to model the bulk seismic properties of antigorite‐rich rocks. The mylonitic samples exhibit incredibly strong bulk anisotropy (10--20% for the antigorite + olivine). Within the nominally undeformed protomylonite, two topotactic relationships were observed: (1) (010)ant//(100)ol with [100]ant//[001]ol and (2) (010)ant//(100)ol with [100]ant//[010]ol. However, the strength of a texture formed by topotactic replacement is markedly weaker than the strength of the textures observed in mylonitic samples. Since antigorite is thought to be rheologically weak, we hypothesise that microstructures formed from topotactic reactions will be progressively overprinted as deformation is localised in regions with greater percentages of serpentine. Regions of highly sheared serpentine, therefore, have the potential to strongly influence seismic wave speeds in subduction settings. The presence of deformed antigorite in a dipping structure is one explanation for observations of both the magnitude and splitting pattern of seismic waves in subduction zones.Horn, C..},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Antigorite is a hydrous sheet silicate with strongly anisotropic seismic and rheological properties. Hydrous minerals such as antigorite have been invoked to explain numerous geologic observations within subduction zones including intermediate‐depth earthquakes, arc volcanism, the persistent weakness of the subduction interface, trench‐parallel S wave splitting, and episodic tremor and slip. To understand how the presence of antigorite‐bearing rocks affects observations of seismic anisotropy, three mylonites from the Kohistan palaeo‐island arc in Pakistan were analysed using electron backscatter diffraction. A fourth sample, which displayed optical evidence for crystallographically controlled replacements of olivine, was also investigated using electron backscatter diffraction to identify potential topotactic relationships. The resulting data were used to model the bulk seismic properties of antigorite‐rich rocks. The mylonitic samples exhibit incredibly strong bulk anisotropy (10--20% for the antigorite + olivine). Within the nominally undeformed protomylonite, two topotactic relationships were observed: (1) (010)ant//(100)ol with [100]ant//[001]ol and (2) (010)ant//(100)ol with [100]ant//[010]ol. However, the strength of a texture formed by topotactic replacement is markedly weaker than the strength of the textures observed in mylonitic samples. Since antigorite is thought to be rheologically weak, we hypothesise that microstructures formed from topotactic reactions will be progressively overprinted as deformation is localised in regions with greater percentages of serpentine. Regions of highly sheared serpentine, therefore, have the potential to strongly influence seismic wave speeds in subduction settings. The presence of deformed antigorite in a dipping structure is one explanation for observations of both the magnitude and splitting pattern of seismic waves in subduction zones.Horn, C.. |
2019
|
Burg, J. P., Bouilhol, P. Timeline of the South-Tibet-Himalayan belt: the geochronological record of subduction, collision, and underthrusting from zircon and monazite U-Pb ages (Article de journal) Dans: Canadian Journal of Earth Sciences, vol. 56, no. 12, p. 1318–1332, 2019. @article{Burg+Bouilhol2019,
title = {Timeline of the South-Tibet-Himalayan belt: the geochronological record of subduction, collision, and underthrusting from zircon and monazite U-Pb ages},
author = {J. P. Burg and P. Bouilhol},
doi = {10.1139/cjes-2018-0174},
year = {2019},
date = {2019-01-01},
journal = {Canadian Journal of Earth Sciences},
volume = {56},
number = {12},
pages = {1318--1332},
abstract = {The textquotelefttextquoteleftexact timingtextquoterighttextquoteright of collision between India and Eurasia is a recurring questions arising in discussions and various publications. Exactitude is a ubiquitous pursuit for all tectonic events. With the example of the Himalaya-South Tibet collision system, a short review of arguments from different approaches suggests that this pursuit is in vain, but that our knowledge is already sufficient to provide an acceptably textquotelefttextquoteleftprecisetextquoterighttextquoteright timing of the main events. We reviewed U-Pb ages of zircons and monazites considering that major tectonic events can produce thermal effects strong enough to be signalled in high-temperature geochronology. This review equally shows that exactitude in timing is beyond the precision of the methods and the rock record. General consistency between geologic and thermochronologic records strengthens previous interpretations of the collisional orogenic system. At variance with most tectonic interpretations, we argue that the Tsangpo Suture in South-Tibet results from two merged subduction zones, and that island arcs may be part of the root of the Eurasian paleo-active margin. The two main collisional events closely followed each other at ca. 50 and 40 Ma.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The textquotelefttextquoteleftexact timingtextquoterighttextquoteright of collision between India and Eurasia is a recurring questions arising in discussions and various publications. Exactitude is a ubiquitous pursuit for all tectonic events. With the example of the Himalaya-South Tibet collision system, a short review of arguments from different approaches suggests that this pursuit is in vain, but that our knowledge is already sufficient to provide an acceptably textquotelefttextquoteleftprecisetextquoterighttextquoteright timing of the main events. We reviewed U-Pb ages of zircons and monazites considering that major tectonic events can produce thermal effects strong enough to be signalled in high-temperature geochronology. This review equally shows that exactitude in timing is beyond the precision of the methods and the rock record. General consistency between geologic and thermochronologic records strengthens previous interpretations of the collisional orogenic system. At variance with most tectonic interpretations, we argue that the Tsangpo Suture in South-Tibet results from two merged subduction zones, and that island arcs may be part of the root of the Eurasian paleo-active margin. The two main collisional events closely followed each other at ca. 50 and 40 Ma. |
Couzinié, S., Laurent, O., Chelle-Michou, C., Bouilhol, P., Paquette, J. L., A.M., Gannoun, Moyen, J. F. Detrital zircon U--Pb--Hf systematics of Ediacaran metasediments from the French Massif Central:Consequence Detrital zircon U--Pb--Hf systematics of Ediacaran metasediments from the French Massif Central: Consequences for the crustal evolution of the north Gondwana margin (Article de journal) Dans: Precambrian Research, vol. 324, p. 269–284, 2019. @article{Couzini_etal2019,
title = {Detrital zircon U--Pb--Hf systematics of Ediacaran metasediments from the French Massif Central:Consequence Detrital zircon U--Pb--Hf systematics of Ediacaran metasediments from the French Massif Central: Consequences for the crustal evolution of the north Gondwana margin},
author = {S. Couzini\'{e} and O. Laurent and C. Chelle-Michou and P. Bouilhol and J. L. Paquette and Gannoun A.M. and J. F. Moyen},
doi = {10.1016/j.precamres.2019.01.016},
year = {2019},
date = {2019-01-01},
journal = {Precambrian Research},
volume = {324},
pages = {269--284},
abstract = {Combining U--Pb and Lu-Hf isotopic data of detrital zircon grains has proven a powerful tool to unravel the provenance of sediments and address continental crust evolution. In this study, we explore the origin of thick siliciclastic metasedimentary units from the high-grade internal domains of the Variscan belt of Europe and examine their significance for Neoproterozoic crust formation and evolution along the North Gondwana margin. We present data and U--Pb/Lu--Hf systematics of detrital zircons from five amphibolite-facies metasediments sampled in the eastern French Massif Central, measured in situ by LA--(MC)--ICP--MS. The sedimentary protoliths were deposited in the Ediacaran as evidenced by field relationships and maximum depositional ages ranging between 592.4 textpm 5.5 and 556.8 textpm 5.1 Ma. All samples contain three main zircon populations in terms of age distribution and Hf isotopes: (i) abundant 0.55--0.65 Ga zircons with considerably scattered $epsilon$Hf(t) from −19 to +14; (ii) varied amounts of 0.65--1.0 Ga zircons showing dominantly positive $epsilon$Hf(t) with the exception of the c. 1.0 Ga zircons; and (iii) �W1.8 Ga zircons (mostly between 1.9--2.1 and 2.5--2.8, up to 3.2 Ga) with $epsilon$Hf(t) ranging between +5 and −7. Multidimensional scaling of our U--Pb dataset and a data compilation of Ediacaran to Lower Cambrian (meta)sediments from the North Gondwana margin reveals that the relative proportions of the three age components carry discriminant provenance information. Geological data indicate that the Ediacaran basins of the eastern French Massif Central collected the erosion products of two main source regions: (i) the Neoproterozoic Cadomian magmatic arc; and (ii) the cratonic hinterland, mostly from the Saharan Metacraton and the Arabian-Nubian Shield. Our dataset attests to the reworking of: (i) juvenile Neoproterozoic crust and, (ii) old Paleoproterozoic to Neoarchean crustal components, either as part of the detritus or incorporated in Neoproterozoic arc magmas. First-order estimates derived from the isotopic signature of S-type granitic magmas sourced in the Ediacaran metasediments suggest that 60--75% of the detritus would correspond to young Neoproterozoic crust and thus represent net additions to the continental crust volume.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Combining U--Pb and Lu-Hf isotopic data of detrital zircon grains has proven a powerful tool to unravel the provenance of sediments and address continental crust evolution. In this study, we explore the origin of thick siliciclastic metasedimentary units from the high-grade internal domains of the Variscan belt of Europe and examine their significance for Neoproterozoic crust formation and evolution along the North Gondwana margin. We present data and U--Pb/Lu--Hf systematics of detrital zircons from five amphibolite-facies metasediments sampled in the eastern French Massif Central, measured in situ by LA--(MC)--ICP--MS. The sedimentary protoliths were deposited in the Ediacaran as evidenced by field relationships and maximum depositional ages ranging between 592.4 textpm 5.5 and 556.8 textpm 5.1 Ma. All samples contain three main zircon populations in terms of age distribution and Hf isotopes: (i) abundant 0.55--0.65 Ga zircons with considerably scattered $epsilon$Hf(t) from −19 to +14; (ii) varied amounts of 0.65--1.0 Ga zircons showing dominantly positive $epsilon$Hf(t) with the exception of the c. 1.0 Ga zircons; and (iii) �W1.8 Ga zircons (mostly between 1.9--2.1 and 2.5--2.8, up to 3.2 Ga) with $epsilon$Hf(t) ranging between +5 and −7. Multidimensional scaling of our U--Pb dataset and a data compilation of Ediacaran to Lower Cambrian (meta)sediments from the North Gondwana margin reveals that the relative proportions of the three age components carry discriminant provenance information. Geological data indicate that the Ediacaran basins of the eastern French Massif Central collected the erosion products of two main source regions: (i) the Neoproterozoic Cadomian magmatic arc; and (ii) the cratonic hinterland, mostly from the Saharan Metacraton and the Arabian-Nubian Shield. Our dataset attests to the reworking of: (i) juvenile Neoproterozoic crust and, (ii) old Paleoproterozoic to Neoarchean crustal components, either as part of the detritus or incorporated in Neoproterozoic arc magmas. First-order estimates derived from the isotopic signature of S-type granitic magmas sourced in the Ediacaran metasediments suggest that 60--75% of the detritus would correspond to young Neoproterozoic crust and thus represent net additions to the continental crust volume. |
Maunder, B., Hunen, J., Bouilhol, P., Magni, V. Modeling slab temperature: A reevaluation of the thermal parameter (Article de journal) Dans: Geochemistry, Geophysics, Geosystems G3, vol. 20, 2019. @article{Maunder_etal2019,
title = {Modeling slab temperature: A reevaluation of the thermal parameter},
author = {B. Maunder and J. Hunen and P. Bouilhol and V. Magni},
year = {2019},
date = {2019-01-01},
journal = {Geochemistry, Geophysics, Geosystems G3},
volume = {20},
abstract = {We reevaluate the effects of slab age, speed, and dip on slab temperature with numerical models. The thermal parameter $Phi$ = t v sin $theta$, where t is age, v is speed, and $theta$ is angle, is traditionally used as an indicator of slab temperature. However, we find that an empirically derived quantity, in which slab temperature T ∝ log (t−av−b) , is more accurate at depths \<120 km, with the constants a and b depending on position within the slab. Shallower than the decoupling depth (textasciitilde70--80 km), atextasciitilde1 and btextasciitilde0, that is, temperature is dependent on slab age alone. This has important implications for the early devolatilization of slabs in the hottest (youngest) cases and for shallow slab seismicity. At subarc depths (textasciitilde100 km), within the slab mantle, atextasciitilde1 and btextasciitilde0 again. However, for the slab crust, now atextasciitilde0.5 and btextasciitilde1, that is, speed has the dominant effect. This is important when considering the generation of arc magmatism, and in particular, slab melting and the generation of slab‐derived melange diapirs. Moving deeper into the Earth, the original thermal parameter performs well as a temperature indicator, initially in the core of the slab (the region of interest for deep water cycling). Finally, varying the decoupling depth between 40 and 100 km has a dominant effect on slab temperatures down to 140‐km depth, but only within the slab crust. Slab mantle temperature remains primarily dependent on age.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We reevaluate the effects of slab age, speed, and dip on slab temperature with numerical models. The thermal parameter $Phi$ = t v sin $theta$, where t is age, v is speed, and $theta$ is angle, is traditionally used as an indicator of slab temperature. However, we find that an empirically derived quantity, in which slab temperature T ∝ log (t−av−b) , is more accurate at depths <120 km, with the constants a and b depending on position within the slab. Shallower than the decoupling depth (textasciitilde70--80 km), atextasciitilde1 and btextasciitilde0, that is, temperature is dependent on slab age alone. This has important implications for the early devolatilization of slabs in the hottest (youngest) cases and for shallow slab seismicity. At subarc depths (textasciitilde100 km), within the slab mantle, atextasciitilde1 and btextasciitilde0 again. However, for the slab crust, now atextasciitilde0.5 and btextasciitilde1, that is, speed has the dominant effect. This is important when considering the generation of arc magmatism, and in particular, slab melting and the generation of slab‐derived melange diapirs. Moving deeper into the Earth, the original thermal parameter performs well as a temperature indicator, initially in the core of the slab (the region of interest for deep water cycling). Finally, varying the decoupling depth between 40 and 100 km has a dominant effect on slab temperatures down to 140‐km depth, but only within the slab crust. Slab mantle temperature remains primarily dependent on age. |
2018
|
Debret, B., Bouilhol, P., Pons, M. L., Williams, H. Carbonate transfer during the Onset of slab devolatilization: New insights from Fe and Zn stable isotopes (Article de journal) Dans: Journal of Petrology, vol. 59, no. 6, p. 1145–1166, 2018. @article{Debret_etal2018,
title = {Carbonate transfer during the Onset of slab devolatilization: New insights from Fe and Zn stable isotopes},
author = {B. Debret and P. Bouilhol and M. L. Pons and H. Williams},
year = {2018},
date = {2018-01-01},
journal = {Journal of Petrology},
volume = {59},
number = {6},
pages = {1145--1166},
abstract = {Long-term carbon cycling is a subject of recent controversy as new mass balance calculations suggest that most carbon is transferred from the slab to the mantle wedge by fluids during subduction, limiting the efficiency of carbon recycling to the deep mantle. Here, we examine the large scale mobility of carbon during subduction using new isotopic tracers sensitive to H--C--O--S--Cl fluids, namely iron and zinc stable isotopes, in samples interpreted to represent residual slab (Queyras, Western Alps) and sub-arc mantle (Kohistan, Himalaya). We show that during subduction there are several stages of carbonate precipitation and dissolution at metasomatic interfaces between metasedimentary and ultramafic rocks in the slab. During the early stages of subduction, before the slab reaches the 300--400C isotherms, the infiltration of sediment-derived fluids into ultramafic lithologies enhances carbonate precipitation in antigorite-bearing serpentinites. Carbonate storage in serpentinites, therefore, acts as a temporary reservoir of carbon in subduction zones. This episode is accompanied by a decrease in serpentinite iron isotope composition (d56Fe), due to interaction with low-d56Fe sediment-derived fluids, and an increase in the concentrations of fluid-mobile elements (e.g. B, Li, As). At higher temperatures (\>400C), carbonate is leached from the serpentinites by fluids. This is accompanied by a decrease in serpentinite zinc isotope composition (d66Zn) which we interpret as the release of a carbonate-bearing fluid with an isotopically heavy d66Zn signature. Thermodynamic modelling shows that the sudden change in fluid carbon mobility is due to a decreasein the aCO2 of the fluids released during slab prograde metamorphism, which shifts from sediment- to serpentinite-dominated dehydration. This demonstrates that slab fluids bearing oxidized carbon (e.g. CO2), associated with isotopically light Fe, heavy Zn and fluid-mobile elements, can be released before the slab reaches eclogite facies P-T conditions. These observations provide strong evidence for the mobility of carbon in fluids during the early stages of subduction.Moreover, the fluids released will act as a potential metasomatic agent for the fore-arc mantle (or slab/mantle interface). The observation of carbonate-bearing metamorphic veins in the Himalayan sub-arc mantle with complementary light d56Fe and heavy d66Zn signatures provides further support for the large scale transfer of both sulphate- and carbonate-bearing fluids during the early stages of subduction. This suggests that the fore-arc may have an important role in delivering water, sulfur and carbon to the source of arc-magmas.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Long-term carbon cycling is a subject of recent controversy as new mass balance calculations suggest that most carbon is transferred from the slab to the mantle wedge by fluids during subduction, limiting the efficiency of carbon recycling to the deep mantle. Here, we examine the large scale mobility of carbon during subduction using new isotopic tracers sensitive to H--C--O--S--Cl fluids, namely iron and zinc stable isotopes, in samples interpreted to represent residual slab (Queyras, Western Alps) and sub-arc mantle (Kohistan, Himalaya). We show that during subduction there are several stages of carbonate precipitation and dissolution at metasomatic interfaces between metasedimentary and ultramafic rocks in the slab. During the early stages of subduction, before the slab reaches the 300--400C isotherms, the infiltration of sediment-derived fluids into ultramafic lithologies enhances carbonate precipitation in antigorite-bearing serpentinites. Carbonate storage in serpentinites, therefore, acts as a temporary reservoir of carbon in subduction zones. This episode is accompanied by a decrease in serpentinite iron isotope composition (d56Fe), due to interaction with low-d56Fe sediment-derived fluids, and an increase in the concentrations of fluid-mobile elements (e.g. B, Li, As). At higher temperatures (>400C), carbonate is leached from the serpentinites by fluids. This is accompanied by a decrease in serpentinite zinc isotope composition (d66Zn) which we interpret as the release of a carbonate-bearing fluid with an isotopically heavy d66Zn signature. Thermodynamic modelling shows that the sudden change in fluid carbon mobility is due to a decreasein the aCO2 of the fluids released during slab prograde metamorphism, which shifts from sediment- to serpentinite-dominated dehydration. This demonstrates that slab fluids bearing oxidized carbon (e.g. CO2), associated with isotopically light Fe, heavy Zn and fluid-mobile elements, can be released before the slab reaches eclogite facies P-T conditions. These observations provide strong evidence for the mobility of carbon in fluids during the early stages of subduction.Moreover, the fluids released will act as a potential metasomatic agent for the fore-arc mantle (or slab/mantle interface). The observation of carbonate-bearing metamorphic veins in the Himalayan sub-arc mantle with complementary light d56Fe and heavy d66Zn signatures provides further support for the large scale transfer of both sulphate- and carbonate-bearing fluids during the early stages of subduction. This suggests that the fore-arc may have an important role in delivering water, sulfur and carbon to the source of arc-magmas. |
Eeken, T., Goes, S., Pedersen, H. A., Arndt, N., Bouilhol, P. Seismic evidence for depth-dependent metasomatism in cratons (Article de journal) Dans: Earth and Planetary Science Letters, vol. 491, p. 148–159, 2018. @article{Eeken_etal2018,
title = {Seismic evidence for depth-dependent metasomatism in cratons},
author = {T. Eeken and S. Goes and H. A. Pedersen and N. Arndt and P. Bouilhol},
doi = {10.1016/j.epsl.2018.03.018},
year = {2018},
date = {2018-01-01},
journal = {Earth and Planetary Science Letters},
volume = {491},
pages = {148--159},
abstract = {The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100--150 km. The metasomatic component can consist of 0.5--1 wt% water bound in amphibole, antigorite and chlorite, �`u0.2 wt% water plus potassium to form phlogopite, or �`u5 wt% CO2plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100--200◦C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100--150 km. The metasomatic component can consist of 0.5--1 wt% water bound in amphibole, antigorite and chlorite, �`u0.2 wt% water plus potassium to form phlogopite, or �`u5 wt% CO2plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100--200◦C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots. |
Jagoutz, O., Bouilhol, P., Schaltegger, U., Müntener, O. The isotopic evolution of the Kohistan Ladakh arc from subduction initiation to continent arc collision (Article de journal) Dans: Geological Society, London, Special Publications, vol. 483, 2018. @article{Jagoutz_etal2018,
title = {The isotopic evolution of the Kohistan Ladakh arc from subduction initiation to continent arc collision},
author = {O. Jagoutz and P. Bouilhol and U. Schaltegger and O. M\"{u}ntener},
doi = {10.6084/m9.figshare.c.4234220},
year = {2018},
date = {2018-01-01},
journal = {Geological Society, London, Special Publications},
volume = {483},
abstract = {Magmatic arcs associated with subduction zones are the dominant active locus of continental crust formation, and evolve in space and time towards magmatic compositions comparable to that of continental crust. Accordingly, the secular evolution of magmatic arcs is crucial to the understanding of crust formation processes. In this paper we present the first comprehensive U--Pb, Hf, Nd and Sr isotopic dataset documenting c. 120 myr of magmatic evolution in the Kohistan-Ladakh paleo-island arc. We found a long-term magmatic evolution that is controlled by the overall geodynamic of the Neo-Tethys realm. Apart from the post-collisionnal melts, the intra-oceanic history of the arc shows two main episodes (150--80 Ma and 80--50 Ma) of distinct geochemical signatures involving the slab and the sub-arc mantle components that are intimately linked to the slab dynamics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Magmatic arcs associated with subduction zones are the dominant active locus of continental crust formation, and evolve in space and time towards magmatic compositions comparable to that of continental crust. Accordingly, the secular evolution of magmatic arcs is crucial to the understanding of crust formation processes. In this paper we present the first comprehensive U--Pb, Hf, Nd and Sr isotopic dataset documenting c. 120 myr of magmatic evolution in the Kohistan-Ladakh paleo-island arc. We found a long-term magmatic evolution that is controlled by the overall geodynamic of the Neo-Tethys realm. Apart from the post-collisionnal melts, the intra-oceanic history of the arc shows two main episodes (150--80 Ma and 80--50 Ma) of distinct geochemical signatures involving the slab and the sub-arc mantle components that are intimately linked to the slab dynamics. |
Jolivet, L., Faccenna, C., Becker, R. H., Tesauro, M., Sternai, P., Bouilhol, P. Mantle flow and deforming continents:From India-Asia convergence to Pacific subduction (Article de journal) Dans: Tectonics, vol. 37, p. 2887-2814, 2018. @article{Jolivet_etal2018,
title = {Mantle flow and deforming continents:From India-Asia convergence to Pacific subduction},
author = {L. Jolivet and C. Faccenna and R. H. Becker and M. Tesauro and P. Sternai and P. Bouilhol},
year = {2018},
date = {2018-01-01},
journal = {Tectonics},
volume = {37},
pages = {2887-2814},
abstract = {The formation of mountain belts or rift zones is commonly attributed to interactions between plates along their boundaries, but the widely distributed deformation of Asia from Himalaya to the Japan Sea and other back-arc basins is difficult to reconcile with this notion. Through comparison of the tectonicand kinematic records of the last 50 Ma with seismic tomography and anisotropy models, we show that the closure of the former Tethys Ocean and the extensional deformation of East Asia can be best explained if the asthenospheric mantle transporting India northward, forming the Himalaya and the Tibetan Plateau, reachesEast Asia where it overrides the westward flowing Pacific mantle and contributes to subduction dynamics, distributing extensional deformation over a 3,000-km wide region. This deep asthenospheric flow partly controls the compressional stresses transmitted through the continent-continent collision, driving crustalthickening below the Himalayas and Tibet and the propagation of strike-slip faults across Asian lithosphere further north and east, as well as with the lithospheric and crustal flow powered by slab retreat east of the collision zone below East and SE Asia. The main shortening direction in the deforming continent between thecollision zone and the Pacific subduction zones may in this case be a proxy for the direction of flow in the asthenosphere underneath, which may become a useful tool for studying mantle flow in the distant past. Our model of the India-Asia collision emphasizes the role of asthenospheric flow underneath continents and mayoffer alternative ways of understanding tectonic processes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The formation of mountain belts or rift zones is commonly attributed to interactions between plates along their boundaries, but the widely distributed deformation of Asia from Himalaya to the Japan Sea and other back-arc basins is difficult to reconcile with this notion. Through comparison of the tectonicand kinematic records of the last 50 Ma with seismic tomography and anisotropy models, we show that the closure of the former Tethys Ocean and the extensional deformation of East Asia can be best explained if the asthenospheric mantle transporting India northward, forming the Himalaya and the Tibetan Plateau, reachesEast Asia where it overrides the westward flowing Pacific mantle and contributes to subduction dynamics, distributing extensional deformation over a 3,000-km wide region. This deep asthenospheric flow partly controls the compressional stresses transmitted through the continent-continent collision, driving crustalthickening below the Himalayas and Tibet and the propagation of strike-slip faults across Asian lithosphere further north and east, as well as with the lithospheric and crustal flow powered by slab retreat east of the collision zone below East and SE Asia. The main shortening direction in the deforming continent between thecollision zone and the Pacific subduction zones may in this case be a proxy for the direction of flow in the asthenosphere underneath, which may become a useful tool for studying mantle flow in the distant past. Our model of the India-Asia collision emphasizes the role of asthenospheric flow underneath continents and mayoffer alternative ways of understanding tectonic processes. |
Kaislaniemi, L., Hunen, J., Bouilhol, P. Lithosphere destabilization by melt weakening and crust-mantle interactions: Implications for generation of granite-migmatite belts (Article de journal) Dans: Tectonics, vol. 37, 2018. @article{Kaislaniemi_etal2018,
title = {Lithosphere destabilization by melt weakening and crust-mantle interactions: Implications for generation of granite-migmatite belts},
author = {L. Kaislaniemi and J. Hunen and P. Bouilhol},
doi = {10.1029/2018TC005014},
year = {2018},
date = {2018-01-01},
journal = {Tectonics},
volume = {37},
abstract = {Orogenic crustal anatexis is a still poorly understood process due to the complexity of the thermal and geodynamical interaction between mantle and crustal processes during and after continental collision. Here we present a novel conceptual model for the formation of granite-migmatite belts: we propose that convective thinning of the lithosphere results in minor amounts of partial melts within the lowermost crust that trigger further instabilities. This will lead to positive feedback effects between melt weakening, mantle upwelling, and wholesale mantle lithosphere removal, causing a strong pulse of mantle and crustal melting. We test this model numerically, and results show that this process, taking between 20 and 50 Myr in total, can explain the temporal evolution of melting in granite-migmatite zones and associated mantle-derived mafic rocks and provides a heat source for crustal melting without the need for other processes, such as slab break-off or increased radiogenic heating. Furthermore, the generation of a refractory residue after mantle and crustal melting is also shown to control the progress of the lithospheric mantle removal, providing another feedback mechanism between melting and lithospheric reequilibration.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Orogenic crustal anatexis is a still poorly understood process due to the complexity of the thermal and geodynamical interaction between mantle and crustal processes during and after continental collision. Here we present a novel conceptual model for the formation of granite-migmatite belts: we propose that convective thinning of the lithosphere results in minor amounts of partial melts within the lowermost crust that trigger further instabilities. This will lead to positive feedback effects between melt weakening, mantle upwelling, and wholesale mantle lithosphere removal, causing a strong pulse of mantle and crustal melting. We test this model numerically, and results show that this process, taking between 20 and 50 Myr in total, can explain the temporal evolution of melting in granite-migmatite zones and associated mantle-derived mafic rocks and provides a heat source for crustal melting without the need for other processes, such as slab break-off or increased radiogenic heating. Furthermore, the generation of a refractory residue after mantle and crustal melting is also shown to control the progress of the lithospheric mantle removal, providing another feedback mechanism between melting and lithospheric reequilibration. |
Riel, N., Bouilhol, P., Hunen, J. Van, Cornet, J., Magni, V., Grigorova, V., Velic, M. Interaction between mantle-derived magma and lower arc crust: quantitative reactive melt flow modelling using STyx (Article de journal) Dans: Geological Society of London Special Publication, vol. 478, 2018. @article{Riel_etal2018,
title = {Interaction between mantle-derived magma and lower arc crust: quantitative reactive melt flow modelling using STyx},
author = {N. Riel and P. Bouilhol and J. Van Hunen and J. Cornet and V. Magni and V. Grigorova and M. Velic},
year = {2018},
date = {2018-01-01},
journal = {Geological Society of London Special Publication},
volume = {478},
abstract = {The magmatic processes occurring in the lowermost arc crust play a major role in the evolution of mantle-wedge-derived melt. Geological evidence indicates that mantle-derived magmas and in-situ products of lower crust partial melting are reacting in a pervasive melt system and are eventually extracted towards higherlevels of the crust. Resolving the relative contribution of mantle-derived magma and partial melting products of pre-existing crust is essential to: (1) quantify crustal growth rate; (2) better understand the compositional range of arc magmatic series; and (3) constrain the chemical differentiation of the lower crust. In this study, we present STyx, a new modelling tool, coupling melt and heat flow with petrology to explore the dynamics of storage, transfer and hybridization of melts in complex liquid/rock systems. We perform three models representing a magmatic event affecting an amphibolitic lower arc crust in order to quantify the relative contribution between partial melting of the pre-existing crust and fractional crystallization from mantle-derived hydrous-magma. Our models demonstrate that most of the differentiated arc crust is juvenile, deriving from the differentiation of mantle melts, and that pre-existing crust does not significantly contribute to the total thickness of magmatic products},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The magmatic processes occurring in the lowermost arc crust play a major role in the evolution of mantle-wedge-derived melt. Geological evidence indicates that mantle-derived magmas and in-situ products of lower crust partial melting are reacting in a pervasive melt system and are eventually extracted towards higherlevels of the crust. Resolving the relative contribution of mantle-derived magma and partial melting products of pre-existing crust is essential to: (1) quantify crustal growth rate; (2) better understand the compositional range of arc magmatic series; and (3) constrain the chemical differentiation of the lower crust. In this study, we present STyx, a new modelling tool, coupling melt and heat flow with petrology to explore the dynamics of storage, transfer and hybridization of melts in complex liquid/rock systems. We perform three models representing a magmatic event affecting an amphibolitic lower arc crust in order to quantify the relative contribution between partial melting of the pre-existing crust and fractional crystallization from mantle-derived hydrous-magma. Our models demonstrate that most of the differentiated arc crust is juvenile, deriving from the differentiation of mantle melts, and that pre-existing crust does not significantly contribute to the total thickness of magmatic products |
2017
|
Freeburn, R., Bouilhol, P., Maunder, B., Magni, V., Hunen, J. Numerical models of the magmatic processes induced by slab breakoff (Article de journal) Dans: Earth and Planetary Science Letters, vol. 478, p. 203–213, 2017. @article{Freeburn_etal2017,
title = {Numerical models of the magmatic processes induced by slab breakoff},
author = {R. Freeburn and P. Bouilhol and B. Maunder and V. Magni and J. Hunen},
doi = {10.1016/j.epsl.2017.09.008},
year = {2017},
date = {2017-01-01},
journal = {Earth and Planetary Science Letters},
volume = {478},
pages = {203--213},
abstract = {After the onset of continental collision, magmatism often persists for tens of millions of years, albeit with a different composition, in reduced volumes, and with a more episodic nature and more widespread spatial distribution, compared to normal arc magmatism. Kinematic modelling studies have suggested that slab breakoff can account for this post-collisional magmatism through the formation of a slab window and subsequent heating of the overriding plate and decompression melting of upwelling asthenosphere, particularly if breakoff occurs at depths shallower than the overriding plate.To constrain the nature of any melting and the geodynamic conditions required, we numerically model the collision of two continental plates following a period of oceanic subduction. A thermodynamic database is used to determine the (de)hydration reactions and occurrence of melt throughout this process. We investigate melting conditions within a parameter space designed to generate a wide range of breakoff depths, timings and collisional styles.Under most circumstances, slab breakoff occurs deeper than the depth extent of the overriding plate; too deep to generate any decompressional melting of dry upwelling asthenosphere or thermal perturbation within the overriding plate. Even if slab breakoff is very shallow, the hot mantle inflow into the slab window is not sustained long enough to sufficiently heat the hydrated overriding plate to cause significant magmatism. Instead, for relatively fast, shallow breakoff we observe melting of asthenosphere above the detached slab through the release of water from the tip of the heating detached slab. Melting of the subducted continental crust during necking and breakoff is a more common feature and may be a more reliable indicator of the occurrence of breakoff. We suggest that magmatism from slab breakoff alone is unable to explain several of the characteristics of post-collisional magmatism, and that additional geodynamical processes need to be considered when interpreting magmatic observations.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
After the onset of continental collision, magmatism often persists for tens of millions of years, albeit with a different composition, in reduced volumes, and with a more episodic nature and more widespread spatial distribution, compared to normal arc magmatism. Kinematic modelling studies have suggested that slab breakoff can account for this post-collisional magmatism through the formation of a slab window and subsequent heating of the overriding plate and decompression melting of upwelling asthenosphere, particularly if breakoff occurs at depths shallower than the overriding plate.To constrain the nature of any melting and the geodynamic conditions required, we numerically model the collision of two continental plates following a period of oceanic subduction. A thermodynamic database is used to determine the (de)hydration reactions and occurrence of melt throughout this process. We investigate melting conditions within a parameter space designed to generate a wide range of breakoff depths, timings and collisional styles.Under most circumstances, slab breakoff occurs deeper than the depth extent of the overriding plate; too deep to generate any decompressional melting of dry upwelling asthenosphere or thermal perturbation within the overriding plate. Even if slab breakoff is very shallow, the hot mantle inflow into the slab window is not sustained long enough to sufficiently heat the hydrated overriding plate to cause significant magmatism. Instead, for relatively fast, shallow breakoff we observe melting of asthenosphere above the detached slab through the release of water from the tip of the heating detached slab. Melting of the subducted continental crust during necking and breakoff is a more common feature and may be a more reliable indicator of the occurrence of breakoff. We suggest that magmatism from slab breakoff alone is unable to explain several of the characteristics of post-collisional magmatism, and that additional geodynamical processes need to be considered when interpreting magmatic observations. |
