Caurant, C.; Debret, B.; Ménez, B.; Nicollet, C.; Bouilhol, P.
Lithos, 2023, 446-447, 107136
Voir en ligne : https://doi.org/10.1016/j.lithos.2023.107136
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 °C 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 (~ FMQ +2) in the deep-seated lithologies made of heazlewoodite-magnetite-olivine assemblages to low fO2 (~ 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.