Fluid-mediated selective dissolution of subducting carbonaceous material: Implications for carbon recycling and fluid fluxes at forearc depths

• Published in: Chemical geology Vol. 549; p. 119682
• Main Authors: Vitale Brovarone, Alberto, Tumiati, Simone, Piccoli, Francesca, Ague, Jay J., Connolly, James A.D., Beyssac, Olivier
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05.09.2020; Elsevier
• Subjects: Deep carbon cycle; Dissolution of organic C; Fluid-rock interactions; Sciences of the Universe; Subducted carbonaceous material; Dissolution of organic C; Deep carbon cycle; Subducted carbonaceous material; Fluid-rock interactions
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2020.119682

Subduction of crustal C governs the long-term global C cycling. The role of carbonates recycling in subduction zones and the related dissolution of C at various depths have been the subject of a large body of literature over the last decades. Much less is known about the contribution of carbonaceous material (CM) to the deep C cycling in subduction zones. This paper presents natural evidence for intense fluid-mediated leaching of CM in pelitic schists at high-pressure/low-temperature conditions relevant to the forearc region of subducting slabs. Manifestations of such process were identified along fluid pathways at various scales in the blueschist-facies subduction complexes of both Alpine Corsica and the Western Alps. Microstructural, whole-rock and Raman analyses across a selected metasomatic aureole were used to quantify the amount and mechanisms of C loss during fluid-rock interaction. In samples affected by intense fluid infiltration, >90% of the initial CM was removed from the rock. Microstructural and micro-Raman data indicate selective leaching of disordered CM relative to nearly crystalline graphite. The collected data allowed constraining the magnitude of fluid fluxes required to bleach the studied CM-bearing lithologies at different P-T-fO2 conditions, which corresponds to rather high time-integrated fluid fluxes in the order of ~106 m3/m2. In settings of large-scale fluid channelization, such as along regional-scale, lithological/tectonic boundaries or at the top of the subducted sedimentary pile, intense dissolution of subducted CM is expected. This process may thus exert a negative feedback on the sink of C phases into the deep mantle over the geological timescales and contribute to the release of isotopically light C from subducting slabs in forearc regions.