Graphite formation by carbonate reduction during subduction

Abstract

International audienceCarbon is transported from Earth's surface into its interior at subduction zones. Carbonates in sediments overlying hydrothermally altered rocks (including serpentinites) within the subducted slab are the main carriers of this carbon1. Part of the carbon is recycled back to the surface by volcanism, but some is transferred to the deep Earth1, 2. Redox transformations during shallow subduction control the transfer and long-term fate of carbon, but are poorly explored1, 3. Here we use carbon stable isotopes and Raman spectroscopy to analyse the reduction of carbonate in an exhumed serpentinite-sediment contact in Alpine Corsica, France. We find that highly crystalline graphite was formed during subduction metamorphism and was concentrated in the sediment, within a reaction zone in direct contact with the serpentinite. The graphite in this reaction zone has a carbon isotopic signature (δ13C) of up to 0.8±0.1‰, similar to that of the original calcite that composed the sediments, and is texturally associated with the calcium-bearing mineral wollastonite that is also formed in the process. We use mass-balance calculations to show that about 9% of the total carbonaceous matter in the sedimentary unit results from complete calcite reduction in the reaction zone. We conclude that graphite formation, under reducing and low-temperature conditions, provides a mechanism to retain carbon in a subducting slab, aiding transport of carbon into the deeper Earth

T1 - Immiscible C-H-O fluids formed at subduction zone conditions

“…This is because the experiments here at 1.5 to 2.5 GPa, 600 to 700 °C, and the Co-CoO to the Re-ReO 2 buffer are all saturated with graphite (Table 1), and the bulk composition and species of C-H-O fluids are fixed at graphite saturation at the given P-T-fO 2 conditions (e.g., Zhang and Duan, 2009). In the subducting slab, graphite could be formed by graphitisation of organic matter or by reduction of carbonates (Galvez et al, 2013;Stagno et al, 2015).…”

CO 2 flow during orogenic gravitational collapse: Syntectonic decarbonation and fluid mixing at the ductile-brittle transition (Lavrion, Greece)

“…Besides H 2 O, CO 2 is a dominant volatile phase commonly observed in metamorphic fluids (Roedder, 1984;Crawford and Hollister, 1986;Diamond, 2001;Kerrick and Connolly, 2001;Yardley and Bodnar, 2014). The knowledge of carbon cycle is of primary importance in climate studies but also in reconstructing geological processes in various environments (Selverstone and Gutzler, 1993;Galvez et al, 2013;Ague and Nicolescu, 2014). The production of CO 2 -rich fluids has been proposed to result from (i) redox reactions or interaction of fluids with carbonaceous material (e.g.…”

Carbonaceous biosignatures of diverse chemotrophic microbial communities from chert nodules of the Ediacaran Doushantuo Formation

“…The structural order of organic matter in sedimentary rocks is controlled by the molecular structures of carbon precursors (Franklin, 1951 (Tuinstra and Koenig, 1970;Beyssac et al, 2002;Rahl et al, 2005;Schiffbauer et al, 2007Schiffbauer et al, , 2012Kouketsu et al, 2014), deformation and shear force (Bustin et al, 1995;Aoya et al, 2010), hydrothermal fluid circulation and graphite deposition (Wopenka and Pasteris, 1993;Luque et al, 2009;Lepland et al, 2011;Galvez et al, 2013), and mineral-templating effect .…”