Role of Magnetite, Rutile and H2S in C-O-H Fluids’ Speciation: Implications For Localized Reduction Zones in the Shallow Upper Mantle

Abstract

Carbon (C) and sulfur (S) bearing volatiles in the mantle are important as they control processes like metasomatism and melting that ultimately influence Earth’s geochemical cycle. Here we report rare shallow mantle fluid inclusions in a pyroxenite, associated with the Kerguelen Plume, from Northeast India. Optical microscopy and Raman spectroscopy reveal primary inclusions of hydrocarbon (CH) fluid and calcite in diopsidic clinopyroxene (Cpx) of this pyroxenite. Another Cpx from the same rock shows a rare occurrence of pseudo-secondary polyphase fluid inclusions of hydrogen sulfide (H2S), carbon monoxide (CO), rutile and calcite. We infer that metasomatic interaction between silicates and C-O-H volatiles generated these fluids, which are trapped as inclusions. We further suggest two factors controlling carbon speciation in these fluids 1) exsolution of magnetite and diffusion of Ti from the Cpx to form rutile and 2) oxidation buffering due to presence of H2S that prevented the formation of CH and formed CO instead. Our study explains the existence of significant reduction zones at localized scale in the oxidized shallow upper mantle.