Redox heterogeneity in lithospheric mantle evidenced by C-O-H-S fluid inclusions entrapped in Kerguelen mantle-plume associated pyroxenites

The C-O-H-S fluids play a significant role in various geological processes. The participation of these fluids in redox reactions remarkably influences carbonation process, melting behaviour, and physicochemical properties of the mantle rocks. In this study, we report rare, reduced fluid inclusions in shallow upper mantle-derived pyroxenites associated with the Kerguelen mantle plume from Northeast India. Raman spectroscopy reveals primary inclusions of hydrocarbon (CH) fluid and calcite along with the nearly perpendicular exsolution lamellae of magnetite in clinopyroxene (referred to herein as Cpx 1) of these pyroxenites. A rare occurrence of pseudo-secondary polyphase fluid inclusions of hydrogen sulfide (H₂S), carbon monoxide (CO), rutile, and calcite was also recorded in other clinopyroxenes (referred to herein as Cpx 2) of these pyroxenites. Mineral chemical data suggests that the studied Cpx are purely diopside and Cpx 1 hosted magnetite exsolutions are coeval with these clinopyroxenes. This is evident from the notable enrichment in MgO in these magnetite exsolutions relative to accessory magnetite, showing that they certainly do not represent a sub-solidus phase. Single Cpx geothermobarometric calculations suggest that these pyroxenites were formed at 1.2- 1.8 GPa pressure and 752- 941 °C temperature. We use a theoretical thermodynamic model to validate our natural observations of fluid inclusions. Overall, our results show that mantle plume-derived oxidized C-O-H-S fluids interacted with crystallizing diopside from pyroxenites in the shallow mantle. During this interaction, these fluids participated in redox reactions leaving the reduced fluid products such as CH, CO and H₂S and minerals such as calcite and rutile trapped as co-genetic inclusions along with magnetite exsolutions in host diopsides at oxygen fugacity conditions equivalent to FMQ-2. Therefore, considering the widely accepted fact that the shallow lithospheric mantle is predominantly oxidized, these natural observations of fluid inclusions, for the first time provide direct evidence of redox heterogeneity of the shallow mantle.