Mantle metasomatism by graphite-saturated COH fluids: an experimental study of solutes generation at high-pressure conditions in subduction zones

• Applicant: Dr. Carla Tiraboschi
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 450056892

COH fluids have a crucial role in a variety of geological processes in subduction zones, where aqueous fluids released from the down going slab infiltrate the overlying mantle wedge and trigger partial melting and arc volcanism. Graphite has been traditionally considered a refractory sink of carbon in the subducting slabs. Nonetheless, recent experimental studies suggest that the presence of graphite modifies COH fluid properties, and result in enhanced mass transport capacity for major and trace elements through complexation with inorganic and organic C-bearing ligands. A quantitative understanding of mantle metasomatism by graphite-saturated COH fluids is, however, still lacking because the speciation of carbon and major solutes in the fluids, and hence their transport mechanisms, have not been identified yet.In this proposal, we focus on the identification of solutes and volatile species in graphite-saturated COH fluids equilibrated with silicate minerals employing hydrothermal diamond anvil cells coupled with Raman spectroscopy. Molecular Dynamic simulations will assist the characterization of dissolved species allowing the interpretation of distinct vibrational contributions of major species in the fluid. We will synthesize COH fluids in equilibrium with graphite and other relevant minerals in subduction mélanges, such as forsterite and quartz, which are representative of the mantle and sedimentary component, respectively. Experiments will be performed at pressure ranging from 1 to 3 GPa and temperature from 400 to 800 °C, to determine the composition of COH fluids released at subsolidus conditions into the mantle wedge. The new speciation results, combined with available data on mineral solubility, will be implemented in an open access thermodynamic model for COH fluids-rock interactions in subduction zones following the formalism of the Deep Earth Water model.The proposed research will provide new constraints for metasomatism in the mantle wedge by solute-rich COH fluids. The retrieved experimental data will represent a significant step forward in the study of deep carbon and will clarify the role of organic/inorganic carbon complexes in the transport of solutes at high-pressure conditions.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Professor Dr. Dimitri Sverjensky, Ph.D.