Influence of H2O and CO2 on deep melting beneath ridges

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ABSTRACT 0623550 (HIRSCHMANN)Intellectual Merit: This works determines the influence of H2O and CO2 on deep melting beneath ocean ridges, with consequent importance to sub-ridge rheology, dynamics, geochemistry, and observational geophysics, is well recognized. Questions addressed include: (1) at what depth does volatile-induced melting begin beneath mid-ocean ridges, and what proportions of melt are generated, (2) what the relationship between melting and dehydration beneath ridges is, and (3) what influence does CO2 have in inciting carbonated silicate partial melting and dehydration deep beneath ridges. Work will involve high pressure and temperature multi-anvil experiments to determine the influence of small amounts of H2O and CO2 on deep melting and dehydration. Experimental targets are the solidus of peridotite with small amounts of added H2O at 3, 4, and 5 Gpa; the extent of melting and the extent of residue dehydration of slightly hydrous peridotite at 3 Gpa; and the effect of CO2 on melting and residue dehydration of slightly hydrous peridotite at 3 and 4 GPa. Run products will be analyzed with electron microscopy, vibrational spectroscopy, and secondary ion mass spectrometry (SIMS). Special attention will be paid to SIMS analysis of H and H/Ce in nominally anhydrous minerals, providing direct evidence of mineral dehydration during partial melting as well as monitoring for potential H2O loss from experiments. The experimental results will provide strong constraints on the relationship between volatiles and melting beneath ridges that should prove useful to geochemists investigating the relationship between melting regime and the volume and composition of melts produced, to geophysicists characterizing melting and temperature anomalies beneath ridges, and to geodynamicists seeking to model the relationship between mantle flow, melting, and development of the oceanic lithosphere. Broader Impacts: The experimental results will be incorporated into the LEPER (Library of Experimental PhasE Relations) database of experimental mineral/melt phase equilibria presently being compiled by the PI and will provide key data of the MELTS algorithm for the effect of H2O and CO2 on mantle melting, for which available constraints are presently sparse. The proposed research will support a faculty member at the University of Minnesota, provide training for a graduate student, and will aid education of undergraduates. In particular, the research activities will be integrated into opportunities for undergraduate education through an NSF-funded REU site (Fluids in the Earth from Surface to Core) and will employ undergraduate research assistants.