Collaborative Research: Partial Melting of the Shallow Mantle from Thermodynamic Modeling of Minerals and Melt

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This grant is to refine and improve the MELTS program for application to problems related to partial melting of the mantle. MELTS is a versatile package of thermodynamic algorithms that allows forward modeling of magmatic processes. Application of MELTS to mantle melting problems has yielded considerable new insight into processes of partial melting and melt transport, particularly related to sub-ridge processes, but imperfections in MELTS have hampered applications to these problems. During the proposed grant period, we will address these defects by increasing the sophistication of the thermodynamic models of silicate liquids and pyroxenes and by binding the resulting model more strongly to high pressure mineral-melt phase equilibria. We will also extend MELTS' capabilities to allow application to depths and conditions where important outstanding problems related to mantle melting processes remain. These include (1) the deeper regions beneath ridges (4-6 GPa), where small quantities of volatiles are believed to generate small-degree melts that have great significance to the geodynamics of sub-ridge processes and to the geochemistry of erupted MORB, and (2) the mantle wedge above subduction zones, where H2O and/or H2O+CO2 play a significant role in determining the locus and style of melting, and the composition and volume of melts. Proposed improvements in MELTS will include (1) changing the model for the entropy of mixing of the liquid from one of molecular mixing of mineral-like components to one of mixing of temperature, pressure and compositionally dependent polymeric units of variable stoichiometry. (2) Completing our already-initiated work of expanding pyroxenes to incorporate Cr and recalibrating the thermodynamic model for pyroxenes with an emphasis on matching high pressure pyroxene-liquid phase equilibria (3) Adding complex C-O-H fluids, both as a distinct phase and dissolved in silicate liquids, as well as associated carbonaceous minerals (carbonates, graphite, diamond). (4) Continuing expansion of the database of mineral-melt phase equilibria experiments used in calibration. (5) Conducting "ground-truthing" phase equilibria experiments to test and improve prototype models.