Phase relations and partial melting of C-bearing sediment under mantle conditions
Final Report Abstract
The goals of this study were i) to estimate phase relationships during the melting of model carbon-bearing pelitic sediments under reduced conditions at pressures from 7-12 GPa, ii) to determine major element compositions and trace element geochemical signatures of the resulting melts (fluids) to evaluate the interaction of sediment-derived fluids (melts) with overlying mantle peridotite, iii) to estimate trace element partition coefficients between residual minerals (garnet, pyroxene, carbonate, kyanite) and volatile-rich melt, and iv) to investigate the conditions of diamond nucleation and the graphite-diamond transformation in partially molten carbonated pelites. The near-solidus mineral assemblage contains coesite/stishovite, garnet, kyanite, clinopyroxene, carbonates (aragonite and Fe–Mg carbonate) and graphite/diamond. At low temperatures, the hydrous phases, phengite and lawsonite occur up to 9 GPa, before they become unstable. At higher pressures, topaz and/or phase EGG are present. Although most hydrous phases disappear at ~900°C, carbonates persist up to 1000–1100°C. At temperatures >1200°C the mineral assemblage consists essentially of coesite/stishovite, kyanite and garnet, along with graphite/diamond. Zircon and rutile are common accessory phases at all pressuretemperature conditions. Melt (fluid) compositions changes gradually with increasing temperature from hydrous carbonate-dominated (<10 wt% SiO2) at 800–1000°C to volatile-rich silicate melts (up to 40 wt% SiO2) at high temperatures, consistent will all experiments lying above the second critical point in this system. . DCpx/L for the REE are very low, which is probably related to the high jadeite content of the clinopyroxenes. On the other hand, partition coefficients for high field strength elements (HFSE) are higher, suggesting that clinopyroxene may be able to fractionate these elements relative to REE during melting of C-bearing sediment. Mg-Fe carbonate and coesite and stishovite all have generally low crystal/melt partition coefficients. On the other hand, aragonite exhibits significantly different partitioning behavior with DArg/L ~1 for the REEs. Considering the bulk element partitioning between solid residual and melt, the melt composition will be strongly enriched in incompatible elements and will display a pronounced negative Nb–Ta anomaly, but without a Zr–Hf anomaly. The Nb–Ta anomaly will be a characteristic as long as rutile remains in the residual assemblage. These trace element characteristics care consistent with signatures of island-arc magmas. Spontaneous diamond nucleation and complete conversion of graphite to diamond transformation occurred at temperatures > 1200–1300°C, which is lower than many other studies of diamond nucleation. A qualitative model for diamond nucleation has been developed involving the relative C concentrations for graphite and diamond saturation and the “overstep” concentration necessary for diamond nucleation to occur. The results from this study will also provide important input data for the interpretation of future experiments investigating the interaction of sediment-derived melts and fluids with peridotite in a temperature gradient.
Publications
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Reduces sediment melting at 7.5–12 GPa: phase relations, geochemical signals and diamond nucleation. Contributions to Mineralogy and Petrology, August 2015, 170:18
Brey G.P., Girnis, A. V., Bulatov, V. K., Höfer ,H. E., Gerdes, A., Woodland, A.
