The Melting of Carbonated Pelites from 70 to 700 km Depth
Grassi, Daniele ; Schmidt, Max W.
In: Journal of Petrology, 2011, vol. 52, no. 4, p. 765-789
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- Phase assemblages, melting relations and melt compositions of a dry carbonated pelite (DG2) and a carbonated pelite with 1·1 wt % H2O (AM) have been experimentally investigated at 5·5-23·5 GPa and 1070-1550°C. The subsolidus mineralogies to 16 GPa contain garnet, clinopyroxene, coesite or stishovite, kyanite or corundum, phengite or potassium feldspar (≤8 GPa with and without H2O, respectively), and then K-hollandite, a Ti phase and ferroan dolomite/Mg-calcite or aragonite + ferroan magnesite at higher pressures. The breakdown of clinopyroxene at >16 GPa causes Na-rich Ca-carbonate containing up to 11 wt % Na2O to replace aragonite and leads to the formation of an Na-rich CO2 fluid. Further pressure increase leads to typical Transition Zone minerals such as the CAS phase and one or two perovskites, which completely substitute garnet at the highest investigated pressure (23·5 GPa). Melting at 5·5-23·5 GPa yields alkali-rich magnesio-dolomitic (DG2) to ferro-dolomitic (AM) carbonate melts at temperatures 200-350°C below the mantle geotherm, lower than for any other studied natural composition. Melting reactions are controlled by carbonates and alkali-hosting phases: to 16 GPa clinopyroxene remains residual, Na is compatible and the magnesio- to ferro-dolomitic carbonate melts have extremely high K2O/Na2O ratios. K2O/Na2O weight ratios decrease from 26-41 at 8 GPa to 1·2 at 16 GPa when K-hollandite expands its stability field with increasing pressure. At >16 GPa, Na is repartitioned between several phases, and again becomes incompatible as at <3 GPa, leading to Na-rich carbonate melts with K2O/Na2O ratios 1. This leaves the pressure interval of c. 4-15 GPa for ultrapotassic metasomatism. Comparison of the solidus with typical subducting slab-surface temperatures yields two distinct depths of probable carbonated pelite melting: at 6-9 GPa where the solidus has a negative Clapeyron slope between the intersection of the silicate and carbonate melting reactions at ∼5 GPa, and the phengite or potassium feldspar stability limit at ∼9 GPa. The second opportunity is related to possible slab deflection along the 660 km discontinuity, leading to thermal relaxation and partial melting of the fertile carbonated pelites, thus recycling sedimentary CO2, alkalis and other lithophile and strongly incompatible elements back into the mantle