THE SOLIDUS OF CARBONATED, FERTILE PERIDOTITE

Abstract

The solidus for a fertile peridotite composition (“Hawaiian pyrolite”) in the presence of CO2 has been determined from 14 to 35 kbar. The solidus shape inP-T space is consistent with previous studies in the model system CaO—MgO—SiO2—CO2. The intersection of the decarbonation reaction Olivine + Diopside + CO2 ⇌ Orthopyroxene + Dolomite with the pyrolite solidus defines the pointI6, which is located at 20.5 kbar, 1050°C. At pressures less thanI6 the solidus is only slightly (20–50°C) below that of the volatile-absent (C—H absent) solidus but at pressures greater thanI6 the solidus curve is approximately 275°C below the volatile-absent solidus. The large depression of the solidus to lower temperatures with little change in pressure occurs at approximately 19 kbar, 1250°C. At 26 kbar the Hawaiian pyrolite-CO2 solidus is approximately 100°C higher than the solidus for Hawaiian pyrolite-H2O—CO2 confirming that the primary sodic dolomitic carbonatite melt at the latter solidus does contain some dissolved water.