Melting phase relations of natural peridotite +CO ₂ as a function of degree of partial melting at 15 and 30 kbar

Geophys. Lab., Washington, D.C., United States

Phase relations of peridotite + CO2 have been determined at 15 and 30 kbar using 185W to monitor the degree of melting. At 15 kbar the initial <5 percent melting interval is affected by the presence of H2O formed by the reaction of some H2 (diffused through the capsule wall) with CO2 to form H2O + CO. This effect becomes insignificant with >5 percent melt ( approximately 1400 degrees C), where the melting (to approximately 20 percent melt) is nearly invariant. In this latter melting interval, the melt coexisting with olivine, orthopyroxene, clinopyroxene, and vapor is tholeiitic. Melting commences more than 200 degrees C lower at 30 kbar than at 15 kbar for both CO2-saturated and CO2-undersaturated conditions. Again, the exact nature of the melting interval from the solidus to about 5 percent melting is uncertain because of the presence of small amounts of H2O in the experimental charges. Dolomite is a solidus phase together with olivine, orthopyroxene, and garnet, and the initial melt is carbonatitic, as evidenced by quenching of the melt to carbonate. With increasing degree of partial melting the melt compositions become richer in silicate components, as evidenced by quenching of the melt to mixtures of silicate glass and carbonate minerals. These results indicate that the principal phase relations in simple systems such as CaO-MgO-SiO2-CO2 apply to melting of natural peridotite. Published experimental results in this system show a great depression of the solidus temperature ( approximately 150 degrees C) as the pressure is raised above that of the carbonation reaction, Di + Fo + V --> En + Dol. At the same time, the liquid composition changes from haplobasalt to haplocarbonatite and therefore serves as a suitable model for liquids produced in the system peridotite + CO2.

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