Phase relations in peridotite+CO2 Systems to 12 GPa: Implications for the origin of kimberlite and carbonate stability in the Earth's upper mantle

The phase relations of synthetic peridotites in the systems CaO-MgO-SiO2-CO2 (CMS-CO2), CaO-MgO-Al2O3-SiO2-CO2 (CMAS-CO2) and CaO-MgO-SiO2-CO2-H2O (CMS-CO2-H2O) were studied from 4 to 12 GPa using a multi-anvil apparatus. Peridotite solidi in both the CMS-CO2 and CMAS-CO2 systems have similar topologies. Carbonates are the first phases consumed at, or above, the solidus in peridotite+CO2 at pressures above 4 GPa. Magnesite solid-solutions are stable subsolidus phases in peridotite+CO2 to pressures of at least 12 GPa. It is demonstrated that if uncontaminated, diamond-bearing kimberlites emplaced in the Earth's crust are primary magmas, then they could be derived by partial melting of carbonated peridotite at 5 to 7 GPa in the upper mantle beneath the cratonic areas of continents. “Proto-kimberlites” may be generated at pressures above 7 GPa in the upper mantle, but no such magmas are represented on the Earth's surface. Mineral chemical data from experimental run products indicate that: 1) the discrete sub-calcic clinopyroxene nodules found in some kimberlites represent phenocrysts which crystallized at high pressure in the kimberlite magma and, 2) the spatial association of kimberlite and melilitite observed in the field is not related to a common source region for both these magmas in the mantle. If the oxygen fugacity recorded in samples from the upper 150 km of the mantle is maintained at greater depths, then carbonates are potential hosts for carbon in the mantle to depths of at least 350 km.