Phase equilibria in a volatile-free kimberlite at 0.1 MPa and the search for primary kimberlite magma

Phase relations at 0.1 MPa over a range of oxygen fugacities (fO2) from 1130 to 1350 °C on a volatile-free synthetic kimberlite analog composition show the crystallization sequence of olivine, monticellite, akermanite, perovskite, clinopyroxene, ulvopsinel and kirschteinite. At low pressures kimberlite liquids show limited change in Ca, Si, Al and Ti, but a large variation in Fe/Mg from the liquidus to ~ 90% crystallization. We observe low KdFeMg ol/liq for kimberlite liquids (0.15 to 0.25). There is limited low pressure differentiation of kimberlites, affecting mostly only FeO and MgO by olivine fractionation or accumulation. The low KdFeMg ol/liq for kimberlite liquids require they crystallize higher Mg# olivine compared to most other mantle-derived magmas. Olivine phenocrysts in kimberlite are dominantly more magnesian (Fo92–94) than typical mantle olivine (Fo90) and with a low KdFeMg ol/liq necessitates the large amounts of this phase in kimberlites precipitated and accumulated at low pressure by decompression induced degassing of H2O or CO2. The comparison of experimental with natural data suggest that kimberlites can be viewed as essentially carbonate liquids with only 10 to 25% SiO2 (nearly carbonatites) that have accumulated olivine.