Eutectic between wollastonite II and calcite contrasted with thermal barrier in MgO-SiO_2-CO_2 at 30 kilobars, with applications to kimberlite-carbonatite petrogenesis

Abstract

In the join CaCO_3-CaSiO_3 at 30 kbars, calcite melts at 1615°C, wollastonite II at 1600°C, and a binary eutectic occurs at 1365°C with liquid composition 43 wt.% CaCO_3, 57 wt.% CaSiO_3. The eutectic liquid quenches to a glass with few quench crystals. In the join MgCO_3-MgSiO_3 at 30 kbars, magnesite melts at 1590°C, enstatite at 1837°C, and the fields for the primary crystallization of magnesite and enstatite are separated by a thermal barrier near 1900°C for the melting of forsterite in the presence of CO_2. Only about 10 wt.% MgSiO_3 dissolves in the carbonate liquid. These data, are considered together with incomplete results for joins CaMgSi2O6-CaMg(CO_3)_2, CaMgSi_2O_6-MgCO_3, CaMgSi_2O_6-CaCO_3, and other published data in the system CaO-MgO-SiO_2-CO_2. A thermal barrier separates the silicate and carbonate liquids in MgO-SiO_2-CO_2 but, in the quaternary system, silicate liquids with dissolved CO_2 can follow fractionation paths around the forsterite field to the fields for the primary crystallization of carbonates. This suggests that fractional crystallization of CO_2-bearing ultrabasic magma at 100 km depth can produce residual carbonatite magma.

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