The join grossularite-calcite through the system CaO-Al_2O_3-SiO_2-CO_2 at 30 kilobars: Crystallization range of silicates and carbonates on the liquidus

Abstract

At 30 kbar, calcite melts congruently at 1615°C, and grossularite melts incongruently to liquid + gehlenite (tentative identification) at 1535°C. The assemblage calcite + grossularite melts at 1450°C to produce liquid + vapor, with piercing point at about 49 wt.% CaCO_3. Vapor phase is present in all hypersolidus phase fields except for those with less than about 7% CaCO_3 or 8% Ca_3Al_2Si_3O_(12). These results, together with known liquidus data for CaO—SiO_2—CO_2 and inferred results for CaO—Al_2O_3—CO_2 and Al_2O_3—SiO_2—CO_2, permit construction of the position of the CO_2- saturated liquidus surface in the quaternary system, and estimation of the positions of liquidus field boundaries separating some of the primary crystallization fields on this surface. The field of calcite is separated from those for grossularite and quartz by a field boundary with about 50% dissolved CaCO_3. Crystallization paths of silicate liquids in the range Ca_2SiO_4—Ca_3Al_2Si_3O_(12)—SiO_2, with some dissolved CO_2, will terminate at a quaternary eutectic on this field boundary, with the precipitation of calcite together with grossularite and quartz, at a temperature below 1450°C. Addition of Al_2O_3 to CaO—SiO_2—CO_2 in amounts sufficient to stabilize garnet thus causes little change in the general liquidus pattern as far as carbonates and silicates are concerned. With addition of MgO, we anticipate that silicate liquids with dissolved CO_2 will also follow liquidus paths to fields for the precipitation of carbonates; we conclude that similar paths link kimberlite and some carnbonatite magmas.

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