Calcite and Dolomite without Portlandite at a new Eutectic in CaO-MgO-CO_2-H2_O with Applications to Carbonatites

Abstract

Bröggers (1921) idea of a carbonatite magma was not widely accepted. Shand (1947, p. 326) concluded that "a carbonate magma…would be a very strange thing indeed". The existence of carbonatite magmas appeared less strange when Wyllie and Tuttle (1960) discovered that melts in the system CaO-CO_2-H_2O precipitate calcite down to temperatures below 650°C at very low pressures. The low-temperature synthetic carbonatite magmas are near a eutectic between calcite and portlandite in the system CaCO_3-Ca(OH)_2. A magmatic origin for most carbonatites was favored by the field evidence summarized in books by Heinrich (1966) and Tuttle and Gittins (1966), although the overprint of the deuteric stage and hydrothermal or carbothermal activity may obscure the interpretation.