Liquidus relationships in the system CaCO_3-Ca(OH)_2-CaS and the solubility of sulfur in carbonatite magmas

Abstract

CaCO_3-Ca(OH)_2-CaS serves as a model system for sulfide solubility in carbonatite magmas. Experiments at 1 kbar delineate fields for primary crystallization of CaCO_3, Ca(OH)_2 and CaS. The three fields meet at a ternary eutectic at 652°C with liquid composition (wt%): CaCO_3 = 46.1%, Ca(OH)_2 = 51.9%, CaS = 2.0%. Two crystallization sequences are possible for liquids that precipitate calcite, depending upon whether the liquid is on the low-CaS side, or the high-CaS side of the line connecting CaCO3 to the eutectic liquid. Low-CaS liquids precipitate no sulfide until the eutectic temperature is reached leading to sulfide enrichment. The higher-CaS liquids precipitate some sulfide above the eutectic temperature, but the sulfide content of the melt is not greatly depleted as the eutectic temperature is approached. Theoretical considerations indicate that sulfide solubility in carbonate melts will be directly proportional to ƒS212 and inversely proportional to ƒO212; it also is likely to be directly proportional to melt basicity, defined here by a CO32−ƒCO2. A strong similarity exists in the processes which control sulfide solubility in carbonate and in silicate melts. By analogy with silicates, ferrous iron, which was absent in our experiments, may also exert an important influence on sulfide solubility in natural carbonatite magmas.

Additional details