Carbon distribution in the stiliwater complex and evolution of vapor during crystallization of stillwater and bushveld magmas

The occurrence and distribution of carbon in the Stillwater Complex have been investigated. In mineralized troctolite and associated rocks of olivine-bearing zone I (OB I), carbon is present as graphitic material and calcite. The assemblage forsterite-antigorite-calcite-graphite and the petro graphic relations indicate equilibration of the carbon-rich phases during serpentinization. Typical OB I troctolite contains 500-1100 ppm wt. carbon, 40-70% of which is in calcite, whereas troctolite from higher stratigraphic positions generally contains <400 ppm carbon. Due to the metamorphism, it is not possible to deduce the extent to which enrichment of carbon in the ore zone is inherited from magmatic processes. In contrast, there is good evidence for magmatic graphite in parts of the Bushveld Complex.The C-O-H-Cl system has been investigated for conditions of Stillwater and Bushveld crystalliz ation. In alkali-poor fluids over a wide range of igneous and metamorphic conditions, the important chlorine species are HCl and CH3Cl The addition of chlorine to a C-O-H fluid in equilibrium with graphite leads to a quantitative increase in HCl+CH3Cl and corresponding decrease in H2O contents, and, when Cl/H exceeds 1, to a CO2+CO-rich fluid with little H2O Similarly, in more reduced fluids, CH4 contents are depressed by the formation of CH3Cl.From consideration of volatile solubilities and abundances in mafic magmas and the nature of the C-O-H-Cl system, it is hypothesized that the first fluid to exsolve from Bushveld and Stillwater intercumulus melt was composed of a mixture of CO2 CO, and HCl with minor amounts of sulfur species and H2O A model is developed for the evolution of such a fluid with cooling. The model assumes that graphite began to precipitate from the fluid at supersolidus temperature and that the system cooled down a T-fo2 path parallel to and >2 log units below that of the Ni-NiO oxygen buffer. Upon the appearance of graphite, the fluid evolved to a more hydrogen-rich composition by graphite precipitation and loss of oxygen to the surrounding silicate-oxide assemblage. Cooling of fluid to 25°C below the first appearance of graphite resulted in reduction in the fluid mass by >70%, thus concentrating chlorine, sulfur and other residual species in the intercumulus fluid and melt. The model explains the presence of chlor-apatite and the enrichment of graphite in the Bushveld Critical Zone and predicts that chlor-apatite-bearing Stillwater rocks were similarly enriched in graphite during crystallization. © 1989 Oxford University Press.

Duke Scholars

Author Alan E. Boudreau Earth and Climate Sciences