An experimental study of the effect of pressure on the formation of chromite deposits

Despite extensive research on massive chromitites, the mechanism(s) that form such anomalous chromite segregations remains uncertain. Recent work that considered a theoretical parental melt to the Critical Zone of the Bushveld Complex applied the MELTS thermodynamic model to propose that reduction of pressure upon magma ascent shifts the silicate-in temperature to lower values, such that chromite is the sole liquidus phase, resulting in formation of chromitites. Herein the effect of pressure on Cr solubility at constant fO2 relative to the FMQ buffer is evaluated through laboratory phase equilibrium experiments done at 0.1 MPa, 0.5 GPa, and 1 GPa. Two bulk compositions were employed: (1) the theoretical melt used in the MELTS modelling study and (2) B1, which is a widely accepted parental composition to the Bushveld Critical Zone. Experiments were conducted at 0.1 MPa by equilibrating compositions on Fe-Ir alloy wire loops from 1170-1300°C in a vertical-tube, gas-mixing furnace for 12-48 hours. Experiments at 0.5 GPa and 1 GPa were conducted with a piston-cylinder apparatus at 1230°C and 1280°C for 4-12 hours using Fe-Ir alloy and graphite-lined Pt capsules. Experiments show that the B1 composition reproduces phase equilibria and mineral compositions observed in the Bushveld whereas mineral compositions produced by the theoretical melt composition used in the MELTS modelling study are too Al-rich, excluding it as viable parental liquid. Results show no significant change in Cr content of the melt at chromite saturation with pressure at constant relative fO2. However, reduction of pressure can promote chromite crystallization, as the modal abundance and DCr(px/liq) of orthopyroxene decrease with pressure in experiments, increasing the availability of Cr for chromite crystallization. While a low-pressure interval of chromite-alone crystallization is plausible, results indicate that significant volumes of unusually Cr-enriched B1 magma would be required to produce the chromitites observed in the Bushveld by the pressure reduction mechanism.