AN EXPERIMENTAL STUDY OF THE EFFECT OF PRESSURE ON THE FORMATION OF CHROMITE DEPOSITS

Abstract

Despite extensive research on the Bushveld Complex chromitites, the mechanism(s) that form such anomalous chromite segregations remains uncertain. Recent work applying the MELTS thermodynamic model proposed that reduction of pressure upon magma ascent shifts silicate-in temperatures to lower values, such that chromite is the sole liquidus phase, resulting in formation of massive chromitites in the Bushveld. This project evaluates this hypothesis by determining the effect of pressure on chromite crystallization through laboratory phase equilibrium experiments done at 0.1 MPa, 0.5 GPa, and 1 GPa, employing two bulk compositions. The first corresponds to the widely accepted parental magma of Bushveld chromitites, termed B1, and the second is the same used in the MELTS modelling study, which contrasts with B1 most significantly in Al2O3 (17.4 wt% vs 11.8 wt% in B1), MgO (6.7 wt% vs 11.9 wt% in B1), and Cr (~680 μg/g vs ~1000 μg/g in B1) contents. Experiments were done by equilibrating compositions at 0.1 MPa in a gas mixing furnace from 1170-1300°C and at 0.5 GPa and 1 GPa in a piston-cylinder from 1230-1530°C, with fO2 corresponding to the fayalite-magnetite-quartz buffer and graphite-carbon dioxide buffer. Results show that the B1 magma reproduces phase equilibria and mineral compositions observed in the Bushveld whereas mineral compositions produced by the melt composition used in the MELTS modelling study are too Al-rich, excluding it as possible parental magma. Results show no significant change in Cr content of the melt at chromite saturation with pressure at constant relative fO2 and that orthopyroxene-in temperatures decrease with falling pressure. Therefore, a low-pressure interval of chromite-alone crystallization is plausible. However, results show 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.