Experiments and Models Bearing on the Role of Magma Mixing and Contamination on Chromite Crystallization in Ultramafic Magmas

Abstract To better understand the origin of stratiform chromitites, we experimentally evaluate the role of bulk compositional shifts arising from mixing or contamination of a mafic magma on both the liquid line of descent and the chromite saturation state. Experiments equilibrated synthetic Cr-bearing komatiite containing 0–50 wt % Cr-free contaminants (granodiorite (GD), Fe-rich shale (Fe-shale), metasediment (MS), natural magnetite (NM)) on Fe-pre-saturated Pt or Ir loops at 1192–1462 °C and 0.1 MPa at the fayalite-magnetite-quartz (FMQ) oxygen buffer. Experimental run products involving mixtures of komatiite with silicate contaminants produced some combination of euhedral olivine, chromite and glass, whereas initial cubes of NM nearly completely dissolved to produce Fe-enriched melt plus Cr-rich magnetite. The addition of contaminants results in the chromium content of the melt at chromite saturation (CCCS) to decrease with increasing melt FeO or decreasing melt SiO2 abundance. Assessment of the temperature dependence reveals that the log of the CCCS decreases linearly with inverse temperature, allowing for the chromite crystallization temperature to be predicted for a given melt Cr concentration. The addition of any of the studied contaminants decreases the modal abundance of olivine.