Seminar James Brenan

Chromitites are segregations of up to 90 vol% of the mineral chromite (FeCr₂O₄), occurring as either layers in stratiform igneous intrusions (stratiform deposits) or as irregular schlieren or polydeformed layers in ophiolites (podiform deposits).  Chromitites are the sole ore of chromium, an important critical metal, with stratiform deposits representing enrichments in chromium by a factor of ~1600 times average crustal abundances.  Both phase equilibria and Cr-MgO systematics for mafic and ultramafic magma compositions indicate a crystallization sequence of olivine or orthopyroxene followed by chromite, with silicate:chromite cotectic proportions of ~50:1.  This is consistent with the typical occurrence of chromite as an accessory phase in mafic/ultramafic lithologies, making massive chromitite an unusual rock-type.

Models for the petrogenesis of stratiform chromitites fall into three general categories: (1) formation by predominantly fluid dynamical processes involving crystal sorting from mafic/ultramafic magmas, (2) interaction/mixing with some combination of contaminants, or more evolved magma compositions during transit through the crust and (3) changes in the physiochemical state of the chromitite-forming system, in terms of oxygen fugacity (fO₂), water content or pressure.  In this talk I will review the experimental constraints for categories (2) and (3), with a focus on the most recent results from my research group.  A fundamental conclusion of our work is that all models of chromitite formation by direct crystallization require large volumes of magma to form the observed thicknesses of chromitite by direct precipitation.  The existence of the large, long-lived magma chambers needed to provide such magma volumes has been questioned by recent results suggesting stratiform intrusions are emplaced as a sequence of sills.  This would suggest that category 1, fluid dynamic processes, may be important for the formation of significant thicknesses of chromitite.