The range of spinel compositions in terrestrial mafic and ultramafic rocks

Compositional fields for spinels from a wide variety of mafic-ultramafic igneous rock types and tectonic environments have been determined from a global database of over 26 000 analyses. These fields are defined using contoured data density plots based on the spinel prism, and plots of T iO(2) vs ferric iron, for mantle xenoliths, ophiolitic rocks, continental layered intrusions, alkalic and lamprophyric rocks, tholeiitic basalts, Alaskan ultramafic complexes and komatiites. Several trends appear regularly in the various environments: a trend of widely variable Cr/(Cr + Al) at low Fe2+/(Mg + Fe2+) (the Cr-Al trend); increasing Fe3+, Fe2+/(Mg + Fe2+) and T iO(2) at constant Cr/(Cr + Al) (Fe-T i trend); a trend found primarily in kimberlites, similar to Fe-T i but at constant Fe2+/(Mg + Fe2+); and an unusual trend of increasing Al found only in layered intrusions. The Cr-Al and Fe-T i trends are both found to varying degrees in tholeiitic basalts. The Cr-Al trend is prevalent in rocks that have equilibrated over a range of pressures, whereas the Fe-T i trend is dominantly due to low-pressure fractionation. The most Cr-rich chromites found in nature occur in boninites, diamond-bearing kimberlites, some komatiites and ophiolitic chromitites. Exceptionally reduced chromites are found in some komatiites and in ophiolitic chromitites. Detrital chromites from the Witwatersrand conglomerates are of komatiitic provenance.