The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks

TLDR

The authors defined compositional fields for spinels using contoured data‑density plots of the spinel prism and TiO₂ versus ferric iron for a wide range of mafic–ultramafic rock types. The analysis reveals distinct compositional trends in spinels: a Cr–Al trend varying with pressure, a Fe–Ti trend linked to low‑pressure fractionation, a similar trend in kimberlites, and an Al‑increasing trend in layered intrusions, with the most Cr‑rich chromites occurring in boninites, kimberlites, komatiites, and ophiolitic chromitites and exceptionally reduced chromites in some komatiites and ophiolitic chromitites.

Abstract

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 iO2 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 iO2 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.