Direct oxygen measurements of Cr-rich spinel; implications for spinel stoichiometry

Abstract

The availability of a synthetic multilayer crystal and accurately calibrated oxide and silicate standards make it possible to use the electron microprobe for precise O analysis of spinel. A requirement of the O measurement routine described is the use of repetitive statistical analyses of the O standards and subsequent corrections and recalibration. A representative set of O analyses for each spinel population studied is essential to obtain reliable data, and the danger of using single datum is emphasized. Magnesiochromite spinel grains, having broad compositional similarities but different petrogenetic and cooling histories, were analyzed for O and their stoichiometry was assessed. Diamond-indicator spinel from the Aries kimberlite and Argyle lamproite is stoichiometric. Spinel inclusions in olivine phenocrysts from Ti-poor tholeiitc from the Hunter Fraction Zone and Ca-rich boninite from the Tonga Trench show a range of nonstoichiometry. High Fe2+/ Fe3+ values calculated assuming stoichiometry for such spinel are invalid. Spinel samples from metamorphosed volcanics from the Peak Hill-Glengarry Basin and the Heazlewood River Ultramafic Complex are also nonstoichiometric. having significant Fe83O4-Cr83O4 components. Our results demonstrate that nonstoichiometry is a common feature of Cr-rich spinel. This has important implications for the use of Fe3+ and Fe2+ concentrations to estimate the oxidation state or temperature of formation.