DIRECT OXYGEN MEASUREMENTS OF CR-RICH SPINEL - IMPLICATIONS FOR SPINEL STOICHIOMETRY

The availability of a synthetic multilayer crystal and accurately cali brated oxide and silicate standards make it possible to use the electr on microprobe for precise O analysis of spinel. A requirement of the O measurement routine described is the use of repetitive statistical an alyses of the O standards and subsequent corrections and recalibration . A representative set of O analyses for each spinel population studie d is essential to obtain reliable data, and the danger of using single datum is emphasized. Magnesiochromite spinel grains, having broad com positional similarities but different petrogenetic and cooling histori es, were analyzed for O and their stoichiometry was assessed. Diamond- indicator spinel from the Aries kimberlite and Argyle lamproite is sto ichiometric. Spinel inclusions in olivine phenocrysts from Ti-poor tho leiite from the Hunter Fraction Zone and Ca-rich boninite from the Ton ga Trench show a range of nonstoichiometry. High Fe2+/Fe3+ values calc ulated assuming stoichiometry for such spinel are invalid. Spinel samp les from metamorphosed volcanics from the Peak Hill-Glengarry Basin an d the Heazlewood River Ultramafic Complex are also nonstoichiometric, having significant Fe8/3O4-Cr8/3O4 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.