A PROTON-MICROPROBE AND ELECTRON-MICROPROBE STUDY OF GALLIUM, NICKEL AND ZINC DISTRIBUTION IN CHROMIAN SPINEL

Chromian spinels from a number of localities were analyzed by proton a nd electron microprobes to examine the variation of trace elements in different geological environments. Chromian spinels studied include th ose representing ophiolitic complexes, layered intrusions in continent al areas, Alaskan-type complexes, komatiitic subvolcanic sills, Ni sul fide ores and metamorphosed mafic and ultramafic rocks. Nickel concent rations range from 30 to 4690 ppm (proton microprobe), with a mean val ue of 943 ppm. Nickel, favouring the octahedral sites in the spinel st ructure, displays an overall increase with a decrease in the Cr/(Cr+Al +Fe3+) ratio. Nickel partitioning between chromian spinel and mantle s ilicates appears to be primarily dependent upon the major element comp osition of the chromian spinel. Zinc concentrations range from 230 to 9810 ppm (proton microprobe), with a cluster around 500 ppm. Ophioliti c chromian spinels display the lowest overall Zn values. Zinc levels a ppear to discriminate between chromian spinels in chromitite samples f rom those occurring as accessory chromian spinel in dunites or other r ock types. Chromian spinels that are enclosed in or adjacent to sulfid e grains display higher concentrations of Zn. Zinc, strongly favouring the tetrahedral sites, displays a crude negative correlation with the Mg/(Mg+Fe2+) ratio and appears to vary as a function of temperature. All the chromian spinel grains analyzed contain measurable quantities of Ga by proton microprobe, ranging from 10 to 208 ppm. There is a rec iprocal relationship between the Ga concentration and the Cr/(Cr+Al+Fe 3+) ratio and a crude positive correlation between the Ni and Ga conce ntrations. Measured Ga concentrations in chromian spinel and whole roc k samples are supportive of the experimental mineral-melt partition co efficient of Malvin and Drake ( 1987). Similar to the peridotite xenol iths, mid-ocean ridge basalts and oceanic-island basalts, Ga correlate s well with Al2O3 in the case of the ophiolitic chromian spinels and m ost of those representing layered complexes. The ophiolitic chromian s pinels seem to have crystallized from melts whose compositions are sim ilar to those of partial melts of the upper mantle.