A proton- and electron-microprobe study of gallium, nickel and zinc distribution in chromian spinel

Chromian spinels from a number of localities were analyzed by proton and electron microprobes to examine the variation of trace elements in different geological environments. Chromian spinels studied include those representing ophiolitic complexes, layered intrusions in continental areas, Alaskan-type complexes, komatiitic subvolcanic sills, Ni sulfide ores and metamorphosed mafic and ultramafic rocks.

Nickel concentrations range from 30 to 4690 ppm (proton microprobe), with a mean value of 943 ppm. Nickel, favouring the octahedral sites in the spinel structure, displays an overall increase with a decrease in the Cr/(Cr + Al + Fe³⁺) ratio. Nickel partitioning between chromian spinel and mantle silicates appears to be primarily dependent upon the major element composition of the chromian spinel.

Zinc concentrations range from 230 to 9810 ppm (proton microprobe), with a cluster around 500 ppm. Ophiolitic chromian spinels display the lowest overall Zn values. Zinc levels appear to discriminate between chromian spinels in chromitite samples from those occurring as accessory chromian spinel in dunites or other rock types. Chromian spinels that are enclosed in or adjacent to sulfide 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 reciprocal relationship between the Ga concentration and the Cr/(Cr +Al + Fe³⁺) ratio and a crude positive correlation between the Ni and Ga concentrations. Measured Ga concentrations in chromian spinel and whole rock samples are supportive of the experimental mineral-melt partition coefficient of Malvin and Drake (1987). Similar to the peridotite xenoliths, mid-ocean ridge basalts and oceanic-island basalts, Ga correlates well with Al₂O₃ in the case of the ophiolitic chromian spinels and most of those representing layered complexes. The ophiolitic chromian spinels seem to have crystallized from melts whose compositions are similar to those of partial melts of the upper mantle.