Chromite spinels from ultramafic xenoliths

Abstract

The spinels in ultramafic xenoliths from kimberlites and alkali olivine basalts show a wide range of compositional variation, far in excess of the ranges shown by their coexisting silicate phases. The chemical variation of the spinels is a function of the host magma, texture and depth of origin. The spinels occur in five textural types: 1. (a) euhedral spinels restricted only to the kimberlites; 2. (b) spinels intergrown with silicates commonly found in kimberlites; 3. (c) exsolved spinel from orthopyroxene in xenoliths from both alkali olivine basalt and kimberlite; 4. (d) interstitial spinel texture generally restricted to xenoliths from alkali olvine basalts; 5. (e) spinels in kelyphitic rims around garnet in xenoliths from kimberlites. The chemistry of the spinels varies systematically with texture. The highest Cr (Cr + Al) ratios and lowest Mg (Mg + Fe 2+) ratios occur in the euhedral spinels, whereas the spinels showing intergrowth and exsolution textures are compositionally intermediate. Spinels from kimberlite xenoliths have a higher and wider range of the ratio Fe 3+ (Cr + Al + Fe 3+) than the spinels from alkali olivine basalt xenoliths. The ratio Cr (Cr + Al + Fe 3+) increases with pressure or depth of origin of the xenoliths. The Al 2O 3 content of the spinels varies sympathetically with the Al 2O 3 content of the coexisting orthopyroxenes, the value being lowest for euhedral spinel-orthopyroxene pairs in kimberlite xenolith and highest for interstitial spinel-orthopyroxene pairs in alkali basalt xenolith. The wide range of chemistry of the spinels and their correspondence with geologic environment suggest that the chromite spinels from ultramafic xenoliths are particularly sensitive minerals in examining the environmental conditions of the Earth's upper mantle.