Significance of Accessory Chrome Spinel in Identifying Serpentinite Paragenesis

Abstract

Chrome spinels are common accessory minerals in a wide series of ultrabasic rocks occurring in various geological settings and formed over a wide P-T range. These rocks include kimberlites, lamproites, and peridotites of orogenic massifs, part of the ophiolitic association. In many cases, these rocks are heavily serpentinized such that alterations cause difficulties in identifying the presence and, in particular, compositions of olivines and pyroxenes. Chrome spinels, however, may be present in heavy concentrates as the sole surviving primary mineral. This study shows that the compositional features of accessory chromites from xenoliths of granular peridotites from the Udachnaya kimberlite pipe, Yakutia, and chrome spinel from kimberlite and lamproite heavy concentrates, display a wide range in Mg# (50-80) and Cr# (20-90). Chromites occurring as inclusions in diamonds are notable exceptions, showing relatively narrow Cr# (mostly from 75 to 90) but a wide range in Mg# (50-80).

Accessory chrome spinels, similar to diamonds in kimberlites and lamproites, and zircons in UHP metamorphic rocks, may be considered as good containers of primary minerals, including olivine, pyroxenes, and amphiboles. This is confirmed for a variety of chrome spinels studied from kimberlites, lamproites, and detrital spinels from unknown sources within the Siberian platform. The examination of chrome spinels from heavily serpentinized peridotites and serpentinites for the presence of mineral inclusions may widen the possibilities of identifying their paragenesis and P-T estimations.