Pyrope Inclusions in Chrome Spinels from Kimberlites and Lamproites and Their Significance for Estimation of the Paragenetic Assemblage and Formation Depth

摘要

Chrome spinels are common accessory minerals in a wide range of ultramafic rocks (dunites, harzburgites, lherzolites, websterites, and wehrlites) from different geological settings and depth facies. Macrocrysts and microcrysts of chrome spinel in the kimberlite ground-mass are characterized by the maximal compositional diversity. The microcrysts usually do not exceed 0.25 mm in size and crystallize at the final (low-pressure) stage of kimberlite evolution. The chrome spinel macrocrysts represent a part of the primary minerals of spinel-containing peridotites of different (spinel, graphite-pyrope, and diamond-pyrope) depth facies. Thus, the macrocrysts are relicts of the completely disintegrated xenoliths of the corresponding peridotites. Along with pyropes and picroilmenites, they are present in the heavy fraction concentrate of kimberlites. However, the chrome spinel macrocrysts prevail over other minerals of the concentrate in lamproites.