PYROPE INCLUSIONS IN CHROME SPINELS FROM KIMBERLITES AND LAMPROITES AND THEIR SIGNIFICANCE FOR ESTIMATION OF THE PARAGENETIC ASSEMBLAGE AND FORMATION DEPTH

Abstract

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 groundmass are characterized by the maximal compositional diversity. The new data reported in the present work once more confirm the validity of the assessment of the boundary of the composition field of the diamond-equilibrated subcalcic pyropes (Cr2O3 ≥5%) associated with the chrome spinels (Cr2O 3≥62%) in the harzburgite assemblage, i.e., pyropes with no less than 15 mol % knorringite. These results refute the simplified approach, according to which any subcalcic pyropes, including the Cr2O3-depleted variety certainly related to the graphite-pyrope facies, i.e., the graphite peridotites, should be referred to as the diamond-equilibrated phases.