EVALUATION OF DIAMOND POTENTIAL FROM THE COMPOSITION OF PERIDOTITIC CHROMIAN DIOPSIDE

Abstract

Chemical variations in experimental and natural, high-pressure chromian diopsides were studied to evaluate the potential use of mantle-derived clinopyroxene as a diamond-indicator mineral during kimberlite and lamproite exploration. A two-step method that involves (i) identification of clinopyroxenes from garnet peridotite and (ii) discrimination of those equilibrated within the diamond stability field, is proposed. The first step is achieved by means of Ramsay's (1995) Cr2O3 vs. Al2O3 classification diagram, and is further refined on the basis of Al2O3 vs. MgO relations. The adopted compositional criteria allow clinopyroxenes from garnet peridotites, which occur as characteristic xenoliths in diamond-bearing magmatic rocks, to be distinguished from those derived from other types of mantle rocks (eclogites, pyroxenites, spinel peridotites) as well as from megacrysts and cognate phenocrysts. Excluding all but clinopyroxenes of garnet-peridotite derivation has the advantage of reducing greatly the variance of the petrological system under consideration. This allows simple chemical parameters to be used as geothermobarometric indicators. As predicted by thermodynamic models on simple systems (CMAS), the Ca content in clinopyroxene is negatively correlated with T and the Al content is negatively correlated with P. Within the compositional ranges observed for natural garnet peridotites, Ca vs. Al relations in the clinopyroxene are only slightly affected by variations in bulk chemistry and thus provide qualitative information on the P-T conditions of equilibration. A Ca vs. Al plot can be used to identify clinopyroxenes that last equilibrated within the stability field of diamond with an "accuracy" of 89 %. Mantle-derived chromian diopside may represent a valid complement to other, widely utilized indicator minerals for the evaluation of the diamond potential of an area affected by deep-seated magmatism. Studying the composition of clinopyroxenes from heavy-mineral concentrates can help to select tal gets for which mantle/magma interaction occurred at sufficient depth for diamond to be sampled and entrained in xenoliths or as xenocrysts.