Petrological conditions for diamond occurrences in Alpine-type ultrabasic rocks

Abstract

Discoveries of diamonds in ultrabasic rocks of the Urals and Armenia have spurred an attempt to estimate the thermodynamic parameters for the formation of Alpine-type ultrabasics. The temperature of formation of the ultrabasic rocks, determined from diagrams for the isotherms of distribution of Mg and Fe in coexisting olivine, ortho-, and clinopyroxene, lies within the narrow limits of 1200-1250°C. Interpolation of these data with I. D. MacGregor's diagram for the solubility of Al2O3 in orthopyroxenes has established a wide range of pressures under which ultrabasic rocks may be formed: from 25 to 65 kbar. Moreover, part of the rocks have been formed in the field of diamond stability, which establishes the possibility of the protomagmatic crystallization of this mineral in the ultrabasic rocks. The presence of juvenile carbon in ultrabasic massifs, the reducing conditions of their crystallization, and the possibility of preserving the diamonds during their rapid passage to the earth's surface provides grou...