Inclusions in Premier mine diamonds

The mineral inclusions in Premier mine diamonds fall predominantly into the peridotitic and eclogitic inclusion suites reported from diamonds world-wide. Sulphides are the commonest inclusions. The eclogitic suite is more abundant by approx 3:2 and apart from sulphides, consists of garnet, clinopyroxene and very rare kyanite and coesite. The garnets have a wide range in compositions which are mainly due to substitution of Fe and Ca for Mg; they contain = or <0.39 wt.% Na₂O. The clinopyroxenes also vary in composition in a systematic manner in which tie-lines between coexisting garnet-clinopyroxene do not cross. The largest variations in chemistry result from substitution of Na-Al for Ca-Mg in the pyroxenes as the Ca-Fe increases in the coexisting garnets. K₂O is present in trace amounts = or <0.33 wt.%. Calculated equilibration T for coexisting mineral pairs are in the range 1199-1363oC at 50 kbar. The inclusions and the diamonds are considered to have crystallized together in eclogite which precipitated from a mantle-derived magma on cooling. Sub-solidus re-equilibration of the eclogite is inferred to account for the differences in composition of the minerals in the rocks and the diamonds. Olivine followed by orthopyroxene are the most abundant peridotitic minerals apart from sulphides. Both silicates show a bimodal distribution which is interpreted as evidence for two distinct harzburgitic parageneses for diamonds. Garnet, chromite and particularly clinopyroxene are rare. The presence of the latter suggests a very minor garnet-lherzolite field. Limited geothermometry indicates equilibration T <1200oC. (Authors' abstract)-C.N.