ARTICLES kimberlite petography

摘要

In a paper entitled "The interpretation of the major element compositions of mantle minerals in diamond exploration", Gurney and Zweistra (1995) refer to a "diamond preservation index" with regard mainly to ilmenite chemistry and diamond resorption. Apart from this, the relationship between ilmenite and diamond is not well constrained, as ilmenite is neither a common inclusion in diamond nor is it a primary component of mantle-derived, diamond-bearing peridotites or eclogites. Current assessments of diamond potential or interest ratings of either prospecting samples or kimberlites themselves are based mainly on the presence or proportions of garnets, chromites and clinopyroxenes that have chemistries indicative of having been derived from within the diamond stability field, the closer these chemistries approach diamond inclusion compositions the better. A diamond preservation or destruction indication is provided by trace element work on garnet (Griffin and Ryan, 1995), which also provides evidence on the status of the palaeo-geotherm. The absence of garnets having high values of yttrium at elevated temperatures is an indication of an absence of excessive destructive metasomatic effects, and the lower the geotherm the better the chances are of derivation of a fair proportion of the garnet population from the diamond stability field.