Pyroxene solid solution in garnets included in diamond

Abstract

In what proved to be a temporary phase of trial mining of the Monastery Mine kimberlite pipe, South Africa, we recently had a brief opportunity to characterize the mineral inclusions in diamonds from this locality. The usefulness of such studies in elucidating mantle processes has already been demonstrated^1–3. We describe here garnets included in diamonds which have a unique range of compositions. They are interpreted as representing the first natural occurrence of garnet hosting a component of pyroxene in solid solution. Their discovery fulfils predictions based on successful laboratory experiments in which pyroxene was dissolved in garnet at very high pressures (50–180 kbar)^4–6. As this solid solution is accompanied by an increase in density, such results have both geophysical and chemical implications for the structure of the mantle. In addition, the pressures inferred are higher than generally proposed for mantle minerals sampled by kimberlites. Recent work has demonstrated that at least some inclusion-bearing diamonds have been derived from Archaean lithosphere⁷. The Monastery garnets in diamonds may indicate that the old lithosphere was very thick^8,9. Alternatively, if they have an asthenospheric origin, they may prove particuarly interesting because of their different source.