Ultra-high pressure garnet inclusions in Monastery diamonds: trace element abundance patterns and conditions of origin
Moore, Rory O.; Gurney, John J.; Griffin, William L.; Shimizu, Nobu
European Journal of Mineralogy Volume 3 Number 2 (1991), p. 213 - 230
published: Apr 18, 1991
manuscript accepted: Dec 19, 1990
manuscript received: Oct 25, 1990
ArtNo. ESP147050302002, Price: 29.00 €
Abstract Eclogitic diamonds predominate over peridotitic diamonds at Monastery in the approximate ratio 9:1. Two distinct populations of eclogitic garnets are present: a small number (Group A) have compositions similar to those described from diamonds worldwide, while the majority (Group B) host variable amounts of pyroxene in solid solution. Formation pressures for Group B garnets are inferred to range from 68 to 154 kbar (200 to 480 km). The Group B garnet inclusions have broadly similar trace element compositions to the Group A garnets at Monastery and to eclogitic garnet inclusions from Argyle. However, they differ in detail from the garnets in potentially diamondiferous eclogite xenoliths at Monastery. No direct evidence has been identified to rule out the possibility that the inclusions and these xenoliths shared a common origin. Compositional differences may simply indicate that the xenoliths have reequilibrated in a lower P,T regime. Temperature and pressure played a significant role in determining trace element behaviour in the garnet inclusions, but bulk composition and crystal chemical effects were also important. The garnet inclusions hosting pyroxene in solid solution (Group B garnets) have HREE-enriched patterns typical of garnet. REE concentrations systematically decrease with increasing inferred pressure of formation indicating that pressure played a major role in determining the REE distribution in these garnets. Calculated REE patterns for liquids potentially in equilibrium with the Group B garnet inclusions are very similar to those of alkaline volcanic rocks including kimberlites and ocean island basalts. Diamonds containing Group B garnet inclusions probably formed in the asthenosphere. It is speculated that they may have crystallised from partial melts within a rising asthenospheric diapir that has underplated onto the lithosphere prior to kimberlite eruption. Alternatively, they may have been sampled by a hot-spot related kimberlite from pre-existing pockets of diamondiferous eclogite over the depth interval indicated by the inclusions (480-150 km).