Ancient metasomatism recorded by ultra-depleted garnet inclusions in diamonds from DeBeers Pool, South Africa

Abstract

Peridotitic garnet inclusions in diamonds from the DeBeers Pool mines represent samples from an extremely depleted mantle source. A large number of garnet inclusions have CaO contents of less than 1.8 wt.% and olivine inclusions are characterized by very high Mg-numbers (average: 94.6). These observations suggest that initial melt depletion in the protolith for the cratonic root beneath Kimberley (Kaapvaal Craton) exceeded orthopyroxene exhaustion (i.e. ≥ 50% melt extraction) and hence went farther than typically observed for cratonic peridotites worldwide. This signature of extreme depletion is not readily apparent from the study of mantle xenoliths from the same area and strongly underlines the unique role that diamond may play as a time capsule. Analyses of rare earth and other trace elements show that DeBeers Pool garnet inclusions, despite their ultra-depleted major element composition, were affected by metasomatic enrichment processes similar to those recorded by inclusions in diamonds worldwide. The DeBeers Pool garnet inclusions have a wide range of REE concentrations with variable sinuosity of their chondrite normalized abundance patterns. Sinuosity of garnet REE N patterns generally decreases with increasing fertility (i.e. higher Ca Ti, Fe, Y, and Zr); however, a distinction between the REE N signatures of harzburgitic and lherzolitic garnets is not possible. The REE signatures indicate that the source rocks of these garnets were affected by two phases of metasomatism. LREE–MREE enrichment is preserved by all garnets and indicates interaction with a fluid of highly fractionated character. The variable LREE–MREE concentrations of the garnet inclusions result from progressive evolution of the fluid to higher LREE/HREE ratios during continued equilibration with or precipitation of garnet. This metasomatic fluid was pervasive and affected the entire depth section of the lithosphere represented by the DeBeers Pool inclusions. A second phase of enrichment by HREE- and other HFSE-carrying melts, that also transported major elements (Ca and Fe), affected only a portion of the diamond source region and was restricted to comparatively shallower lithosphere at depths < 180 km. Diamond precipitation was likely associated with both styles of metasomatic re-enrichment. The similarity of lherzolitic and harzburgitic REE N patterns and their similarly variable REE concentrations suggests diamonds of both parageneses were formed concurrently during both phases of metasomatic re-enrichment.