Evolution of the South African mantle — A case study of garnet peridotites from the Finsch diamond mine (Kaapvaal craton); part 1: Inter-mineral trace element and isotopic equilibrium

Abstract

A thorough assessment of inter-mineral equilibrium is essential for the understanding of trace element partitioning and also for the interpretation of isotopic data. Here we investigated high temperature (~1200°C and 6GPa) garnet peridotite xenoliths from the Kaapvaal craton (Finsch mine, South Africa), with exceptionally well equilibrated mineral major element compositions, for their trace element and isotopic inter-mineral equilibrium. Trace element compositions for all major mineral phases, i.e. olivine, orthopyroxene (opx), clinopyroxene (cpx) and garnet, were analysed by laser ablation inductively coupled plasma mass spectrometry (ICP-MS). Garnet, cpx and opx of selected samples were analysed for their Sm–Nd and Lu–Hf isotope compositions by multi collector ICP-MS.Most important mineral characteristics include: a) olivines from most samples are enriched in high-field-strength elements relative to other incompatible trace elements. Their lithium content correlates negatively with Mg#, indicating a depletion signature; b) all other minerals are depleted in heavy and middle rare earth elements (H- and M-REE) and enriched in light REE and large ion lithophile elements. This implies a complex history of depletion and metasomatic overprint for the Finsch cratonic mantle; c) orthopyroxene has similarly shaped trace element patterns as cpx, with one to two orders of magnitude lower abundances; and d) both, garnet and cpx, display variable, mostly positive εHf coupled with moderate variations in εNd.Trace element partitioning between garnet/cpx, cpx/opx and garnet/opx, displays only a weak pressure and temperature dependency. However, equilibrium partitioning of most trace elements between garnet and cpx shows a strong compositional dependency, i.e. on the Cr- (and Ca-) content of the garnets. Garnet–cpx partition coefficients follow a second grade polynomial correlation with Cr2O3 of garnet, whereby high chromium garnets (Cr2O3>6wt.%) have generally higher partition coefficients for all trace elements than low Cr garnets. Potentially, enhanced trace element compatibility in high-Cr garnets may be related to the formation of a knorringite component and its influence on the garnet crystal structure. However, some elements, especially the quadrivalent Zr and Hf and the small trivalent HREE, show increased scatter indicating deviation from complete inter-mineral equilibrium.Garnet–cpx Sm–Nd isochrons of all analysed samples are consistent with the age of the Finsch kimberlite eruption (~118Ma). This further demonstrates complete isotopic equilibration of Sm and Nd between garnet and cpx for these mantle samples. In contrast to Sm–Nd, the Lu–Hf garnet–cpx isochrons display an age range between 60 and 570Ma, indicating Lu–Hf disequilibrium. Most probably slow diffusion of Hf and Lu, in the absence of a melt or fluid, resulted in incomplete Lu–Hf equilibration and the preservation of an “old” radiogenic Hf in garnet and cpx.