Eclogitic and websteritic diamond sources beneath the Limpopo Belt – is slab-melting the link?

Abstract.

Trace-element concentrations in eclogitic and websteritic inclusions in diamonds from Venetia (South Africa) were analysed using an ion microprobe (SIMS). Garnets of both parageneses show similar, positive LREE_N/HREE_N slopes, but eclogitic garnets have higher MREE and almost flat MREE_N–HREE_N, and are also different in having significantly higher Sr and Zr. The occurrence of negative and positive Eu anomalies in garnets and clinopyroxenes of both parageneses points towards feldspar fractionation and accumulation in a magmatic precursor, suggesting subducted oceanic crust as a common protolith. Assuming equilibrium between clinopyroxene and garnet included in the same diamond, a bulk eclogite was reconstructed from these inclusions plus (expected) accessory rutile. The whole rock has a trace-element pattern lying between oceanic gabbro and EMORB, but is depleted in highly incompatible elements relative to these possible precursors. Quantitative modelling shows that relative and absolute trace-element abundances of the reconstructed eclogite and the hypothetical oceanic precursor agree if the latter is subjected to a loss of partial melts after subduction into the eclogite stability field. Major- and trace-element characteristics of websteritic inclusions could imply a more mafic precursor, which may have been part of a heterogeneous oceanic crust. However, new experimental data show that major- and trace-element compositions of websteritic inclusions in diamond are consistent with a mixing model in which they result from the reaction of slab-derived melts with surrounding mantle peridotite. This reaction generates major element contents that are intermediate between those of eclogitic and peridotitic sources whereas trace-element characteristics, such as Eu anomalies, are inherited from the melt source.