Comment on “Trace-element crystal chemistry of mantle eclogites” by F.A. Caporuscio and J.R. Smyth

Abstract

Caporuscio and Smyth have presented crystal chemical study on eleven mantle-derived eclogites from the Bellsbank (8) and Roberts Victor (3) kimberlites, South Africa. They combine these results with experimental partition coefficients and geochemical modelling to argue for a high pressure igneous cumulate origin from a MORB-like melt for these eclogites. In particular, they highlight the kyanite-bearing eclogites (grospydites), especially the presence of a “middle rare-earth-element (MREE) enrichment”, which may also be considered in terms of LREE and HREE depletions. Caporuscio and Smyth, as well as Smyth et al., cite this phenomenon as evidence for an origin by igneous accumulation of hyperaluminous clinopyroxene at high pressure. However, this type of REE pattern has also been interpreted as depicting a positive Eu anomaly inherited from a subducted, plagioclase-rich oceanic crustal protolith (Shervais et al., Taylor and Neal, Neal et al.).

This manuscript presents an alternative interpretation of the data presented by Caporuscio and Smyth. The results presented here demonstrate that high-pressure igneous accumulation of hyperaluminous pyroxene from a MORB-like liquid, followed by exsolution of major amounts of garnet and kyanite, is unlikely to account for all chemical signatures exhibited by grospydites. Our approach is to undertake quantitative geochemical modelling of these processes using the actual samples and literature values quoted by Caporuscio and Smyth.