Platinum-group element geochemistry of mantle eclogites: a reconnaissance study of xenoliths from the Orapa kimberlite, Botswana

摘要

Eclogite xenoliths recovered from kimberlites and alkali basalts are clinoproxene and garnet rich rocks formed at high pressures within the mantle. They are broadly 'basaltic' in composition and their genesis remains controversial. Early models suggested that they represent the metamorphic end product of (frozen) magma that never made it to shallow depths and instead crystallised at mantle pressures. However, most recent models favour an origin where basalts and gabbros generated at shallow pressures are subducted back into the mantle, converted into eclogite and that subduction is the mechanism by which eclogites become incorporated into the ancient subcontinental lithospheric mantle. The platinum-group elements (PGE) and Au are powerful tracers of mantle processes and their limited mobility during metamorphism and alteration allows them to be used as tools to see through later metamorphic events to the original formation mechanism. The present study presents new geochemical data, including the first complete PGE spectra, for a reconnaissance suite of nine eclogite xenoliths (comprising both group 1 and group 2 types) from the Orapa mine in Botswana. While the bulk chemical data presented here inevitably contain an element of contamination from the kimberlite, three out of the four group 2 eclogites show pronounced enrichment in Nb and Nb/Ta fractionation relative to average mantle that cannot be ascribed to interactions with the kimberlite. None of the group 1 eclogites show this feature. Both groups of eclogites show fractionated patterns, with enrichment in the palladium-group PGE (PPGE) over the iridium-group PGE (IPGE). This is similar to many basalts, and both groups overlap with one another on chondrite normalised plots. Some group 1 eclogites show evidence (hump-shaped patterns) for loss of Pd and Au that maybe related to metamorphism or interactions with the kimberlite. Total PGE (and particularly IPGE) concentrations of eclogites are very low compared with mantle peridoties from similar depths and it is clear that the IPGE enrichment observed in the PGE signatures of on-craton kimberlites probably comes predominantly from assimilated peridotite, even in cases such as the Roberts Victor kimberlite where eclogites dominate the xenolith suite.