Lu?Hf and geochemical systematics of recycled ancient oceanic crust: evidence from Roberts Victor eclogites

Eclogites from the Roberts Victor mine, Kaapvaal craton are classic examples of subducted Achaean oceanic crust brought up as xenoliths by kimberlite. New in situ trace element and oxygen isotope data (18O=3.09-6.99 SMOW) presented here reemphasise their origin from seawater-altered plagioclase-rich precursors. Their Hf-Nd isotopic compositions are not in agreement with compositions predicted by geochemical modelling of the isotopic composition of aged subducted oceanic crust. Instead, Hf isotopic compositions are very heterogeneous, varying between 0.281625 and 0.355077 (-37.8 and +2561 Hf) at the time of kimberlite emplacement (128 Ma) in keeping with equally variable Nd isotopic compositions (0.511124-0.545092; -26.3 to +636 Nd). However, most samples plot on the terrestrial array. The isotopic compositions of some samples are too extreme to play a major role in mixed peridotite-eclogite melting in basalt source regions, whereas the isotopic composition of other samples is reconcilable with a contribution of up to ca. 15% of eclogite partial melt to the MORB source. Most importantly, our results show that ancient subducted oceanic crust is not isotopically homogeneous and should not be treated as a component or reservoir during geochemical modelling. The heterogeneity reflects radiogenic in-growth starting from small compositional heterogeneities in gabbroic protoliths, followed by modification during sea-floor alteration, subduction and emplacement into the subcratonic lithosphere. [PUBLIATION ABSTRACT]