Eclogites and garnet pyroxenites: Problems resolving provenance using Lu-Hf, Sm-Nd and Rb-Sr isotope systems

Cratonic eclogites and garnet pyroxenites from the Kaapvaal craton have heterogeneous Hf-Nd-Sr-(O) isotope ratios that define a positive Hf-Nd isotope array and a negative Nd-Sr isotope array. Isotopic variability encompasses depleted (mid-ocean ridge basalt and ocean-island basalt) to enriched mantle compositions (Group I and II kimberlites) and overlaps with that of the Kaapvaal craton garnet peridotite xenoliths. Isotopic heterogeneity at Roberts Victor is less extreme than previously reported and ranges from eclogites with a highly depleted MORB-like signature to enriched eclogites similar to Group II and transitional kimberlites and Group II megacrysts (ε_Hf=-32·8). Much of this similarity may well be due to partial or complete resetting during entrainment. For the majority of eclogites and garnet pyroxenites the Lu-Hf system records 'older' mantle events than the Sm-Nd system, but neither necessarily records the protolith age. Both the Lu-Hf and the Sm-Nd systems are prone to being reset by entrainment in high-temperature kimberlite and/or basaltic magmas (e.g. Kaapvaal) and emplacement in orogenic belts (e.g. Beni Bousera). In the case of one eclogite from Roberts Victor the Sm-Nd cpx-gt mineral isochron age (963·1±42·3Ma) differs from the Lu-Hf cpx-gt mineral isochron age (1953±13Ma) by 1Ga and the Rb-Sr clinopyroxene model age (3·15 Ga) is 1Ga older than the Lu-Hf age and the reconstructed whole-rock isochron age. Ironically, it may be that, in this instance, the Rb-Sr system gives a better indication of protolith age than Sm-Nd or Lu-Hf. Overall variable resetting of isotope systems between protolith formation in the Archaean (>2·5 Ga) and kimberlite and/or basalt entrainment (≤0·2 Ga) masks our understanding of the exact protolith age of eclogites.