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RESEARCH PRODUCT

Eclogites and Garnet Pyroxenites: Problems Resolving Provenance Using Lu-Hf, Sm-Nd and Rb-Sr Isotope Systems

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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 (eHf1⁄4 ^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 (42·5 Ga) and kimberlite and/or basalt entrainment ( 0·2 Ga) masks our understanding of the exact protolith age of eclogites.