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

• Published in: Journal of petrology 2010-01, Vol.51 (1-2), p.513-535
• Main Authors: Gonzaga, R. G., Menzies, M. A., Thirlwall, M. F., Jacob, D. E., Leroex, A.
• Format: Article
• Language: English
• Subjects: cratonic eclogite; garnet pyroxenite; Kaapvaal; Lu–Hf; Marine; Sm–Nd isotopes
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egp091

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 (εHf = –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 ± 42·3 Ma) differs from the Lu–Hf cpx–gt mineral isochron age (1953 ± 13 Ma) by 1 Ga and the Rb–Sr clinopyroxene model age (3·15 Ga) is 1 Ga 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.