Decoupling of the Lu-Hf and Sm-Nd isotope systems during the evolution of granulitic lower crust beneath Southern Africa

• Published in: Geology (Boulder) 2004-05, Vol.32 (5), p.405-408
• Main Authors: Schmitz, Mark D, Vervoort, Jeff D, Bowring, Samuel A, Patchett, P. Jonathan
• Format: Article
• Language: English
• Subjects: absolute age; Africa; anatexis; Botswana; continental crust; crust; dates; decoupling; garnet group; geochemical anomalies; geochemical indicators; geochemistry; Geology; granulites; hafnium; Hf-177/Hf-176; igneous and metamorphic rocks; inclusions; isotope fractionation; isotope ratios; Isotopes; lower crust; Lu/Hf; Magma; melts; metals; metamorphic rocks; mineral assemblages; Minerals; Nd-144/Nd-143; neodymium; nesosilicates; orthosilicates; oxides; Petrology; Precambrian; rare earths; rutile; silicates; Sm/Nd; South Africa; Southern Africa; stable isotopes; xenoliths; zircon; zircon group
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G20241.1

The Lu-Hf and Sm-Nd isotope systems in kimberlite-borne, Archean to Proterozoic lower-crustal granulite xenoliths from southern Africa display significant parent-daughter fractionations and corresponding isotopic anomalies resulting in the decoupling of these generally well correlated systems. Systematic compositional and mineralogical controls on decoupling have also been identified. Opposite senses of divergent evolution from the terrestrial Hf-Nd isotope array are demonstrated in mafic granulite samples versus felsic and metasedimentary granulite samples. The former have superchondritic Lu/Hf and 176Hf/177Hf ratios at subchondritic Sm/Nd and 143Nd/144Nd ratios, whereas the latter have subchondritic Lu/Hf and 176Hf/177Hf ratios at a wide range of subchondritic to superchondritic Sm/Nd and 143Nd/144Nd ratios. This decoupling is shown to be related to the mineralogical sensitivity of crustal protolith Lu/Hf ratios to fractionation upon anatexis in the presence not only of residual garnet and accessory zircon, but also the residual oxide minerals (rutile versus iron-titanium oxides). The isotope diversity observed in these deep-crustal melt residues supports suggestions that the Hf-Nd systematics of complementary crustal melts may be useful fingerprints for deciphering the mineral assemblage and geochemical character of granitoid magma sources, and that together these systematics can yield insights into the origin and differentiation of continental crust.