On the Lu-Hf isotope geochemistry of silicate rocks

This paper reviews the history (TIMS, hot-SIMS, MC-ICP-MS), significance, g eochemical behaviour and current uncertainties (lambda Lu-176, Hf-Nd Bulk S ilicate Earth) surrounding the Lu-Hf isotope system, and thus marks two dec ades of its application to geochemical problems. An appendix further presen ts (a) improvements to the original chemistry protocol of Blichert-Toft et al. (1997) for application to Mg-rich samples and (b) a compilation of prev iously published and new Hf isotope determinations by MC-ICP-MS for a set o f international rock reference materials. Prior to the advent of multiple-col lector plasma source mass spectrometry (MC-ICP-MS), routine analysis of the Lu-Hf isotope system developed only sl owly because of the extreme difficulty of measuring Hf isotope compositions with thermal ionisation mass spectrometry, caused by the very high first i onisation potential of Hf. However, Hf isotope compositions can be measured relatively easily using MC-ICP-MS and this new technique now provides repr oducible measurements at high precision regardless of the matrix from which Hf is separated. Of the commonly used long-lived radiogenic isotope systems, only the Sm-Nd and Lu-Hf isotope systems are unaffected by parent/daughter fractionations related to volatile nebular processes and core formation. While other syste ms (Rb-Sr, U-Th-Pb, Re-Os) may also be used to investigate the chemical evo lution of the Earth, Moon, Mars and parent bodies of differentiated meteori tes, the larger uncertainties in their bulk chemical and isotopic values li mit their application to determine geochemical budgets and assess planetary mantle-crust evolution. In the study of garnet-bearing rocks, both for dat ing purposes and as an isotopic tracer for source provenance and mantle pro cesses, the Lu-Hf isotope system likewise is of major interest because of t he high partition coefficient of Lu compared to Hf for garnet with respect to other minerals. Furthermore, the larger Lu/Hf fractionation compared to Sm/Nd during melting beneath ridges produces proportionally higher Lu/Hf in the residue and faster in-growth of a radiogenic Hf isotopic signature (co mpared to Nd), which may help shed light on the dynamics of mantle melting. While the chemistry protocol and mass spectrometric technique for high-prec ision Lu-Hf isotope analysis have been resolved in satisfactory ways over t he post five years, more accurate determination of the decay constant for L u-176, at present known with a precision of only about 4%, still needs to b e completed and a consensus reached on which value to use for future Lu-Hf isotope studies. Although the current combined Lu-Hf and Sm-Nd Bulk Silicat e Earth parameters are plagued by possible incompatibilities in chondrite s election and potential interlaboratory biases, a more accurate set of value s may not be readily established owing to heterogeneities in the isotopic c omposition of chondrites that far exceed present analytical accuracy.