On the Lu-Hf isotope geochemistry of silicate rocks

This paper reviews the history (TIMS, hot-SIMS, MC-ICP-MS), significance, geochemical behaviour and current uncertainties (lambda Lu-176, Hf-Nd Bulk Silicate Earth) surrounding the Lu-Hf isotope system, and thus marks two decades of its application to geochemical problems. An appendix further presents (a) improvements to the original chemistry protocol of Blichert-Toft et al. (1997) for application to Mg-rich samples and (b) a compilation of previously published and new Hf isotope determinations by MC-ICP-MS for a set of 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 slowly because of the extreme difficulty of measuring Hf isotope compositions with thermal ionisation mass spectrometry, caused by the very high first ionisation potential of Hf. However, Hf isotope compositions can be measured relatively easily using MC-ICP-MS and this new technique now provides reproducible 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 systems (Rb-Sr, U-Th-Pb, Re-Os) may also be used to investigate the chemical evolution of the Earth, Moon, Mars and parent bodies of differentiated meteorites, the larger uncertainties in their bulk chemical and isotopic values limit their application to determine geochemical budgets and assess planetary mantle-crust evolution. In the study of garnet-bearing rocks, both for dating purposes and as an isotopic tracer for source provenance and mantle processes, the Lu-Hf isotope system likewise is of major interest because of the 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 (compared to Nd), which may help shed light on the dynamics of mantle melting. While the chemistry protocol and mass spectrometric technique for high-precision Lu-Hf isotope analysis have been resolved in satisfactory ways over the post five years, more accurate determination of the decay constant for Lu-176, at present known with a precision of only about 4%, still needs to be 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 Silicate Earth parameters are plagued by possible incompatibilities in chondrite selection and potential interlaboratory biases, a more accurate set of values may not be readily established owing to heterogeneities in the isotopic composition of chondrites that far exceed present analytical accuracy.