An investigation of dissolution methods for Lu-Hf and: Sm-Nd isotope studies in zircon-and garnet-bearing whole-rock samples

Abstract

The distinct chemical behavior of Lu and Hf can limit complete sample dissolution and spike-sample equilibration. Here we investigate the effects that four dissolution methods (hotplate, low-temperature microwave, high-temperature microwave, and steel-jacketed Parr bomb) have on the Sm-Nd and Lu-Hf isotope compositions of rocks. Any of the dissolution methods produce high-quality Sm-Nd isotope data, and hotplate dissolution of young, volcanic rocks produces adequate Lu-Hf data. However, low-temperature microwave and hotplate dissolution methods produce significantly different Lu/Hf and 176Hf/ 177Hf ratios as compared to Parr-bomb dissolution data for old samples or those that contain garnet, rutile, or zircon. A characteristic feature of incomplete spike-sample equilibration and sample dissolution for garnet-bearing samples is anomalously high 17 Lu/17 Hf ratios relative to 176Hf/177Hf ratios. Our results show that for Lu-Hf isotope studies of rocks of any significant age, the errors in calculated initial εHf values may exceed 10 ε Hfunits if complete dissolution and spike-sample equilibration are not attained for garnet-or zircon-bearing rocks. High-temperature and pressure dissolution such as that which can be achieved through Parr-bomb dissolution is critical to ensure complete dissolution of refractory phases such as garnet or zircon. © 2008 by the American Geophysical Union.