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

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 176 Hf/ 177 Hf 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 176 Lu/ 177 Hf ratios relative to 176 Hf/ 177 Hf ratios. Our results show that for Lu‐Hf isotope studies of rocks of any significant age, the errors in calculated initial ɛ Hf values may exceed 10 ɛ Hf units 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.