An Acid-Based Method for Highly Effective Baddeleyite Separation from Gram-Sized Mafic Rocks

Dating mafic igneous rocks (silica-undersaturated) is difficult for the lack of suitable minerals such as zircons (ZrSiO₄) commonly found in the sialic rocks such as granites. In this regard, baddeleyite (ZrO₂) has been long recognized as the most important mineral to serve as a geochronometer for dating silica-undersaturated igneous rocks. However, separating baddeleyite is difficult due to its small grain size, typical tabular morphology, and low abundance in samples. The standard water-based separation technique requires kilogram-sized samples and usually has a very low recovery rate. In this study, a new separation method based on the different solubilities of the minerals within HF + HCl + HNO₃ reagents was developed to achieve a high recovery of baddeleyite. With ∼19 g of diabase powder, the new method recovers 150-160 baddeleyite grains of 10-100 μm length and 4-50 μm width, an order of magnitude improvement over the water-based separation method, which typically recovers 11-12 similarly sized baddeleyite grains out of the ∼19 g sample. Subsequent secondary ion mass spectrometry U-Pb analyses demonstrate that the baddeleyite grains recovered by the new separation method keep the U-Pb system closed, indicating no Pb loss during acid treatment. Thus, this new method enables the most efficient baddeleyite recovery from gram-sized rocks and is anticipated to greatly contribute to the geochronological study of silica-unsaturated mafic rocks.