Progress in techniques and methods for uncontaminated and highly efficient separation of zircon from geological samples

Zircon (ZrSiO4) is the most extensive used mineral for dating, and uncontaminated and efficient extracting zircons from geological samples is a prerequisite for obtaining representative and accurate information. The conventional zircon separation method, which is based on the high density and non-magnetism of zircon, depends on mechanical crushing and different hydrodynamic conditions. However, it is prone to be contaminated in mechanical crushing process and low efficiency in separation process. To solve these two problems, this study reviews and summarizes the previous uncontaminated grinding technology and the high-efficiency mineral separation methods, and combines the high voltage pulse crushing technology, high temperature annealing and acid-based separation method. A new zircon separation process was developed to achieve uncontaminated and a high yield of zircon: high voltage pulse fragmentation -> (annealing) -> HF + HNO3 dissolution -> HCl + H3BO3 dissolution. The different methods were compared by mafic samples with low zircon content. For samples bearing coarse-grained zircon (>= 50 mu m) this new method can recover more than 6 times of zircon as the conventional method. For samples bearing only fine-grained zircon (<50 mu m), this method can successfully recover zircon grains from tens of grams of the sample, while the conventional method cannot recover any zircon grains, even if using kilograms of the sample. The Raman measurements reveal that acid treatment for several hours does not affect the crystallinity of zircon with intermediate crystallinity, and annealing can improve the zircon crystallinity. The Secondary Ion Mass Spectrometry (SIMS) U-Pb analyses demonstrate that the zircon grains recovered by the acid-based and annealing-acid-based separation methods show undetectable Pb loss during acid treatment. Using the new separating process and combined with SIMS U-Pb dating, we report the ages of a flowerlike gabbro from Luoyang (similar to 1831 Ma) and a basalt from the middle part of Yarlung Zangbo River (similar to 164Ma) for the first time, demonstrating that the improved zircon separating process is practical and effective. The popularization of the new zircon separating process is expected to significantly expand the zircon application in the study of mafic rocks.