地质样品无污染高效分选锆石的技术和方法进展

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

锆石是应用最广泛的定年矿物,无污染地从地质样品中高效分选出锆石是获得代表地质样品准确信息的前提条件。传统锆石分选方法基于锆石的高密度和无磁性,主要通过机械破碎和不同水动力条件分选,存在破碎过程易污染和分选流程效率低的问题。针对这两个问题,本研究通过回顾和总结前人地质样品粉碎技术和高效矿物分选的方法,将高压脉冲电动力破碎技术、高温淬火和酸溶法结合,开发了一种能够实现锆石无污染、高回收的操作流程:高压脉冲电动力破碎→(淬火)→HF+HNO 3溶解→HCl+H 3BO 3溶解。本次研究以锆石含量较低的基性样品为对象进行了不同方法的对比,对于含有≥50μm锆石的地质样品,新方法的锆石回收率是传统方法的6倍以上;对于只含有<50μm锆石的地质样品,传统方法无法从几千克的样品中分选出锆石,而新方法可以从几十克样品中分选出几十颗锆石。拉曼光谱分析显示,酸溶数小时并不会显著影响中等结晶度锆石的结晶度,而高温淬火可以明显提高锆石的结晶度。离子探针分析表明,酸溶分选和淬火后酸溶分选的锆石,在测试精度范围内均没有明显Pb丢失。采用新分选方法,并结合离子探针U-Pb定年,本文首次报道了洛阳牡丹石和雅鲁藏布江中段玄武岩的年龄(分别为~1831Ma和~164Ma),以证明改进后的锆石分选方法实用有效。新锆石分选流程的推广将显著扩展锆石在基性岩研究中的应用空间。

Zircon(ZrSiO 4)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+HNO 3 dissolution→HCl+H 3BO 3 dissolution.The different methods were compared by mafic samples with low zircon content.For samples bearing coarse-grained zircon(≥50μm),this new method can recover more than 6 times of zircon as the conventional method.For samples bearing only fine-grained zircon(<50μm),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 flower-like gabbro from Luoyang(~1831Ma)and a basalt from the middle part of Yarlung Zangbo River(~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.