A bulk annealing and dissolution-based zircon concentration method for mafic rocks
A bulk annealing and dissolution-based zircon concentration method for mafic rocks
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DOI：
10.1016/j.chemgeo.2022.120817
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发表时间：
2022-03
期刊：
影响因子：
3.9
通讯作者：
A. L. Oliveira;M. Schmitz;C. Wall;M. Hollanda
中科院分区：
文献类型：
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作者：
A. L. Oliveira;M. Schmitz;C. Wall;M. Hollanda
锆石地质年代学应用于各种地质问题，通过使用键铅衰变精确地确定岩石的年代。锆石在中长英质岩石中最为丰富且易于回收，包括双峰大型火成岩省的硅质喷发物，或镁铁质-超镁铁质杂岩中的分异花斑岩。然而，由于中粒至细粒基性岩的稀有性、粒度小、通常与铁磁矿物伴生和/或被顺磁模态矿物吸藏，通过常规密度和磁选方法浓缩锆石晶体对于中粒至细粒基性岩来说是低效和/或无效的。为了解决这些缺点，我们开发并测试了一种基于物理分离和化学溶解相结合的锆石富集方法。样品最初被研磨成沙子大小的颗粒和通过地下水位密度浓缩的重矿物质。富含重矿物质的馏分在 900°C 下加热 60 小时进行退火，然后进行一系列酸消解：王水、氢氟酸、王水，最后是盐酸。这种新方法可以浓缩几乎纯的锆石等分试样，因为锆石是少数能够幸免于这种大块岩石酸侵蚀的矿物之一。结果是使用数十克岩石样品有效地提取了分析上可行的量的锆石，用于使用键铅地质年代学，这意味着锆石浓缩的传统分离技术的回收率提高了一百倍。
Zircon geochronology is applied to a variety of geological problems to precisely and accurately date rocks via Usingle bondPb decay. Zircon is most abundant and easily recovered in intermediate to felsic rocks, including the silicic eruptives of bimodal large igneous provinces, or fractionated granophyres in mafic-ultramafic complexes. However, the concentration of zircon crystals by conventional density and magnetic separation methods is inefficient and/or ineffective for medium- to fine-grained mafic rocks, due to their rarity, small grain size, common association with ferromagnetic minerals, and/or occlusion by paramagnetic modal minerals. To address these shortcomings, we have developed and tested a zircon concentration method that is based on a combination of physical separation and chemical dissolution. The sample is initially ground to sand-size particles and heavy minerals concentrated via density on a water table. The heavy mineral-rich fraction is annealed by heating at 900 °C for 60 h, and then put through a series of acid digestions: aqua-regia, hydrofluoric acid, aqua-regia and finally hydrochloric acid. This new method allows the concentration of an almost pure zircon aliquot, since zircon is one of just a few minerals that can survive this bulk rock acid attack. The result is an efficient extraction of analytically viable amounts of zircon for Usingle bondPb geochronology using tens of grams of rock sample, representing an increase up to a hundred times the recovery rates of conventional separation techniques for zircon concentration.