Characterizing the U-Pb systematics of baddeleyite through chemical abrasion: application of multi-step digestion methods to baddeleyite geochronology
Characterizing the U-Pb systematics of baddeleyite through chemical abrasion: application of multi-step digestion methods to baddeleyite geochronology
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DOI：
10.1007/s00410-010-0507-1
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发表时间：
2010-11-01
影响因子：
3.5
通讯作者：
Hanson, Richard
中科院分区：
文献类型：
--
作者：
Rioux, Matthew;Bowring, Samuel;Hanson, Richard
铀铅斜纹岩年代学已成为测定镁铁质岩石，特别是与大型火成岩省有关的岩脉的主要工具。然而，在许多情况下，斜锆石和锆石的后结晶铅损失和共生限制了结晶年龄的精度和/或准确性。我们介绍了多步骤消化实验的结果，旨在了解和减少这些影响。实验是在纳米比亚Gannakouriep岩墙群中具有锆石内部和过度生长的新元古代斜纹岩上进行的，以及在南非费拉博瓦碳酸盐岩中的一个大型古元古代斜纹岩上进行的。设计了Phalborwa斜纹岩的多步消解实验，以检验最近发展起来的化学磨蚀技术是否适用于斜纹岩。实验产生了复杂的结果--单独的消化步骤正反不协调，表明在多步消化中铀和铅是解偶的-并表明目前的多步化学磨损形式不是减少斜纹石中铅损失的有效方法。另一组针对Gannakouriep斜德莱石的实验集中于分离复合颗粒中的锆石和斜德莱石成分。对该样品的常规单步消化实验导致了一系列不一致的分析，其中显著的分散归因于内部和过度生长的锆石，并突显了从复合颗粒中获得准确和准确的年龄的难度。为了分离出这些颗粒中的斜晶石和锆石，开发了两步HCl-HF化学磨损工艺来处理这些颗粒。这项技术成功地选择性地溶解了斜斜晶石和锆石成分。受麻粒岩相变质作用影响的样品中可能会出现二次锆石的过渡和过度生长，而新的HCl-HF化学磨损方法为解决这些复合颗粒的火成岩和变质历史提供了一种新的方法。
U-Pb baddeleyite geochronology has become a major tool for dating mafic rocks, especially dikes associated with Large Igneous Provinces. However, in many cases, post-crystallization Pb-loss and intergrowth of baddeleyite and zircon limit the precision and/or accuracy of crystallization ages. We present results from multi-step digestion experiments designed to understand and reduce these effects. Experiments were carried out on Neoproterozoic baddeleyites with zircon inter- and over-growths from the Gannakouriep dike swarm, Namibia, and on fragments of a large Paleoproterozoic baddeleyite from the Phalaborwa carbonatite, South Africa. Multi-step digestion experiments on annealed Phalaborwa baddeleyite were designed to test whether the recently developed chemical abrasion technique for U-Pb zircon geochronology can be applied to baddeleyite. The experiments generated complex results-individual digestion steps were both normally and reversely discordant suggesting that U and Pb were decoupled in the multi-step digestions-and indicate that the current form of multi-step chemical abrasion is not an effective method for reducing the impact of Pb-loss in baddeleyite. A separate set of experiments on the Gannakouriep baddeleyite focused on isolating the zircon and baddeleyite components in composite grains. Conventional single-step digestion experiments for this sample resulted in a discordant suite of analyses with significant scatter attributed to inter- and over-grown zircon and highlight the difficulty of obtaining precise and accurate ages from composite grains. To isolate the baddeleyite and zircon in these grains, a two-step HCl-HF chemical abrasion procedure for annealed grains was developed. This technique was successful at selectively dissolving the baddeleyite and zircon components. Secondary zircon inter- and over-growths of baddeleyite can occur in samples affected by low-temperature alteration to granulite facies metamorphism, and the new HCl-HF chemical abrasion procedure provides a method for resolving both the igneous and metamorphic history of these composite grains.