Automated mining of detrital Hadean zircons from Jack Hills, Western Australia: Flash geochronology with SHRIMP II
Automated mining of detrital Hadean zircons from Jack Hills, Western Australia: Flash geochronology with SHRIMP II
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西澳大利亚州杰克山碎屑玄武岩锆石的自动开采：使用 SHRIMP II 进行快速地质年代学
DOI：
10.1016/j.gca.2006.06.520
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发表时间：
2006
影响因子：
5
通讯作者：
T. Harrison
中科院分区：
文献类型：
--
作者：
P. Holden;T. Ireland;Z. Bruce;T. Harrison
挑战关于后吸积地球最早历史的既定范式需要能够测量地球化学数据的样本。古老的锆石颗粒>；4.2Ga，占碎屑总数的每毫升略低于2。任何有意义的古锆石数量，可以从其中准确地评估D142ND，因此是否已经发生了重大的地壳提取，需要调查数十万颗粒才能开采这些为数不多的珍贵文物。现有的非破坏性离子探测技术一次测量207pb/206pb年龄大约需要30秒，而没有关于目标一致性的信息。更糟糕的是，手动操作仪器将使运行成本膨胀到令人望而却步的水平。我们利用这个项目使ANU虾II多采集器上线，实现了自动化采集，并在不影响分析精度的情况下大幅缩短了搜索时间。我们的无人值守闪光年代学程序确定了约5 S的207Pb/206Pb年龄；如果没有获得规定的3.8Ga年龄，则定位并测量下一个样品。一旦发现“老”颗粒，就进行了206Pb/238U和207Pb/206Pb年龄的完全多收集器测定。与标准(AS3)一起，每天可以运行两个400谷物装载，其中约10%用于进一步分析。锆石排列成网格图案，这样一旦发现古老的颗粒，就可以很容易地提取出来进行进一步分析。到目前为止，我们已经调查了6万多谷物，预计到2007年底将超过15万谷物。在这些样品中，我们发现了年龄在3.8~4.0Ga之间的一致性颗粒800个，4.0~4.2Ga之间的一致性颗粒2100个，大于4.2Ga的一致性颗粒126个。在4.1Ga达到种群最大值之后，在年龄分布上没有看到差距，这表明是连续的增长或再循环，而不是地壳的间歇性发展。一个4.35Ga的小种群最大值可能记录了第一个幸存下来的地壳循环进入地幔。我们没有发现任何超过4.37Ga的颗粒。
Challenging the established paradigms concerning the earliest history of the post-accretional Earth requires samples upon which geochemical data can be measured. Ancient zircon grains> 4.2 Ga, form a little less than 2 per mille of the total detrital population. Any meaningful quantity of ancient zircons from which an accurate assessment of the D142Nd could be made, and hence whether major crustal extraction had already occurred, requires several hundred thousand grains to be surveyed in order to mine those few precious relics. Established non-destructive ion-probe techniques take around 30s for a single measurement of the 207Pb/206Pb age with no information as to the concordance of the target. Worse, manual operation of the instrument would balloon running costs to a prohibitive level. We have used this project to bring the ANU SHRIMP II multi-collector on line, to automate acquisition and to substantially decrease search time without any loss of analytical precision. Our unattended flash geochronology procedure determines a 207Pb/206Pb age in around 5 s; if the specified age of 3.8 Ga is not obtained the next sample is located and measured. Once an ‘‘old’’grain is found a full multi-collector determination of the 206Pb/238U and 207Pb/206Pb ages is undertaken. Together with standards (AS3), two 400 grain mounts can be run in a day with around 10% of these being targeted for further analysis. Zircons are arranged in a grid pattern such that old grains once identified, can be easily extracted for further analysis. So far we have surveyed in excess of 60,000 grains and are on target to exceed 150,000 grains by the end of 2007. Of these we have found 800 concordant grains with ages between 3.8 and 4.0 Ga, 2100 concordant grains between 4.0-4.2 Ga and 126 concordant grains older than 4.2 Ga. After a population maximum at 4.1 Ga, no gaps in the age distribution are seen, suggesting a continuum of growth or recycling rather than episodic crustal development. A small population maximum at 4.35 Ga may record the first crust to survive recycling into the mantle. We have not found any grains older than 4.37 Ga.