Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops.
Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops.
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DOI：
10.1126/sciadv.1601318
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发表时间：
2016-09
期刊：
影响因子：
13.6
通讯作者：
Kylander-Clark AR
中科院分区：
文献类型：
--
作者：
Peterman EM;Reddy SM;Saxey DW;Snoeyenbos DR;Rickard WD;Fougerouse D;Kylander-Clark AR
原子探针产生地质意义的年龄从纳米级铅富集位错环不协调锆石。同位素不一致性是锆石普遍存在的导致错误定年的特征，其原因是放射成因Pb在结晶后地质演化过程中的活化和逃逸。在~10 μm的分析尺度上测得的同位素不一致程度在相邻的分析位置之间经常不同，表明Pb在较短的长度尺度上的不均匀分布。我们使用原子探针显微镜来确定这些位点的性质及其形成机制。我们发现，在一个约21亿年的不协调锆石，变质c。1.5亿年前由两个不同的铅库定义。尽管在峰值变质条件下的整体铅损失，原子探针数据表明，放射性铅的一个组成部分被困在10纳米的位错环，在退火过程中形成的辐射损伤与变质事件。第二个铅成分，发现以外的位错环，代表变质作用后的锆石基质中的放射成因铅的均匀积累。207 Pb/206 Pb的比例测量从8位错环内的不确定性是相等的，并产生一个年龄与原来的结晶年龄的锆石，由激光烧蚀点分析确定。我们的研究结果提供了一个特定的机制，捕获和保留的放射性成因的铅在变质作用，并确认同位素不一致的特点是由离散纳米级水库的铅，记录不同的同位素组成和产量的年龄数据与不同的地质事件。这些数据可能提供了一个框架，解释不一致的锆石离散放射性铅人口的非均匀分布，每个产生地质意义的年龄。
Atom probe yields geologically meaningful ages from nanoscale Pb-enriched dislocation loops in discordant zircon. Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The 207Pb/206Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages.
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影响因子：
4.5
作者：
Geisler, Thorsten;Schaltegger, Urs;Tomaschek, Frank
通讯作者：
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影响因子：
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作者：
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DOI：
10.1073/pnas.1415264112
发表时间：
2015-04-21
影响因子：
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作者：
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通讯作者：
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