Metallic lead nanospheres discovered in ancient zircons
Metallic lead nanospheres discovered in ancient zircons
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DOI：
10.1073/pnas.1415264112
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发表时间：
2015-04-21
影响因子：
11.1
通讯作者：
Marquardt, Katharina
中科院分区：
文献类型：
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作者：
Kusiak, Monika A.;Dunkley, Daniel J.;Marquardt, Katharina
锆石（ZrSiO 4）是最常用的地质年代计，通过广泛的地质过程保存年龄和地球化学信息。然而，锆石U-Pb年代学可能会受到放射性铅的重新分配的影响，这是不相容的晶体结构。这种现象在经历过超高温变质作用的锆石中特别常见，离子成像显示亚微米区域的不均匀分布足以严重干扰年龄，在某些情况下，从较年轻的锆石中产生明显的Hadean（>4 Ga）年龄。记录这些Pb富集区域的成分和矿物学特征对于了解Pb在锆石中的再分布过程及其对年代学的影响至关重要。使用高分辨率扫描透射电子显微镜，我们表明，富铅域以前确定的锆石从东南极麻粒岩是5-30纳米的金属铅纳米球。它们是随机分布的锆石结晶度，它们与富钛和铝的二氧化硅熔体表明，它们代表超高温变质过程中产生的熔体包裹体。金属铅在自然界中极其罕见，以前没有报道与高级变质作用有关。这些金属纳米球在退火锆石内的形成有效地阻止了锆石中放射成因Pb的损失。放射成因铅的再分布和相分离都可能影响高空间分辨率方法获得的U-Pb年龄的精度和准确性。
Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti- and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U-Pb ages obtained by high spatial resolution methods.