Kusiak Monika A, Dunkley Daniel J, Wirth Richard, Whitehouse Martin J, Wilde Simon A, Marquardt Katharina
Institute of Geological Sciences, Polish Academy of Science (ING PAN), 00-818 Warsaw, Poland; Department of Geosciences, Swedish Museum of Natural History, SE104 05 Stockholm, Sweden;
Department of Applied Geology, Curtin University, Perth, WA 6845, Australia;
Proc Natl Acad Sci U S A. 2015 Apr 21;112(16):4958-63. doi: 10.1073/pnas.1415264112. Epub 2015 Apr 6.
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.
锆石（ZrSiO₄）是最常用的地质年代计，它能在广泛的地质过程中保存年龄和地球化学信息。然而，锆石U-Pb地质年代学可能会受到放射性铅再分布的影响，而放射性铅在晶体结构中是不相容的。这种现象在经历过超高温变质作用的锆石中尤为常见，在这种锆石中，离子成像显示亚微米级别的区域分布非常不均匀，足以严重干扰年龄测定，在某些情况下，较年轻的锆石会得出明显的冥古宙（>40亿年）年龄。记录这些富含铅区域的成分和矿物学对于理解锆石中铅的再分布过程及其对地质年代学的影响至关重要。利用高分辨率扫描透射电子显微镜，我们发现先前在东南极麻粒岩锆石中鉴定出的富含铅区域是直径为5-30纳米的金属铅纳米球。它们相对于锆石的结晶度随机分布，并且它们与富含钛和铝的硅熔体的关联表明它们代表超高温变质作用期间产生的熔体包裹体。金属铅在自然界极为罕见，此前尚未有与高级变质作用相关的报道。在退火锆石中形成这些金属纳米球有效地阻止了锆石中放射性铅的损失。以这种方式发生的放射性铅的再分布和相分离都会损害通过高空间分辨率方法获得的U-Pb年龄的精度和准确性。
