A terrestrial perspective on using ex situ shocked zircons to date lunar impacts
A terrestrial perspective on using ex situ shocked zircons to date lunar impacts
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DOI：
10.1130/g37059.1
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发表时间：
2015-11-01
期刊：
影响因子：
5.8
通讯作者：
Moser, Desmond
中科院分区：
文献类型：
--
作者：
Cavosie, Aaron J.;Erickson, Timmons M.;Moser, Desmond
变形的月球锆石U-Pb年龄从4333 Ma到1407 Ma，被解释为对月球的离散影响。然而，月球锆石的年龄重置原因并不明确；作为角砾岩中的异地颗粒，它们缺乏岩性背景，而且大多数不包含陆地锆石中发现的震荡的显微结构诊断。碎屑震荡锆石提供了一种类似于非原地月球颗粒的陆地模拟，既可以识别诊断震荡证据，也可以评估非原地锆石测年影响的可行性。对南非约2020 Ma的Vredefort撞击构造侵蚀的锆石的电子背散射衍射和灵敏的高分辨离子探针U-Pb分析表明，在以微孪晶、平面断裂和低角度边界为特征的微结构域中，没有发生完全的撞击年龄重置，其年龄为2890 Ma~2645 Ma。在粒状锆石中检测到的撞击年龄为1975 Ma+/-39 Ma，其中也包含冲击性微孪晶，这将新母细胞的形成与撞击联系在一起。然而，我们发现，颗粒状结构可以在区域变质过程中形成，因此并不是撞击环境所特有的。这些结果表明，用非原位冲击锆石测定撞击的年代需要识别诊断冲击证据以确定撞击来源，然后瞄准特定的年龄重置显微结构。由于认识到锆石在撞击和岩浆环境中都可以塑性变形，在缺乏诊断冲击变形的月球锆石中进行年龄重置可能记录的是岩浆过程，而不是离散的撞击。因此，识别记录地质年代学分析的完整年龄重置的冲击波微结构，对于建立准确的基于锆石的月球、地球或其他行星体的撞击年代学至关重要。
Deformed lunar zircons yielding U-Pb ages from 4333 Ma to 1407 Ma have been interpreted as dating discrete impacts on the Moon. However, the cause of age resetting in lunar zircons is equivocal; as ex situ grains in breccias, they lack lithologic context and most do not contain microstructures diagnostic of shock that are found in terrestrial zircons. Detrital shocked zircons provide a terrestrial analog to ex situ lunar grains, for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons. Electron backscatter diffraction and sensitive high-resolution ion microprobe U-Pb analysis of zircons eroded from the ca. 2020 Ma Vredefort impact structure (South Africa) show that complete impact-age resetting did not occur in microstructural domains characterized by microtwins, planar fractures, and low-angle boundaries, which record ages from 2890 Ma to 2645 Ma. An impact age of 1975 +/- 39 Ma was detected in neoblasts within a granular zircon that also contains shock microtwins, which link neoblast formation to the impact. However, we show that granular texture can form during regional metamorphism, and thus is not unique to impact environments. These results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance, and then targeting specific age-reset microstructures. With the recognition that zircon can deform plastically in both impact and magmatic environments, age-resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts. Identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon-based impact chronologies for the Moon, Earth, or other planetary bodies.