Radiogenic Pb mobilization induced by shock metamorphism of zircons in the Apollo 72255 Civet Cat norite clast

• 全文链接
• 请求原文

作   者：

Zhang, Bidong;  Lin, Yangting;  Moser, Desmond E.;  Hao, Jialong;  Liu, Yu;  Zhang, Jianchao;  Barker, Ivan R.;  Li, Qiuli;  Shieh, Sean R.;  Bouvier, Audrey; 

作者机构：

Chinese Acad Sci;  Univ Western Ontario;  Inst Geol & Geophys;  London;  Peoples R China;  Canada;  ON N6A 5B7;  Key Lab Earth & Planetary Phys;  Dept Earth Sci;  Beijing 100029;  State Key Lab Lithospher Evolut; 

关键词：

Pb mobilization;  Apollo;  Shock metamorphism;  SIMS;  U-Pb;  Zircon; 

期刊名称：

Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society

i s s n：

0016-7037

年卷期：

2021 年 302 卷

页   码：

175-192

页   码：

摘   要：

In situ U-Pb radiometric dating of zircons is regarded as one of the most widely used and reliable methods to acquire geochronologic ages. However, it has been recently reported that radiogenic Pb (Pb*) mobilization within zircon may, in some cases, cause inaccurate age determinations with no geological significance. Such Pb* mobilization can be caused by deformation, a-coil damage, fluid-assisted annealing, and recrystallization. In this study, we report an investigation of Pb* mobilization in shock metamorphosed lunar zircons. NanoSIMS (nanoscale secondary ion mass spectrometry) and IMS 1280HR ion microprobe dating, EBSD (electron backscatter diffraction) and CL (cathodoluminescence) mapping, and scanning ion imaging (SII) were applied to micro-zircon grains from the Apollo 72255 Civet Cat norite clast. Based on the large number of grains with similarities in internal zoning, habit and trace element geochemistry, and host mineral context, the Civet Cat norite zircons are interpreted to be primary, igneous grains. The chronology obtained for three consecutive surfaces (at different depths) by NanoSIMS, SII, and IMS 1280HR, respectively, indicates that the radiogenic Pb distribution of the Civet Cat norite zircons is heterogeneous among different polished or sputtering surfaces. Forty-two NanoSIMS U-Pb ages (beam size of 5 mu m) are concordant on a Wetherill Concordia diagram, and their corresponding Pb-207/Pb-206 ages spread from 4015 Ma to 4459 Ma. More notably, the six oldest spots of the 42 define a concordant U-Pb age of 4460 +/- 31 Ma (2r, MSWD = 0.47, P = 0.92) and a weighted mean Pb-207/Pb-206 age of 4453 +/- 34 Ma (MSWD = 0.056, P = 0.998). These dates are among the oldest in the lunar highland rocks. However, the Pb-207/Pb-206 ages of repolished surfaces of these zircons by IMS 1280HR (beam size of 5 mu m) do not reproduce the NanoSIMS results (up to 300 Ma younger). The SII (spatial resolution of 2 mu m) confirms a heterogeneous distribution of radiogenic Pb within single grains. The EBSD mapping of these zircon grains shows that they have 3-20 degrees of cumulative lattice misorientation. It is proposed that shock-related deformation has facilitated Pb* migration after primordial crystallization. With currently available data, we cannot preclude the possibility that the large errors of the U-Pb ages obscure reverse discordance that would bias our oldest Pb-207/Pb-206 ages to older values. Conversely, our data could be explained by mixing of Pb-retention and Pb-loss nanodomains as seen in shocked terrestrial zircon such that U-Pb date of 4460 +/- 31 Ma approximates the norite formation. (C) 2021 Elsevier Ltd. All rights reserved.