Screening criteria for reliable U-Pb geochronology and oxygen isotope analysis in uranium-rich zircons: A case study from the Suzhou A-type granites, SE China
Screening criteria for reliable U-Pb geochronology and oxygen isotope analysis in uranium-rich zircons: A case study from the Suzhou A-type granites, SE China
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富铀锆石中可靠 U-Pb 年代学和氧同位素分析的筛选标准：以中国东南部苏州 A 型花岗岩为例
DOI：
10.1016/j.lithos.2014.02.002
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发表时间：
2014-04-01
期刊：
影响因子：
3.5
通讯作者：
Wang, Ya-Fei
中科院分区：
文献类型：
--
作者：
Gao, Yu-Ya;Li, Xian-Hua;Wang, Ya-Fei
锆石在地质年代学中有着广泛的应用，在火成过程中也是一种同位素示踪剂。然而，使用从锆石获得的同位素数据需要评估可能损害其同位素系统稳健性的过程。本文研究了不同铀含量和变质程度对苏州(中国南部)富铀A型花岗岩中锆石-石英对的SIMS、U-Pb和氧同位素特征的影响。SIMS结果表明，低铀锆石的U-Pb年龄为126.1 Ma+/-0.5 Ma，均一地幔8180值为5.34+/-0.46%。(2SD)。相反，高U含量的锆石与U-Pb年龄呈正相关(123~157 Ma)，与8180值呈负相关(范围为5.3~3.7%)。这些观测与共生石英的拉曼光谱和氧同位素分析相结合，揭示了异常高的U-Pb值和低的6180值反映了对锆石的辐射损害程度(用每原子位移的辐射剂量D-dpa来量化)。较老的SIMS U-Pb值是变质岩化引起的基质效应的直接结果，而较低的8180值是与含OH流体相互作用的结果，在辐射损害区有利于其渗透和扩散。共存的石英比高U锆石更好地保存了岩浆的原始氧同位素特征，从而对锆石的氧同位素值提供了限制。这项研究表明，在对高铀含量的锆石进行测年或氧同位素分析之前/之后，评估辐射损伤的程度是必要的。结果表明，对于苏州岩体，D-dpa<；0.03是用原生氧同位素比值识别锆石的稳健判别阈值，D-dpa<；0.08是筛选可靠U-Pb定年的稳健判别阈值。这些不同的屏蔽值反映了氧同位素组成受锆石晶格的晶后扰动程度比U-Pb系统低的证据。(C)2014爱思唯尔B.V.保留所有权利。
Zircon is widely used in geochronology and as an isotopic tracer in igneous processes. However, the use of isotopic data obtained from zircon requires evaluating of processes that might compromise the robustness of its isotopic systems. In this study we have assessed how different U contents and degrees of metamictization have affected the SIMS U-Pb and oxygen-isotope signatures of zircon-quartz pairs from the U-rich A-type granites from Suzhou (southern China). Our SIMS results show that zircons with low U concentrations define a U-Pb age of 126.1 +/- 0.5 Ma and homogeneous mantle 8180 values of 5.34 +/- 0.46%. (2SD). In contrast, zircons with high U contents show a positive correlation between U concentration and U-Pb ages (123-157 Ma) and a negative correlation between U concentration and 8180 values (range from 5.3 to 3.7%.). These observations are integrated with Raman spectra and oxygen-isotope analyses of coexisting quartz to reveal that the anomalously high U-Pb ages and low 6180 values reflect the degree of radiation damage to the zircons (quantified by radiation dose in displacements-per-atom, D-dpa). The older SIMS U-Pb ages are a direct result of the matrix effect caused by metamictization, whereas the low 8180 values result from interaction with OH-bearing fluids, the infiltration and diffusion of which was facilitated in the radiation-damaged areas. Coexisting quartz preserves primary O-isotope signatures of the magma better than high-U zircon, and thus provides a constraint for zircon oxygen-isotope values. This study demonstrates that it is essential to evaluate the degree of radiation damage before/after carrying out dating or oxygen-isotope analysis on zircons with high U contents. The results indicate that for the Suzhou pluton, D-dpa < 0.03 is a robust discriminant threshold to identify zircons with primary oxygen-isotope ratios, and D-dpa < 0.08 is a robust discriminant to screen for reliable U-Pb dating. These different screening values reflect the evidence that oxygen-isotope compositions can be affected at a lower level of post-crystallization disturbance of the zircon lattice than the U-Pb system. (C) 2014 Elsevier B.V. All rights reserved.