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“花岗岩造岩矿物40Ar-39Ar体系的封闭温度≤500℃”质疑——基于角闪石-黑云母40Ar-39Ar年龄与锆石U-Pb年龄对比的证据及Dodson’s公式存在问题剖析

Comments on “The Closure Temperature of 40Ar-39Ar Isotopic System of the Rock-Forming Minerals for Granite ≤500 ℃”: Based on Evidence of Comparison Between the Hornblend-Biotite 40Ar-39Ar Ages and the Zircon U-Pb Ages of Granite Plutons and

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【作者】 章邦桐凌洪飞吴俊奇

【Author】 ZHANG Bang-tong;LING Hong-fei;WU Jun-qi;State Key Laboratory for Mineral Deposits Research,School of Earth Sciences,Nanjing University;

【机构】 内生金属矿床成矿机制研究国家重点实验室,南京大学地球科学与工程学院

【摘要】 通过国内外花岗岩体的412对角闪石-黑云母40Ar-39Ar年龄(tAr)与锆石U-Pb年龄(tZr)之间差值ΔtZr-Ar(tZr-tAr)进行的频数统计分析表明:ΔtZr-Ar呈对称正态分布(偏度系数CSK==-0.276,峰度系数CKU=16.52);ΔtZr-Ar既呈正值又有负值,其众数值为0.70Ma,均值为1.15Ma。采用最小二乘法计算,花岗岩体锆石U-Pb年龄与花岗岩角闪石-黑云母40Ar-39Ar年龄拟合出相关系数很高(R=0.996),回归系数接近l的线性回归方程(tAr=1.00453×tZr-1.932)。这些统计特征表明花岗岩角闪石-黑云母40Ar-39Ar定年测定结果与锆石U-Pb定年测定结果在允许的误差范围内是一致的,不存在花岗岩锆石U-Pb年龄>角闪石40Ar-39Ar年龄>黑云母40Ar-39Ar年龄的规律。结合Dodson’s矿物封闭温度计算公式中存在问题的剖析,本文得出结论,按Dodson’s公式计算得出的花岗岩矿物40Ar-39Ar封闭温度不能代表从花岗岩浆晶出造岩矿物40Ar-39Ar体系的封闭温度。

【Abstract】 The frequency statistics analysis was conducted for 412 pairs of difference values ΔtZr-Arbetween the hornblendbiotite40Ar-39 Ar ages(tAr) and zircon U-Pb ages(tZr) of granite plutons worldwide. The result indicated that the ΔtZr-Ar(tZr-tAr) displays a symmetrical normal distribution with a skewness coefficient of CSK=-0. 267 and a kurtosis coefficient of CKU= 16. 52 in the histogram,both positive and negative tZr-Avalues were observed and their mode and mean are 0. 70 Ma and 1. 15 Ma,respectively. Using the least square regression procedure,a best liner regression equation(tAr= 1. 00453 ×tZr-1. 932) was fitted for zircon U-Pb ages(tZr) and hornblend-biotite40Ar-39 Ar ages(tAr) of granite plutons with a correlation coefficient as high as R = 0. 996 and a regression coefficient closed to 1. These statistical analysis characteristics indicated that the hornblend-biotite40Ar-39 Ar dating results are consistent with zircon U-Pb dating results for granites within the permissible error,therefore the rule of hornblende40Ar-39 Ar dating ages>biotite40Ar-39 Ar dating ages is not statistically correct. It implicated that the40Ar-39 Ar closure temperature of hornblend-biotite is same as the U-Pb closure temperature of zircon. Based on these statistic analyses and the query for Dodson’s Formula,it is concluded that the closure temperature of40Ar-39 Ar isotope system estimated by the Dodson’s formula cannot represent the closure temperature of40Ar-39 Ar isotope system in the rock-forming minerals crystallized from the granite magma.

【基金】 中国核工业地质局十二五科技项目(2011-D03);内生金属矿床成矿机制研究国家重点实验室开放项目(2008-Ⅰ-04)
  • 【文献出处】 矿物岩石地球化学通报 ,Bulletin of Mineralogy,Petrology and Geochemistry , 编辑部邮箱 ,2015年01期
  • 【分类号】P597.3;P588.121
  • 【被引频次】2
  • 【下载频次】297
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