“花岗岩浆晶出锆石U-Pb体系的封闭温度≥850℃”质疑——基于元素扩散理论、锆石U-Pb年龄与全岩Rb-Sr年龄对比的证据

摘要

根据实验条件及元素扩散作用理论分析,本文认为Lee等(1997)和Chemiak等(2000)进行花岗岩锆石中U和Pb扩散系数实验得出的“锆石U-Pb同位素体系封闭温度＞900C”结论只适用于解释源区岩石升温产生部分熔融形成花岗岩浆过程中存在残留锆石的情况,而不表明从花岗岩浆晶出锆石的U-Pb同位素体系封闭温度≥850℃.对花岗岩体的523对锆石U-Pb年龄(tz,)与全岩Rb-Sr等时线年龄(tRb)差值△t(△t=tzr-tRb)进行的频数统计分析表明:①△t既有正值又有负值,△t直方图呈正态分布(偏度系数CSK:-0.205;峰度系数CKU=5.093)；②花岗岩体△t值的分布不存在系统正(或负)偏差,而受偶然因素(实验误差)支配.这些锆石U-Pb年龄和全岩Rb-Sr等时线年龄进行的相关分析拟合出相关系数很高(R =0.997),回归系数接近1的线性方程(tzr=0.999775×tRb+0.06898Ma).这表明花岗岩锆石U-Pb定年结果与全岩Rb-Sr等时线定年结果在误差范围内一致,不存在花岗岩锆石U-Pb同位素年龄大于花岗岩全岩Rb-Sr等时线年龄的规律.%Based on the element diffusion theory and analysis to the experimental conditions, in this paper, it is concluded that the "closure temperature of U-Pb isotope system in zircon >900t" drawn from experiment of U-Pb diffusion in zircon by heating zircon crystals done by Lee et al. ( 1997) and Cherniak et al. ( 2000) may be only applied to understanding and explaining the behaviour of U-Pb system of inherited zircons within granite during the heating and partial melting process in source region of the granite, but not be indicated that the zircon crystallized from the granitic magma has the closure temperature of U-Pb isotope system as ≥850℃. The frequency analysis of 523 coupled age differences △t (△t = tZr -tRb) between the zircon U-Pb age (tZr) and whole rock Rb-Sr age ( tRb ) for granite plutons shows that ① the △t histogram displays a symmetrical normal distribution ( skewness CSK= -0. 205; kutrocess CKU =5.093) , the △t are both positive and negative; ②the distribution of △t not possess eitheir positive deviation or negative deviation and are controlled by accidental factors, i. e. experimental error. Using least squares regression procedure, a best regression equation (tZr =0.999775 ×tRb +0. 06898 Ma) with high correlation coefficient (R =0.997) and regression coefficient closed in 1 (a =0.999775) is fitted to the 523 pairs of zircon U-Pb age and whole rock Rb-Sr age of granite plutons. These statistical characteristics indicate that for granites as a whole both zircon U-Pb dating age and whole-rock Rb-Sr isochron age are consistent within error.