研究花岗岩类成因类型和幔壳成分比的同位素方法探讨

A DISCUSSION ON ISOTOPIC METHODS USED FOR THE STUDY OF GENETIC TYPES AND PERCENTAGES OF MANTLE SUBSTANCE OF GRANITOIDS

Sm-Nd法及其参数ε_(Nd)(O)、ε_(Nd)(T)、T_(CHUR)^(Nd)、ε_(DM)^(Nd)及x,是测定花岗岩形成年龄及研究花岗岩成因类型、模式年龄及地幔物质百分比的重要手段,但因其测试费用高等原因,难以广泛使用。作者提出了Rb-Sm法的相应参数ε_(Sr)(O)、ε_(Sr)(T)、T_(UR)^(Sr)、ε_(DM)^(Sr)及μ做为补充,在没有Sm-Nd同位素资料的情况下,Rb-Sr同位素参数基本上可以代替Sm-Nd同位素参数。经对比,求花岗岩中幔源物质百分比时,Rb-Sr法计算值与Sm-Nd法计算值相比,误差一般不超过10％。此外,作者还提出了计算花岗岩山幔源物质百分比的简化式,使该计算更为简便易行。

Abstract Sm- Nd method and its corresponding parameters such as and X are the significant means for the study of the generation age, genetic type, model age and percentage of mantle?crust substance of granitoids. The shortcomings of the Sm-Nd method are: (1) the higher test costs; (2) the number of the assumed factors for the calculation of the mantle substance percentage in the granitoids is much more and that some parameters are difficult to gain with sufficient accuracy: (3) the overlapping field of (143Nd/ and i(T) from various genetic types of granitoids is greater with 47% and 27% respectively. The authors, therefore, suggest the Rb-Sr method as an assistant one. When the data from Sm-Nd method is wanting, then those from the Rb?Sr method may be used as a substitute. By comparison, the errors of Rb?Sr method to Sm-Nd method are generally lower than 10% during the calculation of the mantle substance percentages in granitoids. The advantages of the Rb?Sr method are: (1) the test costs is lower; (2) the corresponding parameters may be gained easily and have good con'rast; (3) the overlapping field of ( 8 7Sr/8 6Sr), and sSt (71) from various genetic types of granitoids is lower, with 18% and 9 % respectively.The formulas calculating Rd-Sr isotope parameters are as follows:- sample determination value -the intrusive age1 of granitoid body, unit: Ma uantle substance percentage in granitoid Iir initial ratio of granitoid body Ic ?crustal Sr initial ratio during the formation of granitoid body Ic = 0.7138-0.000368 . T (used in I-type granite) Jc = 0.71846-0.000616 . T (used in S-type granite) The intrusive age of granitoid body, unit: 108 ageIm mantle Sr initial ratio during the formation of