裂变径迹定年资料应用中的问题及其地质意义

The Issues of Application for the Fission Track Dating and Its Geological Significance

裂变径迹定年与其他大多数放射性定年法不同 ,它测量的是放射性衰变对矿物晶体的物理损伤 ,而不是另一种同位素。外探测器裂变径迹定年分析中 ,可以获得每一个颗粒径迹年龄。样品裂变径迹年龄表述有组合年龄、平均年龄和中值年龄 ,不论哪种年龄都不具有简单的地质意义。封闭径迹长度及分布记录了岩石经历的最高古地温及热历史过程 ,是裂变径迹分析中最重要的参数。利用裂变径迹参数 ,可进行热史模拟 ,以重建样品的热历史。

Unlike most methods of radiometric dating, fission track dating measures the effect, rather than the products, of radioactive decay events. External detector dating method can calculate individual grain ages. There are three assembled ages for single sample, which are pooled age, mean age and central age. It is very important that the fission track ages have no unambiguous geological significance. Tracks are randomly oriented in three-dimensional space. However, the measurements of track density used in age determinations are based on a two-dimensional sample of tracks intersecting an internal surface. With the increasing of the temperature, the fission tracks are progressively shortened from their ends,resulting in a reduction in the observed area density of the tracks and therefore a reduction in the measured fission track age. Because the fission track age is associated with the length of tracks, the length and distribution of tracks are the most important parameters in the fission track analysis. By modelling of thermal histories from apatite fission track data, the thermal histories of samples can be reconstructed.