Argon Transport in Chain and Sheet Silicates: A Combined Laser Microprobe and TEM

链状硅酸盐和片状硅酸盐是变质岩中的重要矿物，它们普遍存在于变质岩中，在同位素定年中具有重要作用。大多数放射性矿物年代测定法记录的是矿物冷却的时间，这些时间是通过它们对同位素扩散的阻塞温度，而这些时间因不同的同位素和不同的链状和片状硅酸盐而不同。地质学家利用这种可变性来描绘变质地体的“温度-时间”历史。然而，对于许多矿物，例如角闪石和云母中的氩，计算出的阻滞温度很难得到约束，除非假设一个反映这些硅酸盐晶格各向异性的扩散几何形状，以及一个反映其微观结构均匀性的扩散尺寸（或半径）。利用激光微探针测量天然和水热处理的角闪石、黑云母和白云母样品中氩的空间分布，以估计这些性质。这些估计将通过用透射电子显微镜（TEM）观察矿物的微观结构来进一步验证。控制加热实验的结果也将有助于解释40Ar/39Ar激光微探针获得的空间信息。

Chain and sheet silicates are important minerals in metamorphic rocks because of their common occurence and their usefulness in isotopic dating. Most radiometric mineral dates record the times which the minerals cooled through their blocking temperatures for isotopic diffusion and these differ for the various isotopes and the various chain and sheet silicates. Geologists exploit this variability to delineate the "temperature versus time" histories of metamorphic terranes. The calculated blocking temperatures for many minerals, however, such as for argon in hornblende and micas, is poorly constrained unless one assumes a diffusion geometry reflecting the anisotropy of these silicate lattices, and a diffusion dimension (or radius) reflecting their microtextural homogeneity. By measuring the argon spatial distribution in both natural and hydrothermally treated samples of hornblende, biotite, and muscovite with the laser microprobe, it is planned to estimate these properties. These estimates will be further tested by observing microtextures of the minerals with the transmission electron microscope (TEM). Results from the controlled heating experiments will also facilitate interpretation of the spatial information being derived from 40Ar/39Ar laser microprobes.