Ar和H在矿物中的扩散补偿关系及其对扩散系数的预测

对现有实验扩散数据的检查发现 ,Ar和H两种元素在不同矿物中分别存在着扩散补偿关系。孔隙度作为矿物内部离子堆积密度的一种量度 ,它与Ar和H的扩散活化能之间存在负的线性相关性 ,因而可以用于预测元素在矿物中的扩散系数。文中采用离子孔隙度法预测了Ar和H在不同矿物中的扩散系数 ,得到的结果与已有的实验数据在实验误差范围内是一致的。由于扩散补偿效应 ,同一物质在不同矿物中的扩散速率在特定温度下趋于汇聚 ,Ar扩散的转折温度为 (86 0± 10 0 )℃ ,H扩散的转折温度为 (330± 50 )℃。这为地质过程中Ar和H扩散的封闭温度提供了一种借鉴 ,因此伴随Ar扩散的地质时钟在低于 86 0℃时才开始启动 ,共存矿物间的氢同位素地质温度计仅当温度低于 330℃时才是有效的。

ARGON AND HYDROGEN DIFFUSION COMPENSATION AND ESTIMATION OF THEIR DIFFUSIVITY IN NATURAL MINERALS

Inspection of available diffusion data shows that there is diffusion compensation effect of Ar and H in different minerals.There exist negative correlations between the ionic porosity,which is a measure of atomic packing density in minerals,and Ar and H diffusion activation energies.thus ionic porosity can be used to predict element diffusion coefficients.This paper predicts argon and hydrogen diffusivity in minerals by ionic porosity,and the results are internally consistent with the previous experimental data.Because of the diffusion compensation,the diffusion rates of the same species in differnet minerals tend to converge at particular temperature,for instance,(860±100)℃ for argon diffusion and (330±50)℃ for hydrogen diffusion.This provides an insight into closure temperatures for argon and hydrogen diffusion in minerals in geological processes.It is expected that radiometric clocking involving argon diffusion in minerals would be started at temperatures below ca.860℃.Hydrogen isotope geothermometers for coexisting minerals can become valid only at temperatures below ca.330℃.