矿物氧扩散与氧同位素退化交换作用

OXYGEN DIFFUSION IN MINERALS AND ITS BEARING ON RETROGRADE ISOTOPE EXCHANGE IN ROCKS

本文从理论上解析了同位素封闭体系内的矿物氧扩散性质。火成岩从高温冷却或变质岩从高峰变质温度冷却过程中所发生的扩散作用会导致矿物晶体内部及晶粒间的氧同位素重新分配，两种不同的矿物氧扩散一同位素交换模式（Ｇｉｌｅｔｔｉ模式和Ｅｉｌｅｒ模式）部可以用来模拟这种性质。实例研究进一步阐明了扩散对氧同位素组成的影响。

Effects of oxygen self-diffUsion in minerals on oxygen isotope composition have been studied theoretically.When igneous or metamorphic rocks slowly cool under closed system conditions, oxygen isotope redistribution can occur due to diffusion-controlled retrograde isotope exchange.According to experimentally measured data for oxygen self-diffusion in minerals and theoretically calculated oxygen isotope fractionation factors,the oxygen isotope compositions of some case examples' have been estimated for given cooling rates. During cooling of metamorphic and plutonic rocks, oxygen diffusion between minerals can be the principal mechanism to cause retrograde isotope exchange.There are two types of models to simulate the effects of oxygen diffusion:(1) Giletti model,a simple cooling rate model which predicts the effect of slow cooling in a closed-system on the oxygen isotope composition of single minerals;(2)Eiler model, a fast grain boundary model which predicts both the zonation profile of δ18O within mineral grains and the bulk δ18O value of each mineral in the rock. Both models have advantages and disadvantages with respect to reality. However, application of the models will help to interpret possible variations in oxygen isotope composition of minerals and to preclude effects of open-system exchange(e. g', water/rock interaction, magma desassing).Case examples are tested to show the applicability of the diffusion models to natural assemblages.These include the granulite-grade metamorphic rock from Einasleigh in Australia,the tonalite from San Jose in USA and the granite from Suzhou in East China.The theoretical prediction for δ18O agrees well with the experimentally measured Values.For the Suzhou granite,the low δ18O value is interpreted to indicate the interaction of its protolith with meteoric water at high temperatures.The water/rock interaction after granite emplacement can be excluded by the constraints from diffusion-controlled oxygen isotope exchange.