石榴石中Li同位素体系及其地球化学意义

最新研究显示， Li在石榴石中具有非常低的扩散速率，石榴石中的Li环带在高温下仍得以保存，因此石榴石是一种探讨Li在各种高温地质过程中行为的非常有用的矿物。申请人最近对荆山花岗岩中的石榴石的Li及其同位素的初步研究表明，该矿物不仅含有高达50ppm以上的Li，而且Li的分布显示出系统的环带，其式样与主量元素和REE的环带存在相关性；微钻取样的分析数据表明其不同环带间存在系统的Li同位素分馏；这些初步结果显示石榴石对探讨高温下Li行为的重要性。本申请拟在此基础上进一步开展研究，以不同高温过程中的石榴石为研究对象，运用最新发展起来的Li同位素微区分析手段，探讨（1）石榴石中Li及其同位素体系的地球化学行为；（2）地壳岩浆过程中的Li同位素示踪；（3）石榴石中Li同位素信息能能够保存的最高温度；（4）流体对于石榴石中Li同位素体系的影响。相关成果可以为Li同位素示踪应用等基础问题提供限定。

The most recent studies suggest that, lithium (Li) has the lowest rates of diffusion in garnet relative to other rock-forming minerals, and that Li zoning in garnet is retained to comparatively high temperature and thus garnet could be a valuable mineral for monitoring the behavior of Li in high-T processes. The applicant have recently carried out preliminary investigations on a coarse-grained garnet crystal and found that, the mineral has Li content up to 50 ppm and shows systematical Li zoning patterns which are comparable with those of major elements and REE. Lithium isotope data from high precision microsampling of the zoned garnet revealed significant isotopic fractionation between Li zones. Taking garnet porphyroblasts from various high-T geological processes as the study objects and using the newly developed micro-domain analytical methods, this project aims to provide information for: (1) the geochemical behavior of Li and its isotopes in high-T garnets, (2) the behavior of Li in crustal magma process, (3) the temperature upper limit of Li zoning that could resist to intracrystalline diffusion and, (4) the influences of fluid for the Li systematics in garnet. The results are expected to provide constraints on the application of Li isotopes for monitoring high-T processes.