石榴子石-黑云母温度计的实验标定

The Fe-Mg exchange between garnet and biotite is strongly temperature dependent and therefore is widely used as a geothermometer in metamorphic rocks. After the research of more than fifty years, a series of 36 calibrations have been proposed to describe the quantitative relationships between the temperatures and compositions of the equilibrium phases. Yet, the pressure, temperature and composition ranges investigated so far have been very limited: for example, the experimental pressure was limited to 0.207~0.6 GPa; the compositions of garnet and biotite were too simple to simulate natural garnet and biotite solid solutions. In order to improve the garnet-biotite geothermomemeter, the synthetic Fe-Mg-Ca-Mn garnets and Fe-Mg-Al(VI)-Ti biotites will be used as starting materials in our reversed phase equilibrium experiments. The experiments will be conducted at temperatures from 450 to 950 °C and pressures from 0.2 to 4.0 GPa by using the piston-cylinder apparatus. The results of this investigation will be important in understanding the effect of pressure and temperature on compositions and element partitions of garnet and biotite. Additionally, the nonideal mixing in Fe-Mg-Ca-Mn garnets and Fe-Mg-Al(VI)-Ti biotite will be evaluated and a garnet-biotite geothermometer involving the derived mixing properties is established and applied to experimental and natural assemblages.

石榴子石-黑云母温度计是变质泥质岩中最常用的矿物温度计。尽管该温度计经历了长达半个世纪的研究，并出现了至少36种版本，但是，已有实验研究的温度、压力、成分变化非常有限：比如，压力还仅限于0.207~0.6 GPa；石榴子石和黑云母成分简单，与天然矿物成分有明显差别。因此该温度计的改进空间还很大。本项目拟采用人工合成的Fe-Mg-Ca-Mn四元石榴子石固溶体、Fe-Mg-Al(VI)-Ti四元黑云母固溶体为实验材料，在可控的温度、压力、氧逸度条件下，开展逆转相平衡实验，进一步标定该温度计。设计的实验温度范围(450~950°C)、压力范围(0.2~4.0GPa)、矿物成分范围足够宽。由此，可深入定量探讨温度及压力对石榴子石和黑云母化学成分及其离子分配的效应，准确描述石榴子石和黑云母的活度，建立高质量的石榴子石-黑云母温度计，满足国内外地质学界的使用。

高温高压实验和热力学计算模拟这两大研究手段，在岩石矿物相平衡研究中起到重要的作用。本课题按照计划书执行了相关研究内容，重点围绕着三个关键科学问题进行：石榴子石和黑云母矿物的合成以及相平衡实验；固溶体矿物的合成及其溶解热力学特性的研究；高压富钾矿物的相转变边界及其热力学性质的研究。经过3年的工作取得了以下几个方面的进展和成果：.（1）通过高温高压试验合成K2ZrSi3O9-K2TiSi3O9固溶体，利用电子探针、XRD和拉曼光谱等测试分析，认为K2ZrSi3O9-K2TiSi3O9体系可形成连续固溶体，符合理想固溶体混合的拉乌尔定律；基于以上实验结果探讨了钾镁煌斑岩中两种矿物的共存机制。（2）基于密度泛函理论用于第一性原理计算了锰钡矿结构KAlSi3O8（富钾矿物在地幔中的主要赋存性质）的相转变，研究了温度和压力对矿物相的转变影响，获得了锰钡矿结构KAlSi3O8的两种矿物相Holl-I和Holl-II的热力学性质参数，填补了相关数据库的空白。（3）利用活塞-圆筒高温高压实验装置合成了Fe-Mg-Ca-Mn四元石榴子石固溶体和Fe-Mg-Al-Ti四元黑云母固溶体，并对石榴子石-黑云母之间Fe-Mg离子交换，进行了相平衡实验。