高温高压条件下绿帘石族矿物脱水的微观机制与过程

The dehydration of hydrous minerals in plate subduction zones has been believed as the key catalyst of continental margin magmatism, hydrothermal activity, earthquake and mineralization, it is one of the hottest topics in earth sciences society. Epidote group minerals can be stable under very wide pressure and temperature range; it is the constituent of many kinds of metamorphic rocks, igneous rocks, hydrothermal veins and sedimentary rocks, and the main hydrous mineral of altered crustal basalt in oceanic plate. The pressure and temperature fields for epidote group mineral phases in hydrothermal system stable and the structure transition of epidote group minerals phases under single factor system (constant temperature with pressure variation or constant pressure with temperature variation) have been documented in previous literatures, however, the structure changes, dehydration and phase transitions and the micro-mechanisms of epidote group minerals under subduction zone environment with pressure and temperature coupled increasing haven’t been clearly understood. The epidote group minerals are chosen as the object of this research to simulate their structure micro-change, phase transition and dehydration under coupled high pressure and high temperature condition similar with plate subduction zone environment via in-situ high pressure and high temperature experiment and analysis techniques. This project should give the micro-mechanisms of endothermic dehydration, solid volume shrinkage of epidote group mineral phases while fluid expansion in these processes under variation conditions. It should also to quantify the relationship between the structure variations of the epidote group minerals including products of its dehydration and the water content. Our purpose is to provide experimental evidences for discussing the function of the fluid released from subduction zone in the formation of water reservoir in mantle and mineralization, seismicity, and other Deep-Earth processes in continental margins and interior.

含水矿物在板块俯冲带的脱水作用是大陆边缘岩浆-流体-成矿-孕震等地质事件的关键触发因素，是地学界高度关注的研究热点。绿帘石族矿物具有很大温压稳定范围，是变质岩、岩浆岩、热液脉及沉积岩的组成成分，也是大洋板块玄武岩质壳层的主要蚀变含水矿物。前人对绿帘石族矿物在水热体系中稳定的温压条件及高温低压或高压低温单因素影响下绿帘石的结构态进行过研究，但类似俯冲带环境高温高压下双因素耦合对绿帘石族矿物结构变化、脱水与相转变过程的约束及其微观机制尚不清楚。本项目拟以天然绿帘石族矿物为对象，采用原位高温高压实验技术，模拟俯冲带环境下绿帘石族的结构微调、相变和脱水过程及其产物特征；研究绿帘石在不同温压条件下吸热脱水、固相体积收缩和流体膨胀的微观机制；研究其脱水产物的含水量与结构变化的定量关系；为探讨俯冲带所释放流体在地幔水储库形成和大陆边缘及内部破坏-成矿-孕震等深部过程中的作用提供新的高温高压实验依据。

采集了绿帘石单晶标本，黝帘石单晶标本以及合成黝帘石标本，并对这些样品进行了单晶结构分析，粉晶相分析。对初始样品进行了拉曼光谱振动模进行了普查，用于高温高压原位实验进行进一步分析。在高温高压原位拉曼光谱实验中观察到了绿帘石三段式脱水的过程：在高温高压下，选取了常温常压下信噪比比较好的八个晶格振动的频带，165cm-1,224cm-1,422cm-1,449cm-1,562cm-1,597cm-1,885cm-1,913cm-1,以及-OH频带作为对比对象进行了峰位峰高的监控。阶段一：在10.2GPa，温度为450℃条件下，绿帘石完成第一次脱水。在此阶段，位于3379cm-1的-OH强振动带，在高温高压下有微弱的红移，最终完全消失。同时，位于3452 cm-1的振动带强度有减弱，但没有发生消失。阶段二：在12.5GPa，温度825℃条件下，绿帘石发生第二次脱水。在这个阶段，位于3452 cm-1尖锐振动带大幅减弱。强度的缩减约为I/I0 = 1/5。该振动带也有轻微的红移。阶段三：在压力13.0GPa、温度850℃条件下，绿帘石发生第三次脱水，位于3452 cm-1的尖锐振动带在此阶段完全消失。在从阶段二到阶段三的升温升压过程中，常温常压下位于422cm-1, 449cm-1, 562cm-1, 597cm-1的晶格振动带发生了强烈的红移。根据不同含铁量绿帘石族矿物样品的变温实验结果的对比，得出了含铁量对羟基水的激活有较为明显的影响：含铁越多越容易在较低温度下激活（加热后即处在一定激发状态下）；含铁（0.1afu）较少的绿帘石族矿物容易在200℃条件下发生激活，而完全不含铁样品的变温实验中羟基振动峰在300℃条件下发生明显展宽向单肩峰转变。这个现象也在变温XRD研究中也有发现。绿帘石三段式脱水过程的发现，对认识该矿物在地球板块俯冲带环境携带水向地球深部循环，在不同深度的温压条件下使水分段卸载，为深部流体聚集成矿或蕴震提供流体条件，构建板块俯冲带绿帘石脱水模型具有重要意义。绿帘石族矿物含铁量增高导致其脱水温度下降，将这一现象所初步揭示出来的规律置入地球俯冲带环境，可预测该族矿物的铁含量对其携带结晶水从地表向深部俯冲的深度大小，从而细化板块俯冲带绿帘石族矿物脱水模型的构建。