熔体-地幔橄榄岩反应及其对华北克拉通岩石圈地幔演化制约的实验研究

It has been proved by many case studies that the lithospheric mantle of the North China Craton (NCC) has experienced an evolution from a refractory type to a fertile one. Currently the mechanism for this evolution is still strongly controversial. In this project, reaction experiments between several melts with different characters and chemical compositions, including Si-rich, tholeiitic, alkali basaltic, and carbonatitic melts, and a refractory peridotite and its component minerals, olivine and orthopyroxene, from the NCC lithosperic mantle will be carried out on a six-anvil apparatus in which a relatively large loaded capsule can be placed under high temperature and high pressure conditions. These experiments are aimed to study correlation between the starting melts and mineral assemblages and element geochemical characteristics of the newly formed mineral(s) in run products, and to obtain effective mineral and element geochemical signatures for identifying the starting melts involved in reactions with the peridotite. These signatures together with and comparison of the data from case studies will provide direct evidence for exploring the evolution mechanism of the NCC mantle from a refractory type to a fertile one.

大量实例研究证实，华北克拉通岩石圈地幔发生了由难熔型向饱满型的演化，但目前对该演化的机制尚存在激烈的争论。本项目拟在六面顶大腔体压机上开展不同性质和化学组成的熔体（包括富硅熔体、拉斑玄武质熔体、碱性玄武质熔体、碳酸岩熔体）与华北克拉通岩石圈难熔型地幔橄榄岩及其组成矿物橄榄石、斜方辉石反应的高温高压实验，研究初始熔体性质和组成与反应产物矿物组合及新生矿物元素地球化学特征的对应关系，获得有效判定参与地幔橄榄岩反应的初始熔体性质和组成的矿物学和元素地球化学指标。这些指标与已有实例研究资料的综合对比，将为揭示华北克拉通岩石圈地幔由难熔型向饱满型演化的机制提供直接证据。

大量实例研究证实，华北克拉通岩石圈地幔发生了由难熔型向饱满型的演化，但目前对该演化的机制尚存在激烈的争论。本项目利用活塞圆筒压机和六面顶大腔体压机，开展了温度、压力在1100-1450°C、1.0-4.5GPa范围内不同性质和化学组成的熔体（包括富硅的英云闪长质熔体，贫硅的拉斑玄武质、碱性玄武质和碳酸岩熔体）与华北克拉通岩石圈地幔橄榄岩、橄榄石、斜方辉石反应的高温高压实验研究。研究结果显示：(1) 富硅的英云闪长质熔体能够与地幔橄榄岩、橄榄石发生明显的反应，主要生成斜方辉石，无单斜辉石形成；(2) 贫硅的拉斑玄武质熔体、碱性玄武质熔体和碳酸岩熔体能够与地幔橄榄岩中的斜方辉石发生明显的反应，新生矿物相主要为单斜辉石，其化学组成与华北克拉通岩石圈地幔包体的单斜辉石组成基本一致。因此，本项目的实验结果为揭示华北克拉通岩石圈地幔由难熔的方辉橄榄岩型向饱满的二辉橄榄岩型演化的机制提供了重要的直接证据，即通过熔体-地幔岩石（矿物）反应导致的华北克拉通岩石圈地幔演化的熔体，主要是玄武质贫硅熔体、碳酸岩熔体，而不是富硅熔体。