高温高压下橄榄石-顽火辉石二元相体系流变学性质的实验研究

Rheological behavior of upper-mantle rocks is of important significance for understanding the strength of asthenosphere,the mechanics of lithosphere and the dynamics process of tectonic evolution in upper mantle. In present project, we plan to investigate the rheological properties of olivine + enstatite aggregates, which are composed of two major upper-mantle minerals, under uniaxial compression at confining pressures of 300 MPa using the Paterson high pressure testing machine and of 3-7 GPa using D-DIA apparatus. On the basis of a detailed study of enstatite, we will mainly concern with the effect of enstatite content, water content and confining pressure on the rheological properties of olivine + enstatite aggregates. The temperature and strain rate of experiments are controlled in the range of 1200-1600K 10-4－10-6s-1 respectively. SEM, TEM and EBSD techniques are used to study the microstructural feature, CPO pattern and deformation mechanism of run products. Experimental data will be analyzed quantitatively to quantify the rheological behavior of this two-phase mineral assemblage in the upper mantle and hence try to establish its rheological constitutive equations. The results of this project will provide new experimental constrains on the rheological behavior of asthenosphere, lithosphere as well as dynamic processes occurring in upper mantle.

本项目拟采用先进的Paterson气体介质高温高压流变仪和D-DIA固体介质流变仪重点研究在单轴压缩条件下上地幔主要矿物组成橄榄石+顽火辉石二元相体系的流变学性质。在深入研究顽火辉石的基础上，探讨顽火辉石含量、水含量对橄榄石+顽火辉石体系的流变性质的影响及其压力效应-即活化体积。实验的围限压力设定为300MPa（Paterson装置） 和3-7GPa（D-DIA装置），温度范围为1200-1600K，应变速率为10-4-10-6s-1。通过SEM、TEM 和EBSD 等手段观察实验产物的显微构造特征、组构型式和变形机制。根据获得的实验数据定量地分析该二元相体系的流变学行为随压力、温度、辉石含量和水含量和的变化规律，建立流变方程。研究结果将会对我们认识软流圈和岩石圈地幔的流变学行为以及构造演化的动力学机制提供新的实验约束。

上地幔的流变学行为对于认识不同构造环境的地球动力学过程十分重要。因为上地幔岩石是由橄榄石、斜方辉石（顽火辉石）和单斜辉石等主要矿物组成多相矿物集合体，其流变学性质受这些组成矿物制约。然而，长期以来上地幔的流变强度被假定可以用橄榄石的流变强度来代表，缺乏充分的实验依据。针对这一问题，本项目采用Paterson气体介质高温高压流变仪和D-DIA固体介质流变仪研究了顽火辉石和橄榄石+顽火辉石二元相体系的流变学性质。取得的主要研究成果包括：1）量化了顽火辉石的流变学参数，在位错蠕变域（应力指数 n=3）顽火辉石在无水条件和水饱和条件下的活化能分别为560 kJ/mol和 590 kJ/mol, 在扩散蠕变域（应力指数n=1）顽火辉石的活化能E=240 kJ/mol、活化体积为~3.5×10-6 m3/mol、水逸度指数r=0.7±0.2; 2）水的存在会显著降低顽火辉石的有效粘度约两个数量级；3）二辉橄榄岩无论在无水条件和水饱和条件下都比橄榄石的粘度要低，也就是说橄榄石+顽火辉石二元相体系的粘度随着顽火辉石含量增加而降低；4）至少在岩石圈上地幔顶部顽火辉石的粘度低于橄榄石，此时如果顽火辉石含量较高形成弱相支撑则其流变强度主要受斜方辉石的强度制约。这一研究结果对于修正上地幔流变学结构以及认识岩石圈地幔的流变学行为及动力学过程提供了新的实验约束。