含水对富铁橄榄石流变性的影响

橄榄石是下地壳上地幔最丰富的造岩矿物,橄榄石集合体的高温高压流变性对地幔动力学过程有着很大的影响。以往对橄榄石单晶和集合体进行过的流变性实验研究,涉及了温度、压力、氧逸度、含铁量以及含水等因素的影响,本文研究了含水对富铁橄榄石集合体流变性能的影响,首先,利用高精度 Paterson 气体介质变形装置对含水富铁橄榄石集合体(Mg_(0.9),Fe_(0.1))_2SiO_4、(Mg_(0.7),Fe_(0.3))_2SiO_4和(Mg_(0.5),Fe_(0.5))_2SiO_4进行了高温三轴压缩蠕变试验,变形试验条件为:温度1323～1473K,围压300MPa,差应力10～300MPa,应变率10~(-7)～10~(-4)s~(-1),每个试件的最大变形量为20%。利用三维非线性拟合方法对蠕变试验数据进行分析,结果表明,含水富铁橄榄石集合体的微观变形机制为扩散域和位错域,对三种铁含量,扩散域的应力指数为1。位错蠕变域中,Fo_(50)应力指教为3.8,活化能为444kJ/mol;Fo_(70)应力指数为3.7,活化能为479kJ/mol,Fo_(90)应力指数为3.6,活化能为514kJ/mol。将实验结果与不含水富铁橄榄石集合体的强度进行对比,发现含水使富铁橄榄石的强度明显下降,舍铁量相同时,含水橄榄石集合体的流变强度比干燥时小至少一个数量级。从而得到了含水对含铁量不同的橄榄石集合体粘性(强度)影响的初步试验结果。

Effect of water on the rheology of iron rich olivine.

We have undertaken an experimental investigation of the effect of water on the viscosity of iron rich olivine aggregates. Tri-axial compressive creep experiments were performed on hydrous fine-grained olivine samples of Fo50, Fo70 and Fo90. Two drops of water were added for hot-pressing using a telescoping can to prevent the water from escaping during the hot-press. Samples were deformed in a gas-medium apparatus at a confining pressure of 300MPa, a temperature between 1323 and 1473K, a differential stress 10 -300MPa, a strain rate 10^-7 --10^-4s^-1 , and the maximum accumulative strain 20% for each sample. The samples were deformed both in the diffusion and the dislocation creep regime. From three dimensional global non-linear fitting of the test data, the stress exponent for wet iron rich olivine aggregates Fo50, Fo70 and Fo90were determined as 3.8, 3.7 and 3.6 in dislocation creep regime, respectively, and the activation energy for wet Fo50, Fo70 and Fo90 as 444kJ/mol, 479kJ/mol and 514kJ/mol, respectively. These hydrous samples creep at a rate over a factor of 10 faster than those anhydrous samples with the same iron content and under the same deformation conditions. Therefore, this paper gives a preliminary test result of effect of water on the viscosity of iron rich olivine aggregates.