天然麻粒岩高温流变实验研究

本实验在气体介质三轴高温流变仪上,采用怀安瓦窑口麻粒岩,在温度900～1200℃、围压300 MPa、应变速率10~(-4)～10~(-6)/s条件下,开展高温流变实验.实验样品麻粒岩由斜长石(52%)、单斜辉石和斜方辉石(40%)、石英(3%)、磁铁矿和钛铁矿(5%)组成,矿物平均粒度为:斜长石294μm、单斜辉石和斜方辉石282μm、石英97μm、磁铁矿和钛铁矿109μm.利用傅里叶变换红外光谱仪分析获得变形后样品的水含量约为0.17±0.05wt%.实验样品的强度随温度升高而降低,随应变速率降低而降低.基于力学数据,采用稳态流变方程,获得实验样品在900～1000℃时的应力指数为8.1～12.9,在1050～1150℃时的应力指数为4.8～5.8,平均值5.2.应力指数随着温度升高而降低.显微结构和成分分析表明,在900℃时麻粒岩出现矿物压扁与定向拉长特征,样品以位错滑移和微破裂变形为主;在950～1000℃时,麻粒岩样品中颗粒边界变得圆滑,表现出位错攀移特征,辉石和磁铁矿边缘出现微量熔体;在1050～1200℃时麻粒岩出现部分熔融,而且随着温度和实验时间(应变)增加,熔体含量增加,熔体结晶出微粒斜长石、辉石和橄榄石,部分辉石通过固体反应生成橄榄石.颗粒边界熔体和矿物反应促进了扩散作用,导致位错攀移和熔体引起的扩散蠕变共同控制了麻粒岩的高温流变.

Experimental study on the rheology of natural granulite at high temperature

Samples of natural granulite were deformed in a gas medium (Paterson) apparatus to evaluate the flow strength of the lower crust. We performed 40 creep tests with 11 samples at 300 MPa confining pressure, temperatures of 900~1200℃, and strain rates between 10^-6~10^-4/s. The samples were collected at Wayaokou village, located in Huai'an, Hebei province, China. Composition of the sample was about~52 vol% plagioclase,~40 vol% pyroxene,~3 vol% quartz,~5 vol% magnetite and ilmenite, with mean grain sizes of 294 μm, 282 μm, 97 μm and 109 μm for plagioclase, pyroxene, quartz, magnetite and ilmenite, separately. Water content of samples was~0.17±0.05wt% measured by a Fourier transform infrared spectrometer on the deformed samples.The sample strength decreased with increasing of temperature and decreasing of strain rate under experimental conditions. Based on creep data of samples, the stress exponent n was calculated, and the value of n is between 8.1~12.9 at 900~1000℃, and 4.8~5.8 with a mean value of 5.2 at 1050~1150℃. The stress exponents decrease with temperature increasing. Microstructural observations on thin sections parallel to the sample axis using optical microscopy and SEM show that deformed samples are different from the starting materials. At temperature of 900℃, grains are elongated and a shape preferred orientation developed perpendicular to the compression direction, which imply that the sample deformed as dislocation slip with intra-granular micro-cracks. At temperatures between 950~1000℃, grain boundaries of most minerals in deformed sample became round, showing dislocation climb. Trace melt films appeared mainly at some of grain boundaries of pyroxene and magnetite. At temperatures between 1050~1200℃, partial melting occurred at most grain boundaries of deformed samples, the content of melt increases with increasing of temperature and the duration of the experiment. The fine-grained new plagioclase, pyroxene and olivine crystallized from melt, and the solid phase reaction from pyroxene to olivine happened. So, dislocation climb is one of major deformation mechanism, but partial melt and mineral reaction induced diffusion process, and melt and reaction assisted diffusion and dislocation climb controlled the rheology of granulite at high temperature.