水热条件下含橄榄石大洋玄武岩的摩擦特性实验研究

Frictional properties of oceanic olivine-bearing basalt under hydrothermal conditions

玄武岩作为大洋地壳上的主要成分,研究其摩擦特性是更好地探究逆冲板块边界上大地震等力学行为机理的基础.为了研究少量橄榄石矿物是否会影响玄武岩的摩擦特性,本研究选用了国际大洋钻探计划(IODP)349航次获取的玄武岩样品,其中含有少量的橄榄石矿物.本次研究的实验温度范围为100~600℃,施加的有效正压力和孔隙水压分别为150 MPa和100 MPa.实验中玄武岩样品在300~400℃的温度条件下表现出不稳定的准静态震荡现象,并且在高温条件下(T>400℃)其剪切强度表现出显著的位移弱化现象,即摩擦系数随着剪切位移的进行而持续减小.在3 mm的剪切位移范围内,摩擦系数的变化范围为0.7~0.55.通过初步的微观剪切变形研究,我们推测橄榄石在摩擦实验过程中对一种弱矿物的生成可能起到了催化作用,但生成的弱矿物含量不足以显著地弱化断层泥的摩擦强度.同时微观构造分析也发现,随着温度的升高,断层泥的剪切构造由局部剪切向整体弥散性剪切变形转变,同时伴随着孔隙度的显著降低,因此我们认为剪切强度的位移弱化现象与流体参与下的颗粒间的压溶过程的逐步增强有关.

Frictional properties of basalt as the principal component of the oceanic crust provide a basis for a deeper understanding of the mechanical behaviors including large earthquakes on the boundary of the subducting plate. To investigate whether a trace amount of olivine mineral affects the frictional properties of basalt, an olivine-bearing basalt sample obtained from the International Ocean Discovery Program (IODP) voyage 349 drill were selected. In our shearing experiments, temperatures were applied in a range from 100 ℃ to 600 ℃, with effective normal pressure and pore water pressure of 150 MPa and 100 MPa, respectively. In this study, we found that the basalt sample presents unstable oscillation at temperature of 300~400 ℃. At higher temperature (T > 400 ℃), shear of basalt is characterized by significant slip weakening phenomenon of shear strength, which is manifested as a continuous decrease of frictional strength coefficient with the progress of shear displacement. With the shear displacement up to 3 mm, frictional coefficient can decrease from 0.7 to 0.55. Based on preliminary microscopic observations, we speculate that olivine catalyzes the retrograde reaction process to produce weak mineral, but the content of weak mineral seems not enough to significantly weaken the frictional strength of the gouge. Microstructural analysis also indicates that the shear structure of gouge layer changed from localized brittle shear to pervasive distributed shear deformation as temperature increased. This change was accompanied by a significant decrease in porosity, consequently, the slip weakening of shear strength may be related to the progressively enhanced intergranular pressure solution process with the presence of pore water.