高压流体条件下叶蛇纹石摩擦特性及其对俯冲带慢滑移事件的启示

Frictional properties of antigorite under high pore-fluid pressure and implications for slow-slip events in subduction zones

为了探讨在俯冲带构造加载环境中叶蛇纹石矿物的摩擦力学性质,我们使用叶蛇纹石作为实验样品,开展了高压流体条件下的摩擦滑动实验研究.实验在气体介质的高温高压三轴实验系统中进行,实验中的有效正应力为30 MPa,孔隙流体压力为100 MPa,温度范围为100~500℃.实验过程中为了得到叶蛇纹石摩擦强度系数的速度依赖性,我们将轴向加载速率在1.0μm·s^(-1)、0.2μm·s^(-1)和0.04μm·s^(-1)之间进行切换.实验结果发现在实验温度范围内叶蛇纹石的摩擦强度系数随着温度的升高表现出系统的降低趋势,摩擦系数从100℃的0.81降低到500℃的0.41.在高于300℃的温度条件下,叶蛇纹石的剪切力学曲线表现出显著的位移弱化现象.在300℃的温度条件下,叶蛇纹石在0.2~1.0μm·s^(-1)的加载速率范围内表现出速度强化的摩擦行为,而在0.04μm·s^(-1)的加载速率下表现为速度弱化.在100~200℃以及400~500℃的温度条件下皆表现为速度强化.我们的实验研究表明在俯冲带地幔楔附近的高压流体条件和低有效正压力环境下,叶蛇纹石矿物可以促成慢滑移现象,但是其背后的物理机制可能与已有的认知(不稳定的摩擦滑动由叶蛇纹石脱水过程所控制)不同.这为我们探讨俯冲带慢滑移现象的物理过程和机制提供了新的思路.

In order to constrain the frictional properties of antigorite in the tectonic loading environment of subduction zones,we carried out triaxial friction experiments of antigorite gouge under the condition of high pore fluid pressure.The effective normal stress in the experiments was 30 MPa with 100 MPa pore-water pressure,and temperature ranged from 100℃to 500℃.To observe the velocity dependence of the frictional strength of antigorite,we used different axial loading rates of 1.0μm·s^(-1),0.2μm·s^(-1),and 0.04μm·s^(-1).Results show that frictional strength of antigorite exhibits a systematic change with temperature,with coefficient of friction decreasing from 0.81 at 100℃to 0.41 at 500℃.It is worth noting that at temperatures higher than 300℃,antigorite exhibits a significant slip weakening behavior.At temperature of 300℃,antigorite exhibits velocity strengthening at loading rates 0.2~1.0μm·s^(-1),but velocity weakening at loading rates 0.2~0.04μm·s^(-1).Meanwhile,only velocity strengthening behaviors are observed at 100~200℃and 400~500℃.Our experimental study implies that under the condition of high pore-fluid pressure and low effective normal stress near the subduction zone mantle wedge corner,antigorite can exhibit unstable frictional behavior and thus facilitate slow slips,although the underlying physical mechanism may differ from the notion that attributes the unstable slip events to the dehydration product of antigorite.Our results allow us to explore the physical process of the slow-slip phenomenon from a new perspective.