亚稳态橄榄石对俯冲带负浮力的影响及其动力学意义

THE INFLUENCES OF METASTABLE OLIVINE ON THE NEGATIVE BUOYANCY OF SUBDUCTION ZONES AND THEIR GEODYNAMIC SIGNIFICANCE

利用有限元方法 ,计算了不同俯冲速度及热传导系数俯冲岩石层的负浮力 .在h <40 0km和h >740km的深度范围 ,低温高密俯冲带的负浮力随深度单调增加 .因为尖晶石相到后尖晶石相的相变有负的克兰帕龙斜率 ,俯冲带冷的物质在 660km间断面以下不到1 0 0km的深度范围内仍以低密度的低压物质相存在 .所以在该深度范围 ,俯冲带受到了周围地幔阻止其插入下地幔的浮力作用 .在 40 0km <h <660km深度范围 ,由于受橄榄石相变的影响 ,不同计算模型负浮力随深度的变化有明显的不同 .对于可能的相变动力学模型 ,亚稳态橄榄石的存在使负浮力随深度的增加值减小 .其作用是不利于俯冲带直接穿透 660km间断面 。

We calculated the negative buoyancy of the subducting lithosphere by means of 2 D FEM numerical simulations of the temperature fields. The values of the negative buoyancy increase with the depth for h <400km and h >740km. Oh the other hand, there is a decrease of negative buoyancy in depths of 660 to 740km, because there is phase transformation from spinel to perovskite, which prevent slab penetrating into the lower mantle. Metastable olivine changes the distribution of negative buoyancy between the depths of 400 to 660km. Specifically, it drastically alters the trend of increasing negative buoyancy with depth, and leads to a net decrease of the negative buoyancy below 400km depth. Metastable olivine hinders the slab's ability to penetrate through the 660km discontinuity and adjusts the down dip compressional stress.