Olivine crystals align during diffusion creep of Earth's upper mantle

摘要

橄榄石在"位错蠕变"过程中产生的"晶体优先取向"被认为是地球上层地幔中弹性各向异性的主要成因，也经常被地震学家用来确定地幔流动的方向。现在,Tomonori Miyazaki等人发 现，与透辉石或熔岩共存的不含铁的橄榄石颗粒，能够在"扩散蠕变"过程中导致橄榄石的"晶体优先取向"的形成。他们发现，在接近"固相线温度"的温度下，从这种"扩散蠕变"会产生强径向各向异性。%The crystallographic preferred orientation (CPO) of olivine produced during dislocation creep is considered to be the fundamental question remains, however, as to whether the alignment of olivine crystals is uniquely produced by dislocation creep. Here we report the development of CPO in iron-free olivine (that is, forsterite) during diffusion creep; the intensity and pattern of CPO depend on temperature and the presence of melt, which control the appearance of crystallographic planes on grain boundaries. Grain boundary sliding on these crystallography-controlled boundaries accommodated by diffusion contributes to grain rotation, resulting in a CPO. We show that strong radial anisotropy is anticipated at temperatures corresponding to depths where melting initiates to depths where strongly anisotropic and low seismic velocities are detected. Conversely, weak anisotropy is anticipated at temperatures corresponding to depths where almost isotropic mantle is found. We propose diffusion creep to be the primary means of mantle flow.