Single-crystal elasticity of (Al,Fe)-bearing bridgmanite and seismic shear wave radial anisotropy at the topmost lower mantle
Single-crystal elasticity of (Al,Fe)-bearing bridgmanite and seismic shear wave radial anisotropy at the topmost lower mantle
复制标题
DOI：
10.1016/j.epsl.2019.04.023
复制
发表时间：
2019-07
影响因子：
5.3
通讯作者：
S. Fu;Jing Yang;N. Tsujino;T. Okuchi;N. Purevjav;Jung‐Fu Lin
中科院分区：
文献类型：
--
作者：
S. Fu;Jing Yang;N. Tsujino;T. Okuchi;N. Purevjav;Jung‐Fu Lin
在25和35 GPa条件下，利用布里色光散射（BLS）、脉冲受激光散射（ISLS）和x射线衍射（XRD）测量结果，研究了化学成分为Mg 0.95 Fe 0.033 2+ Fe 0.027 3+ Al 0.04 Si 0.96 O 3 （Fe6-Al4-Bgm）和Mg 0.89 Fe 0.024 2+ Fe 0.096 3+ Al 0.11 Si 0.89 O 3 （Fe12-Al11-Bgm）的含（Al, Fe）桥菱石（Bgm）的单晶弹性。基于实验测量的纵波和横波速度（V P, V S）作为方位角的函数，我们可靠地推导出了Fe6-Al4-Bgm和Fe12-Al11-Bgm在两种实验压力下的完整弹性常数。结果表明，与实验压力下的Fe6-Al4-Bgm相比，Fe和Al复合取代使Fe12-Al11-Bgm的V S和V P分别降低了2.6（±0.5）%和1.5（±0.3）%。在两个实验压力下，我们观察到Fe和Al对含（Al, Fe） Bgm的V - S分裂各向异性有很强的影响：Fe6-Al4-Bgm沿[001]方向的V - S分裂各向异性最高，为~ 8.23-9.0%，而Fe12-Al11-Bgm的V - S分裂各向异性介于[100]和[001]方向之间，为~ 7.68-11.06%。结合Fe6-Al4-Bgm和Fe12-Al11-Bgm的单晶弹性数据和文献中相关下地幔压力-温度（P-T）条件下变形Bgm的晶体优选取向（CPO）结果，模拟了俯冲板附近变形（Al, Fe） Bgm在与最上层下地幔相关条件下的地震V - S径向各向异性。考虑到地球最上层下地幔(Al, Fe)- Bgm中随深度变化的Fe和Al含量，我们的模型结果表明，Fe6-Al4-Bgm和Fe12-Al11-Bgm晶体的变形将分别在~ 670和~ 920 km深度产生~ 0.9%和~ 0.8% V S径向各向异性。这些发现为在靠近俯冲板块的下地幔最顶部，特别是汤加-克马德克俯冲区，地震探测到的明显的V - S径向各向异性提供了矿物物理学解释。
In this study, we investigated the single-crystal elasticity of (Al, Fe)-bearing bridgmanite (Bgm) with chemical compositions of Mg 0.95 Fe 0.033 2+ Fe 0.027 3+ Al 0.04 Si 0.96 O 3 (Fe6-Al4-Bgm) and Mg 0.89 Fe 0.024 2+ Fe 0.096 3+ Al 0.11 Si 0.89 O 3 (Fe12-Al11-Bgm) using combined experimental results from Brillouin light scattering (BLS), impulsive stimulated light scattering (ISLS), and X-ray diffraction (XRD) measurements in diamond anvil cells at 25 and 35 GPa. Based on experimentally measured compressional and shear wave velocities (V P, V S) as a function of azimuthal angles within selected crystal platelets that are sensitive to derivation of nine elastic constants for each composition, we reliably derived the full elastic constants of Fe6-Al4-Bgm and Fe12-Al11-Bgm at the two experimental pressures. Our results show that the combined Fe and Al substitution results in a reduction of both V S and V P in Fe12-Al11-Bgm up to 2.6 (±0.5)% and 1.5 (±0.3)%, respectively, compared with those in Fe6-Al4-Bgm at the experimental pressures. In particular, we observed strong combined Fe and Al effects on V S splitting anisotropy of (Al, Fe)-bearing Bgm at the two experimental pressures: Fe6-Al4-Bgm exhibits the highest V S splitting anisotropy of∼ 8.23-9.0% along the [001] direction, while the direction shifts to the midway between [100] and [001] directions for Fe12-Al11-Bgm with V S splitting anisotropy of∼ 7.68-11.06%. Combining the single-crystal elasticity data of Fe6-Al4-Bgm and Fe12-Al11-Bgm with the crystallographic preferred orientation (CPO) results of deformed Bgm at relevant lower-mantle pressure-temperature (P-T) conditions from literature, we modeled the seismic V S radial anisotropy of deformed (Al, Fe)-bearing Bgm near a subducting slab at conditions relevant to the topmost lower mantle. Taking into account the Fe and Al contents in (Al, Fe)-bearing Bgm with depth in the Earth's topmost lower mantle, the results of our model show that the deformation of Fe6-Al4-Bgm and Fe12-Al11-Bgm crystals would produce∼ 0.9% and∼ 0.8% V S radial anisotropy at depths of∼ 670 and∼ 920 km, respectively. These findings provide mineral physics explanations to the distinct seismically-detected V S radial anisotropies at the topmost lower mantle near subducted slabs, especially in the Tonga-Kermadec subduction region.