The seismically fastest chemical heterogeneity in the Earth's deep upper mantle—implications from the single-crystal thermoelastic properties of jadeite
The seismically fastest chemical heterogeneity in the Earth's deep upper mantle—implications from the single-crystal thermoelastic properties of jadeite
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DOI：
10.1016/j.epsl.2020.116345
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发表时间：
2020-08
影响因子：
5.3
通讯作者：
M. Hao;Jin S. Zhang;C. Pierotti;Wencai Zhou;Dongzhou Zhang;P. Dera
中科院分区：
文献类型：
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作者：
M. Hao;Jin S. Zhang;C. Pierotti;Wencai Zhou;Dongzhou Zhang;P. Dera
硬玉是大陆成因俯冲沉积物/地壳中的主要矿物相（高达50 vol%），是地球内部不相容元素的主要非均质性和重要富集地球化学储层（如EM-1和EM-2）。识别和定位富集的地球化学非均质性需要了解相关矿物相在高温高压条件下的弹性性质。不幸的是，翡翠的单晶弹性特性从未在高压条件下测量过，部分原因是其晶体对称性低。在这项研究中，我们已经在实验上确定了硬玉的单晶弹性模量在高压下首次高达18 GPa，在环境温度条件下使用布里渊光谱。用三阶有限应变状态方程拟合得到KS 0′= 3.9（1），G 0′= 1.09（4），ρ 0= 3.302（5）g/cm 3，KS 0 = 138（3）GPa，G 0= 84（2）GPa.此外，我们还进行了高达25 GPa和700 K的同步辐射单晶X射线衍射实验。Holland-Powell型热压Birch-Murnaghan状态方程的拟合结果为KT 0′= 3.8（2）和α 0= 3.4（5）× 10− 5 K− 1。根据获得的翡翠热弹性参数，模拟了200 ~ 500 km深度范围内陆源沉积物/地壳的密度和地震速度。俯冲大陆沉积物/地壳的地震波速度在深度超过10300 km时变得非常快，比周围地幔的Vp和Vs分别快11.8%和14.7%，比俯冲洋壳的Vp和Vs分别快5.6%和7.3%。即使是少量的俯冲大陆沉积物/地壳的存在也会导致地球内部强烈的地震异常。
Jadeite is a major mineral phase (up to 50 vol%) in the subducted sediments/crust with continental origin, which are one of the major heterogeneities and important enriched geochemical reservoirs (such as EM-1 and EM-2) for incompatible elements in the Earth's interior. Identifying and locating the enriched geochemical heterogeneities requires knowledge of the elastic properties of relevant mineral phases at high pressure-temperature conditions. Unfortunately, the single-crystal elastic properties of jadeite have never been measured at high-pressure conditions, partially due to its low crystal symmetry. In this study, we have experimentally determined the single-crystal elastic moduli of jadeite at high pressures for the first time up to 18 GPa at the ambient temperature condition using Brillouin spectroscopy. Fitting the third-order finite strain equation of state to the velocity-pressure data yields K S 0′= 3.9 (1), G 0′= 1.09 (4) with ρ 0= 3.302 (5) g/cm 3, K S0= 138 (3) GPa, and G 0= 84 (2) GPa. In addition, we have also conducted synchrotron single-crystal X-ray diffraction experiments up to 25 GPa and 700 K. The fitting of a Holland-Powell type thermal-pressure Birch-Murnaghan equation of state yields K T 0′= 3.8 (2) and α 0= 3.4 (5)× 10− 5 K− 1. Based on the obtained thermoelastic parameters of jadeite, the density and seismic velocities of continent-derived sediments/crust are modeled at the depth range from 200 to 500 km. The seismic velocities of the subducted continental sediments/crust become extremely fast at depths greater than∼ 300 km, up to 11.8% and 14.7% faster than the Vp and Vs of the ambient mantle, and 5.6% and 7.3% faster than the Vp and Vs of the subducted oceanic crust. The existence of even a small amount of the subducted continental sediments/crust can result in strong seismic anomalies in the Earth's interior.