Elasticity of Hydrated Al‐Bearing Stishovite and Post‐Stishovite: Implications for Understanding Regional Seismic VS Anomalies Along Subducting Slabs in the Lower Mantle
Elasticity of Hydrated Al‐Bearing Stishovite and Post‐Stishovite: Implications for Understanding Regional Seismic VS Anomalies Along Subducting Slabs in the Lower Mantle
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DOI：
10.1029/2021jb023170
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发表时间：
2022-04
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通讯作者：
Yanyao Zhang;S. Fu;S. Karato;T. Okuchi;S. Chariton;V. Prakapenka;Jung‐Fu Lin
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文献类型：
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作者：
Yanyao Zhang;S. Fu;S. Karato;T. Okuchi;S. Chariton;V. Prakapenka;Jung‐Fu Lin
地震研究发现，一些俯冲板块在700-1900公里深度处存在−2~−12%的地震散射体。俯冲的大洋中脊玄武岩(MORB)中的铁弹性后辉石转变与这些地震特征有关，但高压下铝、含氢辉石和后辉石的纵波速度、横波速度(Vp和Vs)以及全弹性模数(CIj)仍不确定。本文用拉曼光谱和X射线衍射相结合的方法，测定了两种水合铝方铅矿晶体Al_(1.3)-SiO_2(1.34m ol%Al和0.55m ol%H)和Al_(2.1)-SiO_2(2.10 m o l%Al和0.59 m o l%H)在∼70 Gpa以下的拉曼光谱和状态方程参数。实验数据用伪正则朗道理论进行了模拟，得到了在高压下辉石后转变的完整的CIJ和声速。铝和氢在辉石中的溶解使Al1.3-SiO_2和Al_(2.1)-SiO_2的转变压力分别降低到21.1 Gpa和16.1 Gpa，约为还原压力的29%。考虑到俯冲地幔中铝含量约为1.3m ol%，H含量约为0.6m ol%的辉石可能占到俯冲后地幔中20%的体积分数，因此在约1060 km处将发生克拉珀龙斜率为65K/Gpa的含Al、H后方辉转变，−为7(4)%VS异常。含铝、氢后辉石过渡带上的VS异常有助于解释包括汤加俯冲板块在内的顶中下地幔部分俯冲板块上地震观测到的深度相关VS异常。
Seismic studies have found seismic scatterers with −2 to −12% shear velocity anomalies along some subducting slabs at 700–1900 km depth. The ferroelastic post‐stishovite transition in subducted mid‐ocean ridge basalt (MORB) has been linked to these seismic features, but compressional and shear wave velocities (VP and VS) and full elastic moduli (Cij) of Al,H‐bearing stishovite and post‐stishovite at high pressure remain uncertain. Here we have determined Raman shifts of optic modes and equation of state parameters of two hydrated Al‐bearing stishovite crystals, Al1.3‐SiO2 (1.34 mol% Al and 0.55 mol% H) and Al2.1‐SiO2 (2.10 mol% Al and 0.59 mol% H), up to ∼70 GPa in diamond anvil cells coupled with Raman spectroscopy and X‐ray diffraction. The experimental data are modeled using a pseudoproper Landau theory to derive full Cij and sound velocities across the post‐stishovite transition at high pressure. The Al and H dissolution in stishovite significantly reduces the transition pressure to 21.1 GPa in Al1.3‐SiO2 and to 16.1 GPa in Al2.1‐SiO2, where the transition is manifested by approximately 29% VS reduction. Considering that stishovite with approximately 1.3 mol% Al and 0.6 mol% H could account for 20 vol% in subducted MORB at the top‐lower mantle, the Al,H‐bearing post‐stishovite transition with a Clapeyron slope of 65 K/GPa would occur at about 1060 km depth with −7(4)% VS anomaly. The VS anomalies across the Al,H‐bearing post‐stishovite transition can help explain the seismically‐observed depth‐dependent VS anomalies along some subducting slabs in the top‐ to mid‐lower‐mantle depths including the Tonga subducting slab.