High pressure-temperature single-crystal elasticity of ringwoodite: Implications for detecting the 520 discontinuity and metastable ringwoodite at depths greater than 660 km
High pressure-temperature single-crystal elasticity of ringwoodite: Implications for detecting the 520 discontinuity and metastable ringwoodite at depths greater than 660 km
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DOI：
10.1016/j.epsl.2021.117359
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发表时间：
2022-02
影响因子：
5.3
通讯作者：
Wencai Zhou;Jin S. Zhang;Quancheng Huang;X. Lai;Bin Chen;P. Dera;B. Schmandt
中科院分区：
文献类型：
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作者：
Wencai Zhou;Jin S. Zhang;Quancheng Huang;X. Lai;Bin Chen;P. Dera;B. Schmandt
520 km 不连续性（520）和 660 km 不连续性（660）主要分别是由瓦兹利石到尖木石和尖木石到桥锰石 + 方镁石相变引起的。全球地震研究表明 520 存在显着的区域变化，这可能是由于地幔过渡带 (MTZ) 的化学和热不均匀性造成的。然而，化学成分和温度对 520 检测能力的影响尚不清楚。此外，目前尚不清楚冷且快速俯冲板片核心中可能存在的亚稳尖角橄榄岩是否能够在下地幔顶部附近产生可检测到的地震特征。由于缺乏同时高压-温度 (P-T) 条件下尖伍德石的单晶弹性测量，我们对这两个问题的理解受到阻碍。在本研究中，我们通过布里渊光谱测量了无水含铁尖锥石的单晶弹性，最高可达 32 GPa 和 700 K，然后对尖锥石的成分相关弹性特性进行建模，以计算成分对 520 处速度跳跃的影响。我们发现，与 Fe 的影响相反，水增强了 Vp（P 波速度）跳跃，但降低了 Vs（S 波速度）跳跃。 520 跨越瓦兹利石到尖伍德石的过渡。较高的温度会增加 520 范围内的 Vp 和 Vs 对比。在下地幔 660-700 km 深度之间，亚稳态尖角橄榄岩的存在可能只会导致约 1-2% 的低速异常，这在地震上很难解决。由于下地幔条件下尖角橄榄石中的Vs对压力的依赖性较弱，由亚稳尖角橄榄岩引起的低速异常在750 km深度处增加到5-7%，但它是否可以通过地震探测取决于俯冲板片中足够冷以容纳亚稳尖角橄榄岩的区域的延伸。
The 520 km discontinuity (the 520) and the 660 km discontinuity (the 660) are primarily caused by the wadsleyite to ringwoodite and ringwoodite to bridgmanite + ferropericlase phase transitions, respectively. Global seismic studies show significant regional variations of the 520, which are likely due to chemical and thermal heterogeneities in the Mantle Transition Zone (MTZ). However, the effects of chemical composition and temperature on the detectability of the 520 are unclear. Additionally, it remains unknown whether the possibly existing metastable ringwoodite in the core of the cold and fast subducting slabs could create a detectable seismic signature near the top of the lower mantle. Our understanding of both issues is hindered by the lack of single-crystal elasticity measurements of ringwoodite at simultaneous high pressure-temperature (P-T) conditions. In this study, we measured the single-crystal elasticity of an anhydrous Fe-bearing ringwoodite up to 32 GPa and 700 K by Brillouin spectroscopy, and then modeled the composition-dependent elastic properties of ringwoodite to calculate the compositional effects on the velocity jumps at the 520. We found that opposite to the effect of Fe, water enhances theVp (P-wave velocity) jump, yet decreases theVs (S-wave velocity) jump of the 520 across the wadsleyite to ringwoodite transition. Higher temperature increases bothVp andVs contrasts across the 520. At depths between 660-700 km in the lower mantle, the existence of metastable ringwoodite may only result in ∼1-2% low velocity anomaly, which is seismically difficult to resolve. The low velocity anomaly caused by metastable ringwoodite increases to 5-7% at 750 km depth due to the weak pressure dependence ofVs in ringwoodite at lower mantle conditions, but whether it is seismically detectable depends on the extension of the regions in subducted slabs that are sufficiently cold to host metastable ringwoodite.