Discriminating lower mantle composition
Discriminating lower mantle composition
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DOI：
10.1016/j.pepi.2020.106552
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发表时间：
2020-11-01
影响因子：
2.3
通讯作者：
Wentzcovitch, R. M.
中科院分区：
文献类型：
--
作者：
Houser, C.;Hernlund, J. W.;Wentzcovitch, R. M.
限制地球的整体成分对于理解地球的形成和演化至关重要。虽然下地幔占地球硅酸盐的大部分，但它也是最难进入的。随着较低地幔温度和压力下矿物弹性的实验和理论矿物物理约束的改善，假设的整体成分和一维地震模型的预测地震速度和密度剖面之间的比较已经变得普遍。然而，并不总是报告给定组合物比另一种组合物更好或更差的拟合程度，也没有讨论假设的温度分布和其他不确定性的影响。在这里，我们比较了使用先进的从头计算技术计算出的钙钛矿、硫铁矿和方辉石块状成分的地震速度和密度，以探讨相关的不确定性在多大程度上影响我们区分候选成分的能力。我们发现模型成分之间的预测差异通常小于温度不确定性的影响，因此这些比较缺乏区分力。无法区分成分主要是由于地震特性对温度的高度敏感性，伴随着地幔地温的不确定性，再加上地震速度对地幔底部成分的敏感性降低。一个重要的例外是 (Mg,Fe)O-铁方镁石中的自旋跃迁，预计它会产生压缩波速度的明显变化，从而区分相对铁镁质和富含二氧化硅的成分。然而，全球一维地震剖面中缺乏明显的自旋跃迁特征是地球物理学中一个尚未解决的重大问题，并且具有重要的地球化学意义。我们在此提出的用于建立此类比较的判别力的方法可应用于地震速度和相关不确定性的任何估计，并提供了评估模型比较稳健性的简单工具。
Constraining Earth's bulk composition is fundamental to understanding our planet's formation and evolution. While the lower mantle accounts for a majority of the bulk silicate Earth, it is also the least accessible. As experimental and theoretical mineral physics constraints on mineral elasticity at lower mantle temperatures and pressures have improved, comparisons between predicted seismic velocity and density profiles for hypothesized bulk compositions and 1D seismic models have become commonplace. However, the degree to which a given composition is a better or worse fit than another composition is not always reported, nor are the influences of the assumed temperature profile and other uncertainties discussed. Here we compare seismic velocities and densities for perovskitite, pyrolite, and harzburgite bulk compositions calculated using advanced ab initio techniques to explore the extent to which the associated uncertainties affect our ability to distinguish between candidate compositions. We find that predicted differences between model compositions are often smaller than the influence of temperature uncertainties and therefore these comparisons lack discriminatory power. The inability to distinguish between compositions is largely due to the high sensitivity of seismic properties to temperature accompanied by uncertainties in the mantle geotherm, coupled with diminished sensitivity of seismic velocity to composition toward the base of the mantle. An important exception is the spin transition in (Mg,Fe)O-ferro-periclase, which is predicted to give a distinct variation in compressional wave velocity that should distinguish between relatively ferro-magnesian and silica-rich compositions. However, the absence of an apparent spin transition signature in global 1D seismic profiles is a significant unresolved issue in geophysics, and it has important geochemical implications. The approach we present here for establishing discriminatory power for such comparisons can be applied to any estimate of seismic velocities and associated uncertainties, and offers a straightforward tool to evaluate the robustness of model comparisons.