Seismic tomography has revealed the existence of large‐scale velocity heterogeneities in the mantle. The interpretation of seismic velocity anomalies in terms of temperature and chemical composition is nonunique. We use geodynamic observations including gravity, plate motions, dynamic topography, and excess ellipticity of the core‐mantle boundary (CMB) combined with seismic observations to investigate the thermo‐chemical structure of the mantle through joint inversions. An outstanding issue, however, is the physical connection between mantle density anomalies and the surface geodynamic observations, which requires knowledge of the mantle viscosity structure. Here we perform joint inversions assuming different viscosity profiles and examine the dependence of the results on the viscosity. We first assume mantle heterogeneity is due to thermal variations which places a constraint on the relation between seismic velocity and density, and we subsequently relax the constraint to allow for potential non‐thermal effects. In all of our joint inversions, a non‐thermal origin of density anomalies is required to explain the geodynamic data, though the amount varies with the assumed viscosity structure. A common observation is a high‐density chemical signal in the center of the Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle resulting in a near neutral or slightly dense overall buoyancy there. Using the derived density models and their corresponding viscosity profiles, we also calculate instantaneous mantle flow fields. The predicted flow fields derived from joint inversions are generally similar but are quite different from flow fields using density models derived from a‐posteriori scaling of pure seismic tomography models.

中文翻译：

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地震和地球动力学数据联合反演对地幔粘度的敏感性

地震层析成像揭示了地幔中存在大规模速度非均质性。关于温度和化学成分对地震速度异常的解释是唯一的。我们结合重力观测，板块运动，动态地形和岩心-地幔边界（CMB）的超椭圆率等地球动力学观测资料，结合地震观测资料，通过联合反演研究地幔的热化学结构。然而，一个突出的问题是地幔密度异常与地表地球动力学观测之间的物理联系，这需要了解地幔粘度结构。在这里，我们在假定不同粘度曲线的情况下进行联合反演，并检查结果对粘度的依赖性。我们首先假设地幔非均质性是由于热变化而造成的，该变化对地震速度和密度之间的关系施加了限制，然后我们放宽了该限制，以考虑潜在的非热效应。在我们所有的联合反演中，都需要密度异常的非热成因来解释地球动力学数据，尽管数量会随假定的粘度结构而变化。常见的观察结果是地幔底部大低剪切速度省（LLSVPs）中心出现高密度化学信号，导致那里的总体浮力接近中性或略密。使用导出的密度模型及其相应的粘度曲线，我们还可以计算瞬时地幔流场。纯地震层析成像模型的后验定标。