Mantle viscosity controls a variety of geodynamic processes such as glacial isostatic adjustment (GIA), but it is poorly constrained because it cannot be measured directly from geophysical measurements. Here we develop a method that calculates viscosity using empirical viscosity flow laws coupled with mantle parameters (temperature and water content) inferred from seismic and magnetotelluric (MT) observations. We find that combining geophysical constraints allows us to place significantly tighter bounds on viscosity estimates compared to using seismic or MT observations alone. In particular, electrical conductivity inferred from MT data can determine whether upper mantle minerals are hydrated, which is important for viscosity reduction. Additionally, we show that rock composition should be considered when estimating viscosity from geophysical data because composition directly affects seismic velocity and electrical conductivity. Therefore, unknown composition increases uncertainty in temperature and water content, and makes viscosity more uncertain. Furthermore, calculations that assume pure thermal control of seismic velocity may misinterpret compositional variations as temperature, producing erroneous interpretations of mantle temperature and viscosity. Stress and grain size also affect the viscosity and its associated uncertainty, particularly via their controls on deformation regime. Dislocation creep is associated with larger viscosity uncertainties than diffusion creep. Overall, mantle viscosity can be estimated best when both seismic and MT data are available and the mantle composition, grain size and stress can be estimated. Collecting additional MT data probably offers the greatest opportunity to improve geodynamic or GIA models that rely on viscosity estimates.

中文翻译：

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利用地震和大地电磁数据推断的温度和含水量约束上地幔粘度

地幔粘度控制着各种地球动力学过程，例如冰川等静压调整（GIA），但由于不能直接从地球物理测量中测量，因此受到的约束很差。在这里，我们开发了一种方法，该方法使用经验粘度流动定律以及从地震和大地电磁 (MT) 观测推断的地幔参数（温度和含水量）来计算粘度。我们发现，与单独使用地震或 MT 观测相比，结合地球物理约束允许我们对粘度估计设置更严格的界限。特别是，从 MT 数据推断的电导率可以确定上地幔矿物是否水合，这对于降低粘度很重要。此外，我们表明，在从地球物理数据估计粘度时应考虑岩石成分，因为成分直接影响地震速度和电导率。因此，未知成分增加了温度和含水量的不确定性，并使粘度更加不确定。此外，假设对地震速度进行纯热控制的计算可能会将成分变化误解为温度，从而产生对地幔温度和粘度的错误解释。应力和晶粒尺寸也会影响粘度及其相关的不确定性，特别是通过它们对变形状态的控制。与扩散蠕变相比，位错蠕变与更大的粘度不确定性相关。总体而言，当地震和 MT 数据都可用并且地幔组成时，可以最好地估计地幔粘度，可以估计晶粒尺寸和应力。收集额外的 MT 数据可能为改进依赖粘度估计的地球动力学或 GIA 模型提供了最大的机会。