Constraining Thermo-Chemical Mantle Convection from Observations of Mantle Plumes and Upper Mantle Temperature

最近的地震观测表明，下地幔的底部与上地幔的成分不同，这表明地幔的分层模型可能与地球化学观测相一致。然而，对分层地幔每一层的组成和体积以及分层对地表观测的影响仍然知之甚少。本研究试图通过羽流热通量、羽流过热温度和上地幔温度的观测来约束地幔的热化学对流。建立了等化学对流（即全地幔对流）和热化学对流（即层状地幔对流）的三维区域球形模型，探讨了地幔柱热通量、地幔柱过热温度和上地幔温度的控制因素。假设这些观测值的控制参数是地幔柱起源于热边界层之上的层（即层状地幔的顶层）的内部加热速率，而包括瑞利数在内的其他参数起次要作用。这一假设得到了研究者初步建模计算的支持。在一个大的模型参数空间上进行模型计算，以检验这一假设。这些模式计算确定了在不同层深的等化学模式和热化学模式中再现所有这三个观测值所需的内部加热速率。本研究探讨了地幔顶层所需的内部加热速率对地幔长期冷却、放射性元素分布、地幔组成、地核热通量和冷却以及地幔分层等一系列地球物理和地球化学问题的影响。

Recent seismic observations indicate that the bottom part of the lower mantle is compositionally distinct from the overlying mantle, suggesting a layered mantle model that may be consistent with geochemical observations. However, the composition and volume of each layer of the layered mantle and consequences of the layering to surface observations remain poorly understood. This study seeks to constrain the thermochemical convection of the mantle by using observations of plume heat flux, plume excess temperature, and the upper mantle temperature. 3-D regional spherical models of isochemical (i.e., whole mantle convection) and thermochemical (i.e., layered mantle convection) convection are formulated to examine controls on plume heat flux, plume excess temperature, and the upper mantle temperature. It is hypothesized that the controlling parameter on these observables is internal heating rate for the layer (i.e., the top layer of a layered mantle) overlying the thermal boundary layer from which mantle plumes are derived, while other parameters including Rayleigh number play secondary roles. This hypothesis is supported by the investigator's preliminary modeling calculations. Model calculations over a large model parameter space are to be performed to test this hypothesis. These model calculations determine the internal heating rate that is required to reproduce all these three observations for isochemical models and thermochemical models with different layering depths. This study explores the implications of the required internal heating rate for the top layer for a range of geophysical and geochemical problems including mantle secular cooling, distribution of radioactive elements, mantle composition, core heat flux and cooling, and mantle layering.