Small‐scale convection in the D″ layer
Small‐scale convection in the D″ layer
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D”层小规模对流
DOI：
10.1029/2000jb000063
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发表时间：
2002
影响因子：
--
通讯作者：
Louis Moresi
中科院分区：
文献类型：
--
作者：
V. Solomatov;Louis Moresi
[1]小尺度对流被认为是地幔D″层地震非均质性的可能解释。最近开发的具有实际粘度的对流标度定律允许对这一假设进行定量评估。在核幔边界的热边界层上，温度和粘度的巨大差异表明，在整个热边界层变得不稳定之前，小尺度对流始于热边界层底部并向上传播。由于对流层内的混合，对流边界很可能变成化学边界。这意味着D″不连续可以代表对流边界和化学边界，并且小尺度对流的存在或不存在可能是观测到的D″不连续的间歇性的原因。D″层的横向非均质性主要集中在对流边界附近，与地震资料一致。横向温度变化的长度尺度、核幔边界的地形和D″层的黏度与观测约束基本一致。在对流层底部形成的二次热边界层厚度与超低速区厚度相近。温度的横向变化幅度对地震速度的横向变化只能起到很小的作用。对流边界地形的变化和化学非均质性可能是更重要的因素。超低速带的大地震速度下降不能仅仅用热效应来解释，而必须由部分熔化等其他因素引起。
[1] Small-scale convection has been suggested as a possible explanation for seismic heterogeneities in the D″ layer of the Earth's mantle. Recently developed scaling laws for convection with realistic viscosities allow quantitative assessment of this hypothesis. Large temperature and viscosity contrasts across the thermal boundary layer at the core-mantle boundary suggest that small-scale convection starts at the bottom of the thermal boundary layer and propagates upward before the thermal boundary layer as a whole becomes unstable. The convection boundary is likely to become a chemical boundary as a result of mixing within the convective layer. This implies that the D″ discontinuity can represent both convective and chemical boundary and that the presence or absence of small-scale convection can be responsible for the observed intermittent nature of the D″ discontinuity. Most lateral heterogeneities in the D″ layer are concentrated near the convection boundary, consistent with seismic data. The length scale of lateral temperature variations, the topography of the core-mantle boundary, and the viscosity of the D″ layer are in agreement with observational constraints. The thickness of the secondary thermal boundary layer formed at the bottom of the convective layer is similar to the thickness of the ultralow-velocity zone. The magnitude of lateral variations in temperature can only marginally be responsible for lateral variations in seismic velocities. Variations in the topography of the convection boundary and chemical heterogeneities are likely to be more important factors. A large seismic velocity drop in the ultralow-velocity zone cannot be explained by thermal effects alone and must be caused by other factors such as partial melting.