THE EFFECTS OF CHEMICAL DENSITY DIFFERENCES ON CONVECTIVE MIXING IN THE EARTH'S MANTLE
THE EFFECTS OF CHEMICAL DENSITY DIFFERENCES ON CONVECTIVE MIXING IN THE EARTH'S MANTLE
复制标题
化学密度差异对地幔对流混合的影响
DOI：
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发表时间：
1986
期刊：
影响因子：
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通讯作者：
M. Gurnis
中科院分区：
文献类型：
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作者：
M. Gurnis
俯冲的岩石圈和地壳在化学上与周围的地幔不同，一些人假设由此产生的异常浮力可能会影响混合，特别是可能导致化学不均匀性持续存在。为了探索这种效应，进行了二维对流混合计算。模拟板块运动学特征的边界条件已被施加，负浮力示踪剂被放置在会聚板块边缘下方以模拟俯冲。感兴趣的主要参数是密度异常比：Δρ/(ρ_o α ΔT)，即化学物质与热浮力的比率。对于适合将洋壳与岩石圈其余部分完全分离的密度异常比，对于内部加热流，示踪剂有可能偏析到底部附近的温暖区域并中和那里的热浮力，并且对于底部加热流，示踪剂可能偏析到对流上升分支的底部。然而，对于更可能的情况是洋壳和岩石圈在俯冲后保持其分层，既不存在热浮力的分离也不中和。平均停留时间是示踪剂从俯冲到在山脊下方采样的平均时间，如果地壳和岩石圈保持分层，则最多可以比被动情况增加 4%。即使在地壳与岩石圈分离的更极端情况下，停留时间也仅比被动情况增加了 15%。
Subducted lithosphere and crust are chemically distinct from ambient mantle, and some have hypothesized that the resulting anomalous buoyancy may affect mixing and, in particular, may cause chemical heterogeneity to persist. To explore this effect, two-dimensional convective mixing calculations have been carried out. Boundary conditions mimicking features of plate kinematics have been imposed and negatively buoyant tracers placed beneath converging plate margins to simulate subduction. The principal parameter of interest is the density anomaly ratio: Δρ/(ρ_o α ΔT), the ratio of the chemical to the thermal buoyancy. For a density anomaly ratio appropriate to complete separation of the oceanic crust from the rest of the lithosphere, it is marginally possible for the tracers to segregate into the warm region near the bottom and neutralize the thermal buoyancy there, for internally heated flows, and to segregate to the base of the uprising limbs of convection, for bottom heated flows. However, for the more likely scenario of oceanic crust and lithosphere maintaining their layering after subduction, there is neither segregation nor neutralization of thermal buoyancy. The average residence time is the average time tracers remain in the box from subduction to sampling beneath ridges and could be increased by 4% from the passive case at the very most if the crust and lithosphere remain layered. Even in the more extreme case of separation of crust from lithosphere, the residence times are only increased by 15% from the passive case.