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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Quantifying mixing and age variations of heterogeneities in models of mantle convection: Role of depth-dependent viscosity
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Quantifying mixing and age variations of heterogeneities in models of mantle convection: Role of depth-dependent viscosity

机译：定量地幔对流模型中非均质的混合和年龄变化：深度相关粘度的作用

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Using a two-dimensional finite element model of mantle convection containing over a million tracer particles, we examine the effects of depth-dependent viscosity on the rates and patterns of mixing. We simulate the processes of recycling crust at subduction zones and the homogenization of recycled material (by dispersion and by melting at mid-ocean ridges). Particles are continually introduced at downwellings and destroyed when they either are so thoroughly dispersed that it mould be impossible to measure their presence in the geochemical signature of mid-ocean ridges or oceanic islands, or when they are close to spreading centers, at which point melting would "reset" the geochemical clock. A large number of factors influence the flow pattern and thus the rate at which heterogeneities are dispersed by convection. We examine the effect of increasing the viscosity with depth, and determine how both the residence time of heterogeneities and the extent of lateral mixing and exchange between the upper and lon;er mantle vary with the viscosity profile of the mantle. We determine the particle distribution resulting from convection models with three viscosity profiles: uniform viscosity, a smooth increase of viscosity with depth, and an abrupt jump in viscosity between the upper and lower mantle. We characterize the resulting distribution of heterogeneities in space and time by examining the age distribution of particles and their locations relative to others introduced into the flow at separate downwellings. Mixing rates in the three models are calculated as a function of the number of particles removed from the flow through time. Tie found that an increase of viscosity at depth does not induce age stratification in which older particles stagnate in the lon;er mantle, and it does not produce an upper layer (the source pf mid-ocean ridge basalt) that is well-mixed compared to the deeper regions. However, pronounced lateral heterogeneity is evident in the distribution of particles of different ages and starting locations that is not apparent from the particle positions alone. [References: 65]
机译：使用包含超过一百万个示踪粒子的地幔对流的二维有限元模型，我们研究了深度相关粘度对混合速率和混合模式的影响。我们模拟了俯冲带的地壳再循环过程和再循环材料的均质化过程（通过分散和洋中脊融化）。粒子被连续地引入到井下，并在它们被彻底分散以致无法测量它们在大洋中脊或大洋岛的地球化学特征中的存在时，或者当它们靠近扩散中心而融化时被破坏。将“重置”地球化学时钟。大量因素影响流型，从而影响非均质性通过对流扩散的速率。我们研究了随着深度增加粘度的影响，并确定了非均质性的停留时间以及上地幔与地幔之间的横向混合和交换程度如何随地幔的粘度变化而变化。我们确定具有三种粘度曲线的对流模型所产生的颗粒分布：均匀的粘度，随着深度的增加粘度的平滑增加以及上下地幔之间的粘度突变。我们通过检查颗粒的年龄分布及其相对于在单独的下行井中引入流中的其他颗粒的位置来表征时空分布在时间和空间上的不均匀性。这三个模型中的混合速率是根据从流经时间中除去的颗粒数量计算的。 Tie发现，深处粘度的增加不会引起年龄分层，在该分层中，较老的颗粒停滞在lon er地幔中，并且不会产生上层（中洋脊玄武岩的来源）到更深的地区。但是，明显的横向异质性在不同年龄和起始位置的颗粒分布中很明显，而这仅从颗粒位置来看是不明显的。 [参考：65]

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