Mineralogical and Dynamic Origin of the South African Superplume

来自地震层析成像的全球模型显示，从核-地幔边界延伸到约1000公里以上的南部非洲地下具有广泛的低剪切波速度(VS)特征。这一特征还表现为相对于一维参考剖面，体积声速v_Φ与体积声速v_s呈反相关关系，具有较高的体积和较低的剪切模量。利用旅行时间异常进行的详细研究进一步证明，这一特征具有非常尖锐的、接近垂直边界的特征。尽管在过去的几年里，地震学对这一南非超烟柱的制约一直很强烈，但这种异常的物理或化学来源还没有得到充分的调查。例如，到目前为止，还不清楚这是地幔中的动态特征还是停滞特征。人们试图将其起源归因于热上涌，并因停滞不前的覆盖非洲板块的隔热作用而稳定下来，而非洲板块因富含铁和/或俯冲物质而保持中性浮力。然而，用于这类动力学研究的矿物学模型是在特别的基础上编制的，竞争效应没有以系统的方式进行研究。在这里，我们建议使用嵌入三维对流模型的地幔矿物学热力学模型来测试竞争的热和化学对地幔下部广泛的上升流特征浮力的影响。我们将估计，在什么成分、大小和形状的边界条件下，非洲超级烟柱可以保持中性漂浮，并确实是“地球上存在的最古老的地质特征之一”。

Global models from seismic tomography show an extensive low shear wave velocity (vS) feature beneath southern African that extents from the core-mantle boundary for ~1000 km upwards. This feature is also characterized by anticorrelation of vS and the bulk sound velocity vΦ, relative to 1-dimensional reference profiles, with a high bulk and low shear modulus. Detailed studies using travel time anomalies were able to further demonstrate that this feature has very sharp, near vertical boundaries. While seismoloigcal constraints on this South-African superplume have been robust over the past couple of years, the physical or chemical origin of this anomaly has not adequately been investigated. For example, it is not clear to date if this is a dynamic or stagnant feature in the mantle. Attempts have been made to ascribe its origin to thermal upwellings, stabilized by the insulating effect of the stagnant overriding African plate, kept neutrally buoyant by enrichment of the lowermost mantle in terms of iron content and/or subducted material. However, the mineralogical models used in such dynamic studies are compiled on an ad-hoc basis, and competing effects are not investigated in a systematic way. Here, we propose to use a thermodynamic model of mantle mineralogy embedded in a 3-dimensional convection model to test the competing thermal and chemical influences on buoyancy of a broad upwelling feature in the lower most mantle. We will estimate under what boundary conditions of composition, size and shape the African superplume could stay neutrally buoyant and be indeed one of the ‘oldest geological features existing on our planet’.