Mineralogical and Dynamic Origin of the South African Superplume

来自地震层析成像的全球模型显示，非洲南部下方存在广泛的低剪切波速度（vS）特征，从核幔边界向上延伸约1000 km。该特征还通过相对于一维参考剖面的vS和体声速vΦ的关系表征，具有高体模量和低剪切模量。利用走时异常进行的详细研究能够进一步证明该特征具有非常尖锐的近垂直边界。虽然在过去几年中，对这一南非超级羽流的地震学限制一直很强，但尚未对这一异常的物理或化学起源进行充分调查。例如，至今尚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升，稳定的停滞压倒非洲板块的绝缘效应，保持中性浮力的最低地幔富集铁含量和/或俯冲材料。然而，在这种动态研究中使用的矿物学模型是在特设的基础上编制的，竞争效应没有以系统的方式进行研究。在这里，我们建议使用一个热力学模型的地幔矿物学嵌入在一个三维对流模型来测试竞争的热和化学的影响浮力的一个广泛的上涌功能在最低的地幔。我们将估计在什么样的成分，大小和形状的边界条件下，非洲超级羽流可以保持中性浮力，并且确实是我们星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’.