Mineralogical and Dynamic Origin of the South African Superplume

点击翻译按钮获取中文摘要

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’.