Abstract
AbstractDeep‐rooted mantle plumes are thought to originate from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle. Visible in seismic tomography, the LLSVPs are usually interpreted to be intrinsically dense thermochemical piles in numerical models. Although piles deflect lateral mantle flow upward at their edges, the mechanism for localized plume formation is still not well understood. In this study, we develop numerical models that show plumes rising from the margin of a dense thermochemical pile, temporarily increasing its local thickness until material at the pile top cools and the pile starts to collapse back toward the core‐mantle boundary (CMB). This causes dense pile material to spread laterally along the CMB, locally thickening the lower thermal boundary layer on the CMB next to the pile, and initiating a new plume. The resulting plume cycle is reflected in both the thickness and lateral motion of the local pile margin within a few hundred km of the pile edge, while the overall thickness of the pile is not affected. The period of plume generation is mainly controlled by the rate at which slab material is transported to the CMB, and thus depends on the plate velocity and the sinking rate of slabs in the lower mantle. A pile collapse, with plumes forming along the edges of the pile's radially extending corner, may, for example, explain the observed clustering of Large Igneous Provinces (LIPs) in the southeastern corner of the African LLSVP around 95–155 Ma.
Highlights
We develop numerical models that show plumes rising from the margin of a dense thermochemical pile, temporarily increasing its local thickness until material at the pile top cools and the pile starts to collapse back toward the core-mantle boundary (CMB)
One example where this process may be especially relevant is the clustering of Large Igneous Provinces (LIPs) in the southeastern corner of the African Large Low Shear Velocity Provinces (LLSVPs) mentioned above (Figure 1), where at least eight plumes erupted within about 60 Myr (Torsvik et al, 2016, 2019)
We have shown that plumes at the CMB can be triggered by two ways of increasing the thickness of the lower thermal boundary layer (TBL)
Summary
All seismic tomography models throughout the last decades show a similar degree-2 pattern for the Earth's lowermost mantle (e.g., Dziewonski et al, 2010; Garnero et al, 2016; Hager et al, 1985), dominated by the presence of two so-called Large Low Shear Velocity Provinces (LLSVPs) (Dziewonski et al, 2010; Garnero & McNamara, 2008; Garnero et al, 2016) centered beneath Africa and the Pacific (Cottaar & Lekic, 2016; Garnero & McNamara, 2008; Lekic et al, 2012). A mechanism for plume initiation directly at the pile margin may be better suited to explain the observed correlation between LIPs and the LLSVP margins This may be especially relevant since the presence of weak post-bridgmanite is likely to aid transforming subducted slabs into a broad and spread-out downwelling of cold material in the lowermost mantle, potentially erasing any “memory” of distinct slabs. K ρcP and equals we investigate controls on the periodicity of this process and discuss how we may use plume observations to develop new constraints on the viscosity structure of the lower mantle and the LLSVPs
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