The Stability of Dense Oceanic Crust Near the Core‐Mantle Boundary

Abstract

Seismic tomography models have discerned the presence of two mantle structures close to the core-mantle interface characterized by low seismic velocities. These large low-shear velocity provinces (LLSVPs) sit beneath Africa and the Pacific plate and exhibit a shear velocity anomaly of δlnV s ≈ −2%. Spatially, they are both voluminous and broad (Cottaar & Lekić, 2016), with seismic tomography models showing high power in spherical harmonic degree 2 and to a lesser extent degree 3 structures (Koelemeijer et al., 2016;Ritsema et al., 2011) in the lower mantle. These structures exist within and above the D" layer and may extend >1,000 km into the mantle from the core-mantle boundary (CMB) (He & Wen, 2009;Lekic et al., 2012), with the African LLSVP being significantly taller than the Pacific LLSVP (Ni et al., 2002;Yuan & Li, 2022). Mantle plumes are claimed to cluster around their edges (Thorne et al., 2004) potentially entraining material to be sampled by melting beneath ocean islands. Despite the morphology of LLSVPs being well constrained, increasingly accurate measurements of their material properties and possible direct links to the surface via plumes, their origin and composition remain a matter of debate. This is because the very characteristics that define LLSVPs have a non-unique source; the velocity anomaly may originate thermally, chemically, or thermo-chemically. Consequently there are competing theories over the nature of LLSVPs, are they of a predominantly thermal origin (D. R. Davies et al., 2015;

Unique composition and evolutionary histories of low velocity mantle domains

“…Previous work by Panton et al [30] shows that for buoyancy numbers in the range of 0.55 -0.66, SOC segregated to the lowermost mantle is in a quasi-steady state -it is not immediately entrained, but it also remains mobile enough that it does not completely blanket the CMB. Assuming a coe cient of thermal expansion of ⇠ 1.2 ⇥ 10 5…”

“…3f). A steady rate of replenishment of young SOC [15,30] therefore enables the Pacific S-LLVP to be maintained. Accumulations of SOC beneath present-day Africa continue to migrate south and are almost completely removed from the lowermost mantle, except beneath the Southern Ocean.…”

“…The depth and bulk composition of particles a↵ects the density field, which is quantified by the buoyancy number (Equation 4). We vary the buoyancy number in some simulations (Table 4) to investigate the e↵ect of di↵erent intrinsic densities of SOC [30], for which estimates vary between 0.5 and 5% more dense than average mantle [38,41,62].…”

“…A linear solidus, dependent on depth (z) and bulk composition, controls melting in the simulations [30,32,33]:…”

“…6.1). This long model duration is important for replenishing pile material after entrainment [15,30]. Kinematic reconstructions of plate motions [31] are used to constrain surface plate velocities and the paleo-locations of subduction zones and spreading ridges.…”