Abstract
Abstract. The nature and origin of the two large low-velocity provinces (LLVPs) in the lowest part of the mantle remain controversial. These structures have been interpreted as a purely thermal feature, accumulation of subducted oceanic lithosphere or a primordial zone of iron enrichment. Information regarding the density of the LLVPs would help to constrain a possible explanation. In this work, we perform a density inversion for the entire mantle, by constraining the geometry of potential density anomalies using tomographic vote maps. Vote maps describe the geometry of potential density anomalies according to their agreement with multiple seismic tomographies, hence not depending on a single representation. We use linear inversion and determine the regularization parameters using cross-validation. Two different input fields are used to study the sensitivity of the mantle density results to the treatment of the lithosphere. We find the best data fit is achieved if we assume that the lithosphere is in isostatic balance. The estimated densities obtained for the LLVPs are systematically positive density anomalies for the LLVPs in the lower 800–1000 km of the mantle, which would indicate a chemical component for the origin of the LLVPs. Both iron-enrichment and a mid-oceanic ridge basalt (MORB) contribution are in accordance with our data, but the required superadiabatic temperature anomalies for MORB would be close to 1000 K.
Highlights
Seismology has systematically revealed more and more of the heterogeneity in the mantle
In the upper mantle above the transition zone (410–670 km depth) much of the seismic image can be linked to tectonic features: thick and cold lithospheric roots underlie the cratonic cores of the continents and can extend up to 300 km deep (Schaeffer and Lebedev, 2015); the oceans are characterized by a velocity trend related to cooling of the lithosphere (Priestley and McKenzie, 2006); and slabs formed during subduction sink through the mantle, sometimes stagnating at various depths and sometimes passing through the transition zone to the lowermost mantle
Comparing our results with published core–mantle boundary (CMB) topography estimates based on seismological methods (Morelli and Dziewonski, 1987; Doornbos and Hilton, 1989; Sze and van der Hilst, 2003; Tanaka, 2010), we find that none of the models contain the first-order pattern of CMB depressions beneath Africa and the central Pacific, seen in our maps (Fig. 10c, d)
Summary
Seismology has systematically revealed more and more of the heterogeneity in the mantle. Interpretations of seismic images often aim at determining the density of features, because buoyancy is the driver of mantle dynamics. van der Meer et al, 2018). At greater depths such interpretation becomes more difficult, due to reduced resolution and lack of corroborating surface evidence. The most striking features of the lowermost mantle are probably the large low-velocity provinces (LLVPs) (Garnero et al, 2016). The two LLVPs are antipodal regions of decreased seismic velocity that extend from the core–mantle boundary about 400–800 km into the mantle and cover about 25 % of the surface of the core–mantle boundary (Garnero et al, 2016). It has been suggested that plumes are formed preferentially at the edges of the LLVPs, based on the reconstructed locations of large igneous provinces and kimberlites (Burke et al, 2008; Conrad et al, 2013), which imply that the LLVPs are stable over long time periods
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