Abstract
Mantle tomography reveals the existence of two large low-shear-velocity provinces (LLSVPs) at the base of the mantle. We examine here the hypothesis that they are piles of oceanic crust that have steadily accumulated and warmed over billions of years. We use existing global geodynamic models in which dense oceanic crust forms at divergent plate boundaries and subducts at convergent ones. The model suite covers the predicted density range for oceanic crust over lower mantle conditions. To meaningfully compare our geodynamic models to tomographic structures, we convert them into models of seismic wavespeed and explicitly account for the limited resolving power of tomography. Our results demonstrate that long-term recycling of dense oceanic crust naturally leads to the formation of thermochemical piles with seismic characteristics similar to the LLSVPs. The extent to which oceanic crust contributes to the LLSVPs depends upon its density in the lower mantle for which accurate data is lacking. We find that the LLSVPs are not composed solely of oceanic crust. Rather, they are basalt rich at their base (bottom 100–200 km) and grade into peridotite toward their sides and top with the strength of their seismic signature arising from the dominant role of temperature. We conclude that recycling of oceanic crust, if sufficiently dense, has a strong influence on the thermal and chemical evolution of Earth’s mantle.
Highlights
Tomographic models reveal the presence of two large anomalous structures in the lowermost mantle. These large low-shear velocity provinces (LLSVPs) are surrounded by high shear-wave velocity regions that are associated with past subduction (Fig. 1; Tanaka et al 2009; Ritsema et al 2011; and and Cottaar and Lekic 2016)
We find that thermal heterogeneity plays a dominant role in LLSVP formation, our study is in contrast with previous work suggesting that the LLSVPs could be purely thermal (Schubert et al 2004; Schuberth et al 2009; Bull et al 2009; Davies et al 2012, 2015; Koelemeijer et al 2018) as we find that some dense compositional component is required
In summary, we have shown that oceanic crust formation and recycling as modeled by BB08 can satisfactorily explain the LLSVPs in both the size and amplitude of the shear-wave anomalies as long as oceanic crust is denser than ambient mantle by 3% or more
Summary
Tomographic models reveal the presence of two large anomalous structures in the lowermost mantle. These large low-shear velocity provinces (LLSVPs) are surrounded by high shear-wave velocity regions that are associated with past subduction (Fig. 1; Tanaka et al 2009; Ritsema et al 2011; and and Cottaar and Lekic 2016). We are motivated in particular by geodynamical studies that show that the long-term recycling of oceanic crust can significantly contribute to the formation of warm but dense pools of compositionally distinct material at the base of the mantle (Fig. 2; Christensen and Hofmann 1994; Brandenburg and van Keken 2007; Brandenburg et al 2008; and Nakagawa and Tackley 2008). We will test how well such velocity structures are resolved in the shear-wave velocity model, S40RTS (Ritsema et al 2011), and the combined shear- and compressional-wave velocity model, SP12RTS (Koelemeijer et al 2016)
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