Geochemical Constraints on the Structure of the Earth's Deep Mantle and the Origin of the LLSVPs

Matthew Gleeson,Sanne Cottaar,Simon Matthews,Caroline Soderman,Sally Gibson

doi:10.1029/2021gc009932

Abstract

AbstractGeophysical analysis of the Earth's lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core‐mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep‐seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep‐seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in ocean island basalts provides constraints on the structure of the Earth's lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high‐MgO basalts, that the isotopically enriched south‐western region of the Galápagos mantle—assigned to melting of LLSVP material—displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north‐central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations.

Highlights

Volcanic archipelagos such as the Galápagos, Hawai’i and Samoa represent the surface expression of deep-seated mantle plumes that likely originate near the core-mantle boundary (Morgan, 1971; Wilson, 1973)
We focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of Large Low Shear Velocity Provinces (LLSVPs) material in the underlying plume
It is unclear how the shallow level (

Summary

Introduction

Volcanic archipelagos such as the Galápagos, Hawai’i and Samoa represent the surface expression of deep-seated mantle plumes that likely originate near the core-mantle boundary (Morgan, 1971; Wilson, 1973). Seismic models highlight the presence of two ‘superstructures’ on the core mantle boundary that are characterized by lower shear wave velocities than the surrounding mantle (Cottaar and Lekic, 2016; Dziewonski and Woodhouse, 1987; Garnero et al, 2016; Ritsema et al, 2011) and are argued to have higher densities than surroundings (Lau et al, 2017; Moulik and Ekström, 2016) at least at their base (Davaille and Romanowicz, 2020; Richards et al, 2021) These superstructures, known as Large Low Shear Velocity Provinces (LLSVPs), are located beneath Africa and the Pacific and play a critical role in mantle dynamics, the rise of mantle plumes (Heyn et al, 2020), and the configuration of True Polar Wander events (Steinberger et al, 2017). The origin of the LLSVPs remain enigmatic, with their presence having previously been assigned to piles of subducted oceanic crust (Brandenburg and van Keken, 2007; Niu, 2018) or primordial material that has undergone differentiation early in Earth’s history (such as magma ocean cumulates; Deschamps et al, 2012; Labrosse et al, 2007; Peters et al, 2018)

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