Compositional and thermal state of the lower mantle from joint 3D inversion with seismic tomography and mineral elasticity

Abstract

The compositional and thermal state of Earth’s mantle provides critical constraints on the origin, evolution, and dynamics of Earth. However, the chemical composition and thermal structure of the lower mantle are still poorly understood. Particularly, the nature and origin of the two large low-shear-velocity provinces (LLSVPs) in the lowermost mantle observed from seismological studies are still debated. In this study, we inverted for the 3D chemical composition and thermal state of the lower mantle based on seismic tomography and mineral elasticity data by employing a Markov chain Monte Carlo framework. The results show a silica-enriched lower mantle with a Mg/Si ratio less than ~1.16, lower than that of the pyrolitic upper mantle (Mg/Si = 1.3). The lateral temperature distributions can be described by a Gaussian distribution with a standard deviation (SD) of 120 to 140 K at 800 to 1,600 km and the SD increases to 250 K at 2,200 km depth. However, the lateral distribution in the lowermost mantle does not follow the Gaussian distribution. We found that the velocity heterogeneities in the upper lower mantle mainly result from thermal anomalies, while those in the lowermost mantle mainly result from compositional or phase variations. The LLSVPs have higher density at the base and lower density above the depth of ~2,700 km than the ambient mantle, respectively. The LLSVPs are found to have ~500 K higher temperature, higher Bridgmanite and iron content than the ambient mantle, supporting the hypothesis that the LLSVPs may originate from an ancient basal magma ocean formed in Earth’s early history.

Formation of deep mantle heterogeneities through basal exsolution contaminated magma ocean

“…Phase equilibrium calculations suggest that the solidi cation of BMO quickly depletes the silicate melts of Si due to the crystallization of the rst liquidus phase, bridgmanite, leading to a 1000-km bottom layer that contains approximately 20 wt% of ferropericlase 20 . However, this contrasts sharply with inferences drawn from seismic tomography which favor a low ferropericlase content of ~6-10 wt% 21,22 . If LLSVPs originate from the BMO, a mechanism enriching Si content is required to replenish the crystallizing BMO and suppress the production of ferropericlase.…”

“…Consequently, silicate phases dominate most depths, with ferropericlase concentrated towards the very bottom of the mantle. As a result, the solidi cation of the BECMO can simultaneously explain the SiO 2 and FeO enrichment in the LLSVPs as well as the Fe-rich and dense ultralow-velocity zones in the base of the mantle, as inferred from the tomography models 21,22 . We note that the amount of MgO and SiO 2 added to the mantle by exsolution could be as large as ~ 10 23 kg (Supplementary Information), which is a few percent of the present-day mantle mass.…”

“…If the shallow LLSVPs is intrinsically lighter, to generate the observed density anomaly, the remaining 14% of the primordial light layer may mix with the ambient mantle in a ratio of 3:1 by volume. On the other hand, if the shallow LLSVPs are intrinsically denser 21 and the apparent small density is due to thermal buoyancy, mixing with the dense base layer is required.…”

Ancient Stratified Thermochemical Piles Due To High Intrinsic Viscosity

“…Alternatively, water carried in deep‐subducted slabs (Ohtani, 2020) may cause hydrolytic weakening (Karato et al., 1986) of ROC, although the magnitude of this effect may be negligible in the lower mantle (Muir & Brodholt, 2018). In both cases, basalt would not be a good candidate to make up the majority of stratified piles, and a bridgmanite‐enriched composition of LLVPs would be more likely (Deng et al., 2023; Deschamps et al., 2012; Vilella et al., 2021). Dominantly bridgmanitic LLVPs may originate from ancient BMO cumulates (Labrosse et al., 2007), depending on its starting composition: if the BMO is Si‐enriched compared to pyrolite (Ballmer et al., 2017), the cumulate sequence would consist of strong minerals (i.e., bridgmanite (+stishovite)) (Boukaré et al., 2015; Tsujino et al., 2022).…”

“…The most prominent structures in this region are the Large Low Velocity Provinces (LLVPs), two continent‐scale anomalies of slow seismic wave speed with controversial origin (Cottaar & Lekic, 2016). The LLVPs are likely composed of hot, compositionally‐distinct material (Deng et al., 2023; Richards et al., 2023), originating from early‐Earth processes—for example, basal‐magma‐ocean (BMO) cumulates (Labrosse et al., 2007)– or the long‐term accumulation of Recycled Oceanic Crust (ROC) (Brandenburg & Van Keken, 2007) at the Core‐Mantle Boundary (CMB). Particularly, the formation of piles mostly formed of ROC hinges on how efficiently this segregates and settles.…”