The long‐wavelength geoid from three‐dimensional spherical models of thermal and thermochemical mantle convection

Abstract

The Earth's long‐wavelength geoid anomalies have long been used to constrain the dynamics and viscosity structure of the mantle in an isochemical, whole mantle convection model. However, there is strong evidence that the seismically observed large low shear velocity provinces (LLSVPs) in the lower mantle underneath the Pacific and Africa are chemically distinct and likely denser than the ambient mantle. In this study, we have formulated dynamically self‐consistent 3‐D spherical mantle convection models to investigate how chemically distinct and dense piles above the core‐mantle boundary may influence the geoid. Our dynamic models with realistic mantle viscosity structure produce dominantly spherical harmonic degree‐2 convection, similar to that of the present‐day Earth. The models produce two broad geoid and topography highs over two major thermochemical piles in the lower mantle, consistent with the positive geoid anomalies over the Pacific and African LLSVPs. Our geoid analysis showed that the bottom layer with dense chemical piles contributes negatively to the total geoid, while the layer immediately above the chemical piles contributes positively to the geoid, canceling out the effect of the piles. Thus, the bottom part of the mantle, as a compensation layer, has zero net contribution to the total geoid, and the thickness of the compensation layer is ~1000 km or 2 to 3 times as thick as the chemical piles. Our results help constrain and interpret the large‐scale thermochemical structure of the mantle using surface observations of the geoid and topography, as well as seismic models of the mantle.

Stabilizing Effect of Compositional Viscosity Contrasts on Thermochemical Piles

“…In contrast, the overall density of the LLSVPs has been estimated by Koelemeijer et al () and Lau et al (), who argue for a neutral buoyancy of the LLSVP volume or a small excess density of about 0.5–1% in the lower part of the LLSVPs, respectively. This also agrees with geoid observations indicating that the upwelling material above the LLSVPs has to overcompensate dense material at the CMB to produce a geoid high (e.g., Liu & Zhong, ). However, these measured densities are smaller than those typically required for stability in numerical models of thermochemical convection, even when accounting for both chemical density and thermal expansion.…”

Dynamic topography, gravity and the role of lateral viscosity variations from inversion of global mantle flow

“…For example, the model parameterization, including the number and depth of layers (Rudolph et al, 2015), or the smoothness of the viscosity (Constate, 1987) may affect the inversion. Possible composition heterogeneity in the mantle (Ballmer et al, 2015, Liu andZhong, 2015) may also affect the inversion. Also, due to the limitation of tomography models and the simplified mantle viscosity and temperature structure, high-accuracy residual topography at discrete observation points (Hoggard et al, 2016).…”

Lower Mantle Dynamics Perceived With 50 Years of Hindsight From Plate Tectonics

“…They found that LLSVPs less dense than surrounding material fit better than denser LLSVPs. Moreover, by showing both good and bad fits to the data, they made a convincing case for low‐density LLSVPs, but as they admit, they “cannot rule out the presence of a ~100‐km‐thick denser‐than‐average basal structure.” Concurrently, using tides, Lau et al () concluded that the LLSVPs are 0.5% more dense than surroundings, though they admitted that most of the excess density “may be concentrated towards the very base of the mantle.” A safe summary might be that all data allow for a relatively dense D ″ beneath the LLSVPs, but at hundreds of kilometers above the core‐mantle boundary, the LLSVPs appear to be less dense than surrounding material, as suggested by bad fits presented by Koelemeijer et al () and by fits to the geoid (Liu & Zhong, , ).…”