The dynamics and impact of compositionally originating provinces in a mantle convection model featuring rheologically obtained plates

Abstract

SUMMARY Previous geodynamic studies have indicated that the presence of a compositionally anomalous and intrinsically dense (CAID) mantle component can impact both core heat flux and surface features, such as plate velocity, number and size. Implementing spherical annulus geometry mantle convection models, we investigate the influence of intrinsically dense material in the lower mantle on core heat flux and the surface velocity field. The dense component is introduced into a system that features an established plate-like surface velocity field, and subsequently we analyse the evolution of the surface velocity as well as the interior thermal structure of the mantle. The distribution and mobility of the CAID material is investigated by varying its buoyancy ratio relative to the ambient mantle (ranging from 0.7 to 1.5), its total volume (3.5–10 per cent of the mantle volume) and its intrinsic viscosity (0.01–100 times the ambient mantle viscosity). We find at least three distinct distributions of the dense material can occur adjacent to the core–mantle boundary (CMB), including multiple piles of varying topography, a core enveloping layer and two diametrically opposed provinces (which can on occasion break into three distinct piles). The latter distribution mimics the morphology of the seismically observed large low shear wave velocity provinces (LLSVPs) and can occur over the entire range of CAID material viscosities. However, diametrically opposed provinces occur primarily in cases with CAID material buoyancy numbers of 0.7–0.85 (corresponding to contrasts in density between ambient and CAID material of 130 and 160 kg m−3, respectively) in our model (with an effective Rayleigh number of order 106). Steep and high topography piles are also obtained for cases featuring buoyancy ratios of 0.85 and viscosities 10–100 times that of the ambient mantle. An increase in relative density, as well as larger volumes of CAID material, lead to the development of a core enveloping layer. Our findings show that when two provinces are present core heat flux can be reduced by up to 50 per cent relative to cases in which CAID material is absent. Surface deformation quantified by Plateness is minimally influenced by variation of the properties of the dense material. Surface velocity is found to be reduced in general but mostly substantially in cases featuring high CAID material viscosities and large volumes (i.e. 10 per cent) or buoyancy ratios.

The evolution of basal mantle structure in response to supercontinent aggregation and dispersal

“…S4 ). Colder and stiffer slabs push the thermochemical structures to form steep ridges along their margins, as observed in previous studies 2 , 6 , 46 . An illustrative example is the concave-upward tops of the superocean structure at 800 Ma (Fig.…”

“…This is consistent with the idea that colder slabs tend to drive the formation of the steep edges of thermochemical structures 19 . Importantly, we find that the slope of the edge of thermochemical structures is transient and dynamic, and that it decreases likely because the slabs that push it heat up as they interact with the edge of hot structures over time 6 (Fig. 6 ).…”

“…For example, increasing the temperature dependence of viscosity tends to decrease the viscous coupling between basal thermochemical structure and surround mantle: the larger the temperature dependence, the less viscously coupled the two materials are 50 , 74 . A larger compositional viscosity for basal thermochemical structures that makes them more viscous than ambient mantle would also contribute to their long-term survival and stability 6 , 46 , 62 .…”

“…For example, they are thought to be the reservoir for ocean island basalts 4 , responsible for the degree-two geoid anomaly 5 , and to affect the velocity of tectonic plates 6 . The relationship between LLSVPs and plate motion is controversial, with some models implying that LLSVPs have remained stable over long time periods, while others suggest that they could be shaped by subducted slabs that have sunk into the deep mantle 6 – 11 . Characterizing the plate-basal mantle relationship is important to better understand Earth’s long-term evolution.…”

The Thermo‐Chemical Evolution of Mars With a Strongly Stratified Mantle

“…In the above equation, α is the thermal expansion and Δ T is the characteristic temperature scale, which is chosen as the superadiabatic temperature difference across the entire mantle. The large values of B d are sufficient to prevent convective mixing between the enriched layer and the overlying mantle despite its relatively large HPE content (Langemeyer et al., 2020; Lebars & Davaille, 2002; M. Li & McNamara, 2018; Y. Li et al., 2014; Limare et al., 2019; McNamara & Zhong, 2005; Nakagawa & Tackley, 2004; Olson, 1984; Plesa et al., 2014; Samuel & Farnetani, 2003; Tosi, Plesa, et al., 2013; Trim et al., 2014).…”