Variable dynamic styles of primordial heterogeneity preservation in the Earth’s lower mantle

Abstract

The evolution of the system Earth is critically influenced by the long-term dynamics, composition and structure of the mantle. While cosmochemical and geochemical constraints indicate that the lower mantle hosts an ancient primordial reservoir that may be enriched in SiO2 with respect to the upper mantle, geophysical observations and models point to efficient mass transfer and convective mixing across the entire mantle. Recent hypotheses of primordial-material preservation in a convecting mantle involve delayed mixing of intrinsically dense and/or intrinsically strong heterogeneity. Yet, the effects of composition-dependent rheology and density upon heterogeneity preservation and the dynamics of mantle mixing remain poorly understood. Here, we present two-dimensional numerical models in spherical geometry, investigating the preservation styles of primordial material as a function of its physical properties (i.e., viscosity and density contrasts). We establish multiple regimes of primordial-material preservation that can occur in terrestrial planets. These include (1) efficient mixing, (2) double-layered convection with or without topography, and (3) variable styles of partial heterogeneity preservation (e.g., as diffuse domains, piles or viscous blobs in the lower mantle). Some of these regimes are here characterised for the first time, and all regimes are put into context with each other as a function of model parameters. The viscous-blobs and diffuse-domains regimes can reconcile the preservation of primordial domains in a convecting mantle, potentially resolving the discrepancy between geochemical and geophysical constraints for planet Earth. Several, if not all, regimes characterised here may be relevant to understand the long-term evolution of terrestrial planets in general.

D" Reflection Polarities Inform Lowermost Mantle Mineralogy

“…(2021) show that an increase of bm with an increase of Fe has been suggested to be responsible for the LLSVPs. Their composition is similar to BEAMs (bridgmanite enriched ancient material, Ballmer et al., 2017) and if the BEAMs exist near the CMB (Gülcher et al., 2020) this could explain case 2b in Table 2. Our thermochemical modeling suggests that an increase in bm has the largest effect, but it may also be accompanied with a decrease of FeO (Figure 7f).…”

Geodynamic and Isotopic Constraints on the Genesis of Kimberlites, Lamproites and Related Magmas From the Finnish Segment of the Karelian Craton

“…Assuming these “polluting” components do not mix and homogenize with the ambient mantle (i.e., they remain as physical entities mingled with ambient mantle material) the correlation between εHf (i) and kimberlite emplacement age (Figure 6) also requires that any entrainment/sampling of this perturbing component must intensify with time and potentially also lower the f O 2 of the kimberlite source (Figure 7). Indeed, numerical geodynamic models suggest that streaks or “blobs” of geochemically enriched material can persist in the mantle for timescales up to the age of the Earth (e.g., Ballmer et al., 2017; Gülcher et al., 2020; Jones et al., 2021).…”

Narrow, Fast, and “Cool” Mantle Plumes Caused by Strain‐Weakening Rheology in Earth's Lower Mantle

“…This result may be attributed to the lack of effective strain weakening in the low‐strain downwelling regions. Only if both upwellings and downwellings were significantly weaker than the regions in‐between, efficient preservation may occur in these in‐between regions (Ballmer et al., 2017; Gülcher et al., 2020). For example, it has been proposed that grain‐size reduction in cold slabs that enter the lower mantle causes local weakening (Dannberg et al., 2017; Ito & Sato, 1991; Karato et al., 2001; Yamazaki et al., 2005).…”

“…Subducted oceanic crust at lower mantle conditions does not contain any ferropericlase, but instead, contains much cubic CaSiO 3 perovskite (Hirose, Sinmyo, & Hernlund, 2017; Tschauner et al., 2021; Wicks & Duffy, 2016), which may be intrinsically weak (Immoor et al., 2022). It is further interesting to test this composition‐dependent weakening in combination with the existence of ancient bridgmanite‐enhanced regions in the mid‐mantle (as in Gülcher et al., 2020; Gülcher et al., 2021), which should not exhibit SW due to the absence of ferropericlase, hence promoting their preservation. Moreover, it remains to be explored how strain‐dependent and grain size‐dependent rheology interact with each other in the lower mantle (see Section 4.1).…”

“…The weakening of the bulk rock with accumulating strain (“strain weakening”) implies that deformation may localize in the lower mantle, analogous to localized shear zones in crustal rocks. Such strain localization may potentially explain the isolation of large unmixed domains in the lower mantle, which may host primordial (or “hidden”) geochemical reservoirs away from regions of localized deformation (Ballmer et al., 2017; Chen, 2016; Gülcher et al., 2020; Mundl et al., 2017) (see Figure 1b). However, the effects of strain weakening on lower‐mantle convection patterns and mixing dynamics have not yet been studied using global‐scale geodynamic models.…”