Coupled dynamics and evolution of primordial and recycled heterogeneity in Earth's lower mantle

Abstract

Abstract. The nature of compositional heterogeneity in Earth's lower mantle remains a long-standing puzzle that can inform about the long-term thermochemical evolution and dynamics of our planet. Here, we use global-scale 2D models of thermochemical mantle convection to investigate the coupled evolution and mixing of (intrinsically dense) recycled and (intrinsically strong) primordial heterogeneity in the mantle. We explore the effects of ancient compositional layering of the mantle, as motivated by magma ocean solidification studies, and of the physical parameters of primordial material. Depending on these physical parameters, our models predict various regimes of mantle evolution and heterogeneity preservation over 4.5 Gyr. Over a wide parameter range, primordial and recycled heterogeneity are predicted to co-exist with each other in the lower mantle of Earth-like planets. Primordial material usually survives as medium- to large-scale blobs (or streaks) in the mid-mantle, around 1000–2000 km depth, and this preservation is largely independent of the initial primordial-material volume. In turn, recycled oceanic crust (ROC) persists as large piles at the base of the mantle and as small streaks everywhere else. In models with an additional dense FeO-rich layer initially present at the base of the mantle, the ancient dense material partially survives at the top of ROC piles, causing the piles to be compositionally stratified. Moreover, the addition of such an ancient FeO-rich basal layer significantly aids the preservation of the viscous domains in the mid-mantle. Finally, we find that primordial blobs are commonly directly underlain by thick ROC piles and aid their longevity and stability. Based on our results, we propose an integrated style of mantle heterogeneity for the Earth involving the preservation of primordial domains along with recycled piles. This style has important implications for early Earth evolution and has the potential to reconcile geophysical and geochemical discrepancies on present-day lower-mantle heterogeneity.

Dynamics and Evolution of Venus’ Mantle Through Time

“…Later-stage partial melting induced by hot anomalies in a largely solidified mantle leads to small-scale heterogeneity as different rock components melt under different conditions and have different affinity to partition into the molten phase (e.g., Hofmann 1997). Such processes compete with mantle convective stirring that tends to homogenise lateral variation, but the mixing time scales in the mantle are large and compositional heterogeneity occurs likely across multiple scales in the Earth's present mantle (e.g., Tkalčić et al 2015), as is also supported by modelling studies (Gülcher et al 2021). However, terrestrial plate tectonics continuously induces new heterogeneity by deep crustal subduction, which may be less relevant to Venus without plate tectonics.…”

The Stability of Dense Oceanic Crust Near the Core‐Mantle Boundary

“…While we have not included the effects of a dense, primordial layer in our simulations, we do not refute the potential for such compositions to exist in the mantle and play a role in LLSVP formation. Studies which combine both primordial and recycled OC material (Ballmer et al., 2016; Gülcher et al., 2021; Jones et al., 2021) will also benefit from including the effects of radiogenic heating in each of the compositions (Citron et al., 2020).…”

“…The first is that LLSVPs are composed of dense primordial material (Deschamps et al., 2011; Garnero & McNamara, 2008; Labrosse et al., 2007; M. Li & McNamara, 2018) and the second is that they formed from the accumulation of recycled oceanic crust (henceforth referred to as OC) (Christensen & Hofmann, 1994; Coltice & Ricard, 1999; Tackley, 2011). A third scenario arises from this, where LLSVPs may comprise heterogeneity from both recycled OC and primordial material, a so‐called basal melange (Gülcher et al., 2021; Jones et al., 2021; Tackley, 2012).…”

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

“…Giuliani et al ( 2021 ) noted that the PREMA component found in most kimberlite sources may be contained within LLSVPs. However, the distribution of PREMA in the mantle remains uncertain; for example, "blobs/streaks" of ancient material are also modeled to exist and persist throughout the ambient mantle ( Ballmer et al, 2017;Becker et al, 1999;Brandenburg et al, 2008;Gülcher et al, 2021;Jones et al, 2021 ).…”