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 Gyrs. Over a wide parameter range, primordial and recycled heterogeneity are predicted to coexist with each other in the lower mantle of Earth-like planets. Primordial material usually survives as mid-to-large scale blobs (or streaks) in the mid-mantle, around 1000–2000 km depth. This preservation is largely independent on 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 a dense FeO-rich layer that is initially present at the base of the mantle, the FeO-rich material partially survives at the top of ROC piles, causing the piles to be compositionally stratified. Moreover, the addition of an ancient FeO-rich basal layer in the lowermost mantle significantly aids the preservation of the viscous domains in the mid-mantle. Primordial blobs are commonly (but not always) directly underlain by thick ROC piles, and aid their longevity and stability. The preservation of primordial domains along with recycled piles is relevant for Earth as it may reconcile geophysical and geochemical constraints on lower mantle heterogeneity.

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

“…Our results further contribute to the ongoing debate about whether thermochemical piles are intrinsically stable features in the deep mantle, which spatially determine mantle convective patterns (Dziewonski et al, 2010;Torsvik et al, 2014) or are instead pushed around by subduction zones (McNamara and Zhong, 2005;Zhang et al, 2010). In our models, piles and blobs both mostly reside well away from lower-mantle downwellings, as they are pushed away from them (videos in the Supplement, Gülcher, 2021). Indeed, as noted in previous studies (e.g., McNamara and Zhong, 2005;Zhang et al, 2010;Schierjott et al, 2020), we observe that downgoing slabs are mainly controlling the spatial distribution of piles (as well as viscous blobs) and not the other way around.…”

“…Video supplement. Video Supplements are available at Zenodo under the identifier https://zenodo.org/record/4767426 (Gülcher, 2021).…”

“…Model predictions for this sub-regime are also consistent with the stagnation of some slabs in a depth range that is similar to primordial-domain roofs, while other slabs readily sink into the deep mantle (Fukao and Obayashi, 2013), as seen in Figs. 2, 7, and videos in the Supplement (Gülcher, 2021). Furthermore, the presence of viscous domains in the midmantle may explain the observation of sharp seismic velocity contrasts in the uppermost lower mantle (850-1100 km depth), usually occurring away from major upwellings and downwellings (Jenkins et al, 2017;Waszek et al, 2018).…”

Continental Magmatism: The Surface Manifestation of Dynamic Interactions Between Cratonic Lithosphere, Mantle Plumes and Edge‐Driven Convection

“…In particular, melting at depth relies on a batch melting parameterization of a peridotite assemblage, and our implementation does not account for changes in material properties, such as density and viscosity, that arise through melting. As such, we neglect complexities associated with multi‐component melting (e.g., Nebel et al., 2019; Shorttle et al., 2014) and potentially important feedbacks between melting, lithospheric deformation and mantle dynamics (e.g., Burov & Guillou‐Frottier, 2005; Gerya et al., 2015; Gülcher et al., 2021; Lavecchia et al., 2017). Furthermore, we do not simulate the effects of melt extraction, melt transport and preferential melt migration pathways (Dannberg & Heister, 2016; Ghods & Arkani‐Hamed, 2002; Jain et al., 2019; Keller et al., 2017), which needs to be considered when comparing our predicted melting rates with observations from the geological record.…”

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

“…(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).…”