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.
Highlights
The lower mantle is the largest geochemical reservoir in the Earth’s interior and controls the style of mantle convection and planetary evolution
In contrast to previous work (Gülcher et al, 2020), we do not observe any models with pervasive convective mixing of recycled oceanic crust (ROC) and primordial mantle materials in this study
Despite no detection of models in which both primordial and recycled materials are efficiently mixed throughout the mantle, we expect such a behavior to occur for small rheological contrasts of primordial material in combination with lower-density anomalies of ROC than modeled here (Nakagawa and Tackley, 2014)
Summary
The lower mantle is the largest geochemical reservoir in the Earth’s interior and controls the style of mantle convection and planetary evolution. Despite efficient stirring by vigorous convection over billions of years, the Earth’s lower mantle appears to be chemically heterogeneous on various length scales (e.g., Allegre and Turcotte, 1986; van Keken and Ballentine, 1998). On relatively small scales (mm to km), the concept of a “marble cake” mantle has gained wide acceptance, emphasizing that much of the mantle is made out of recycled oceanic lithosphere, deformed into narrow streaks of depleted and enriched compositions (Fig. 1a). Partial melting in the upper mantle creates heterogeneity between basaltic (magma) and harzburgitic (residue) end-members, forming a physically and chemically layered oceanic lithosphere. Subsequent injection into the mantle during subduction causes a non-equilibrated mantle that is a mechanical mixture of basalt and harzburgite
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