Preprints
https://doi.org/10.5194/se-2020-205
https://doi.org/10.5194/se-2020-205

  16 Dec 2020

16 Dec 2020

Review status: this preprint is currently under review for the journal SE.

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

Anna Johanna Pia Gülcher1, Maxim Dyonis Ballmer2,1, and Paul James Tackley1 Anna Johanna Pia Gülcher et al.
  • 1Institute of Geophysics, Department of Earth Sciences, ETH Zürich, Zürich, Switzerland
  • 2Department of Earth Sciences, University College London, London, UK

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.

Anna Johanna Pia Gülcher et al.

 
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Anna Johanna Pia Gülcher et al.

Video supplement

Coupled dynamics and evolution of primordial and recycled heterogeneity in Earth's lower mantle - Supplementary Videos Anna J. P. Gülcher, Maxim D. Ballmer, and Paul J. Tackley https://doi.org/10.5281/zenodo.4298777

Anna Johanna Pia Gülcher et al.

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Short summary
The lower mantle extends from 660–2890 km depth, making up > 50 % of the Earth's volume. Its composition and structure, however, remain poorly understood. In this study, we investigate several hypotheses with computer simulations of mantle convection that include different materials: recycled, dense rocks and ancient, strong rocks. We propose a new integrated style of mantle convection including piles, blobs, and streaks, which agrees with various observations of the deep Earth.