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Burying Earth's primitive mantle in the slab graveyard
  • Timothy David Jones,
  • Nathan Sime,
  • Peter van Keken
Timothy David Jones
University of California San Diego

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Nathan Sime
Carnegie Institution for Science
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Peter van Keken
Carnegie Institution for Science
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The evolution of mantle composition can be viewed as process of destruction whereby the initial chemical state is overprinted and reworked with time. Analyses of ocean island basalts reveals that some portion of the mantle has survived this process, retaining a chemically ‘primitive’ signature. A question that remains is how this primitive signature has survived four and half billion years of vigorous convection. We hypothesize that some of Earth’s primitive mantle is buried within a slab graveyard at the core-mantle boundary. We explore this possibility using high‐resolution finite element models of mantle convection, in which oceanic lithosphere is produced at zones of plate spreading and subducted at zones of plate convergence. Upon subduction, dense oceanic crust sinks to the base of the mantle and gradually accumulates to form broad and robust thermochemical piles. Sinking oceanic crust entrains the surrounding mantle whose composition is predominantly primitive early in the model’s evolution. As a result, thermochemical piles are initially supplied with relatively high concentrations of primitive material –summing up to ~30% their total mass. The dense oceanic crust that dominates the piles resists efficient mixing and preserves the primitive material that it is intermingled with. The significance of this process is shown to be proportional the rate of mantle processing through time and the excess density of oceanic crust at mantle pressures and temperatures. Unlike existing theories for the survival of Earth’s primitive mantle, this one does not require the early Earth to have anomalously high density or large scale viscosity contrasts.
Mar 2021Published in Geochemistry, Geophysics, Geosystems volume 22 issue 3. 10.1029/2020GC009396