title: Compositional mantle layering revealed by slab stagnation at ~1000-km depth.
abstract: Improved constraints on lower-mantle composition are fundamental to understand the accretion, differentiation, and thermochemical evolution of our planet. Cosmochemical arguments indicate that lower-mantle rocks may be enriched in Si relative to upper-mantle pyrolite, whereas seismic tomography images suggest whole-mantle convection and hence appear to imply efficient mantle mixing. This study reconciles cosmochemical and geophysical constraints using the stagnation of some slab segments at ~1000-km depth as the key observation. Through numerical modeling of subduction, we show that lower-mantle enrichment in intrinsically dense basaltic lithologies can render slabs neutrally buoyant in the uppermost lower mantle. Slab stagnation (at depths of ~660 and ~1000 km) and unimpeded slab sinking to great depths can coexist if the basalt fraction is ~8% higher in the lower mantle than in the upper mantle, equivalent to a lower-mantle Mg/Si of ~1.18. Global-scale geodynamic models demonstrate that such a moderate compositional gradient across the mantle can persist can in the presence of whole-mantle convection.
authors.affiliation: Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8551, Japan.; School of Ocean and Earth Sciences and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA.
authors.forename: Maxim D
authors.initials: MD
authors.lastname: Ballmer
authors.affiliation: Department of Geology, University of Maryland, College Park, MD 20742, USA.
authors.forename: Nicholas C
authors.identifier: 0000-0002-3256-1262
authors.initials: NC
authors.lastname: Schmerr
authors.affiliation: Department of Mathematical Science and Advanced Technology, Japan Agency for Marine-Earth Science and Technology, Yokohama 236-0001, Japan.
authors.forename: Takashi
authors.identifier: 0000-0003-3179-6462
authors.initials: T
authors.lastname: Nakagawa
authors.affiliation: Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, USA.
authors.forename: Jeroen
authors.identifier: 0000-0003-4287-7639
authors.initials: J
authors.lastname: Ritsema
keywords: bulk silicate Earth; mantle composition; mantle convection; stagnant slab; subducted slab
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