Mantle structures and their origins
Bernhard Steinberger
GFZ German Research Centre for Geosciences, Potsdam, Germany

Figure from Deschamps, Li & Tackley, 2015

Sendai, Summer 2017• Mantle contains thermal and chemical structure• Convection creates thermal structure; erases
chemical structure through mixing• Melting separates materials and causes
chemical structure• Large uncertainties on mantle rheology

Example for stagnating slab: Japan

Example for penetrating slab: South America

Cross sections from SubMachine (Hosseini et al.) https://www.earth.ox.ac.uk/~smachineExample for stagnating slab: Europe

Example for penetrating slab: South America

→ s-wavespeed anomalies inlowermost mantle,→ reconstructed LIPs (green) →likely deep hotspots (stars) → afterTorsvik et al. (2006)

Subduction locations andamounts (color intensity)from a global plate
reconstruction (seeSteinberger and Torsvik, 2012, fordetails)

timeFrom Mulyukova et al., JGR: Piles from subducted slabs accumulating over Gyrs

• Zoom-in on one of the piles• plumes forming at cusps
near margins• plumes entraining pilematerials

“Hawaii plume” afterSteinberger and Torsvik (2012)

Hotspots located near margins of Large Low Shear Velocity Provinces (LLSVPs)“Jason” and “Tuzo”

• Same is true also for reconstructed LIP locations• Indicates spatial stability of LLSVPs back to at least 200 Myr, perhaps longer

Plume-Lithosphere interaction– intraplate: Hotspot track and swell:- Interaction with ridge(Gassmöller et al., G-Cubed, 2016)

Interaction with moving lithosphereof variable thickness (Steinbergeret al., NGeo, 2019)

TX2019slab-S (Lu et al., 2019)

14Upper mantle
flow (650 kmdepth)Viscosity
η2

53

6

Plume head rises (vrise ~ 1/η)gets advected by large-scale flow(density anomalies from tomography)
How fast?
→ vary total rise time (prescribed)

5cm/yrPlume conduit also rises(but less fast) +gets advected by large-scale flow

Lower mantle flow
(2600 km depth)low viscosity
Conduitshape for

30 Ma

60 Ma

plume headrise time

90 Ma

120 Mahighviscosity

Plumeheadrisetime
30 Ma
60 Ma
90 Ma
120 MaHawaiiNSLa Reunionsmean2

ComoresSt Helena

Canary ( / Cape Verde )Radial viscosity:• Postglacial rebound• Geoid modelling• Slow motion of hotspots

Steinberger & Calderwood(2006)

Temperature dependent viscosity
Causing narrow plume conduits

Lin and van Keken (2006)

Composition dependent viscosity: bridgmanite-enriched ancient mantle structures

Ballmer et al., NGeo, 2017

Cartoon view of BEAMS

Ballmer et al., NGeo, 2017

Suggested distribution:
Between dashed lines, below arrows

Integrated strain dependent viscosity
LBF=Load-bearing framework

ILLUSTRATION: C. BICKEL/SCIENCE
Illustrating Girard et al., Science, 2016

• Where Bridgmanite isinterconnected, mantle isstronger.
• However, strong shear may
lead to interconnection ofweaker Magnesiowustite

Summary• Thermal structure created through convection: slabs going down,plumes coming up.
• Chemical structure: Crust generated through melting.
Accumulated in piles on base of mantle? Entrained in plumes?
• Ancient (primordial) structure in piles? In BEAMS?
• Complicated rheology: Viscosity depth dependent, temperaturecomposition- strain(rate), strain history dependent?