Structure and dynamics of earth’s lower mantle Edward J. Garnero and Allen K. McNamara Presented by: David de Vlieg Folkert van Straaten

Research on lower most mantle:  This part of the mantle has influence on the convection and chemistry of the entire mantle  It plays an important role in the heat release of the core  It has influence on thermal structure and evolution of the earth

Key scientific areas to study the lower mantle  Seismology  Mineral physics  Geodynamics  Geochemistry  to get a better insight into the lower mantle, it is important to combine these areas

Different theories to explain the lower mantle anomalies  Anomalies are caused by a  Temperature effect  Chemical effect  It is very difficult to determine how important each effect is and how they influence each other  During the remainder of the presentation we focus on the different theories explaining the properties of the lower mantle

Historical perspective lower most mantle research  Discovery of a reduced seismic velocity gradient as function of depth  This was interpreted as a lower most mantle thermal boundary layer above a hot core  1980’s: seismologists also observed a first order discontinous increase in velocity between 250 km and 350 km above the core-mantle boundary (CMB)  This was named the D” discontinuity

Anomalies in shear velocity Lower shear velocity Higher shear velocity

The D” discontinuity  D’’discontinuity does not have a specific structural characteristic, but is more a general depth shell of a few hundred kilometers  It shows a connection with subduction and Hot spot regions above it  This can be used as an argument for total mantle convection  Convergent plate boundaries overlie D″ regions with higher than average velocities  hot-spot volcanoes overlie D″ regions with lower than average velocities.  combined with evidence for high P- and S-wave velocities mimicking subduction slab shapes

The LLSVP’s (large low-shear- velocity province’s)  Below Africa and the Pacific regions two broad regions of lower shear velocity and higher than average density are observed  African region is ca km across and 1000 km high  Pacific region is ca km across and 500 km high  Both show sharp edges with normal mantle

What are these LLSVP’s?  No agreement  Geodynamical view: Higher density material will go to upwelling regions by convection  LLSVP’s have stable densities  Too low density will cause buoyancy  Too high density will flat out or even let the structures disappear

Other way to look at LLSVP’s  Thermochemical view: LLSVP’s are in essence superplumes in different stadia, and due to a thermochemical balance very stable  thermochemical superplumes may heat up and rise because of excess thermal buoyancy  then cool and sink due to decreased thermal buoyancy  Smaller plumes with the denser material can form at the top of these structures

 Mantle piles are piles with specific chemical properties  They are accumulated in the Pacific and African region, which are dominant upwelling centers Mantle Piles  Piles are passively swept and shaped by mantle convection  Plumes maybe originate from pile tops, in particular at peaks and ridges

Causes of this lower-mantle chemical heterogeneity  Lower mantle heterogeneity could be explained by:  remnants of primordial material  the result of chemical reaction products from the CMB  remnants of subducted oceanic material

A way to recognise the chemical properties of a pile  Piles composed of a long-lived primordial layer will likely have sharp contacts at their top surface  Piles composed of accumulated subducted material may have a rough or diffusive top

Chemistry of llsvp’s  Volcanic hot spots tend to overlie LLSVP edges rather than their interiors  consistent with edges and ridges of thermochemical piles forming in regions of return flow and initiating plumes  This is still controversial  Because numerical models of mantle convection show that plume morphologies are often more complicated than simple vertically continuous whole-mantle conduits  Further geochemical research on ocean island basalts (OIB’s) is necessary

Cause of D” discontinuity  Lateral variations in deep-mantle temperature are expected but should be smooth  hence they do not explain a step velocity increase  D″ has interpreted as chemical dregs from subduction,  as a region of chemical reaction between the core and mantle,  Today most preferred: as a boundary between isotropic and anisotropic fabrics, or as a solid-state phase change

D” discontinuity and chemical properties of LLSVP”s (1)  D’’-discountinuity could be the result of the transition from perovskite into post-perovskite  This transitions has a positive Clapeyron curve  So when temperature increases the pressure needed for the transition must be higher

Double crossing Perovskite, Post- Perovskite From: Ferroir

D” discontinuity and chemical properties of LLSVP”s (2)  Due to this positive Clapeyron relation the discontinuity should deepen or even vanish in hot area’s  Near the core double crossing  This is not the case: Clear evidence is present for an S-wave discontinuity within the Pacific LLSVP  Proof for a different chemical composition! ( maybe higher iron content)

D” discontinuity and chemical properties of LLSVP”s (3)  Perovskite to Post perovskite: exothermic reaction  Resulting in Plume formation  Higher convection leads to lower temperatures  Lower temperatures reaction

D” discontinuity and chemical properties of LLSVP”s (4)  To determine which of the possibilities is the most probable you need to measure the discontinuities perfectly  Measuring anisotropy using horizontal and vertical components of shear waves is a way to do this

Anisotropy and measuring the D’’ discontinuity (1)  If the D’’ anisotropy is the result of the change from perovskite into post perovskite an offset of depth between the onset of the anomaly and the discontinuity is expected  This is because the preferred lattice orientation is only visible after a sufficient amount of deformation

Anisotropy and measuring the D’’ discontinuity (2)  may explain seismic observations under the central Atlantic which thought to be away from current downwellings  which there is evidence for a D″ discontinuity  but a weak seismic anisotropy

Ultra-low velocity zones (1)  Directly above the CMB  5 to 40 km thick thin patches in which P- and S-wave velocities are reduced by up to 10% and 30%, respectively  Partial melt and a density increase up to 10%

Ultra-low velocity zones (2) These ULVZ’s can be used to say something about LLSVP’s:  If the most lower mantle has an isochemical composition ULVZ’s should be the thickest in the middle of a LLSVP (hottest region)  If a LLSVP has a thermochemical structure the hottest regions should be at their edges and ULVZ’s should be the thickest here

Ultra-low velocity zones (3)  Most proof that llsvp’s have a thermochemical structure instead of a isochemical structure

Thank you for listening  Are there still questions?