Structure and dynamics of earth’s lower mantleEdward J. Garnero and Allen K. McNamara

Presented by:

David de VliegFolkert 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. 15000 km across and 1000 km high

Pacific region is ca. 15000 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?