Post on 26-Dec-2015
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
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
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