Transcript Structure and dunamics of earth’s lower mantle

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-shearvelocity 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

Mantle piles are piles with
specific chemical
properties

They are accumulated in
the Pacific and African
region, which are
dominant upwelling
centers

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, PostPerovskite
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 Pand S-wave velocities are reduced by up
to 10% and 30%, respectively
 Partial
10%
melt and a density increase up to
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?