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Seismological observations Earth’s deep interior, and their geodynamical and mineral physical interpretation. Arwen Deuss, Jennifer Andrews University of Cambridge, UK John Woodhouse University of Oxford, UK. Global tomography. Velocity heterogeneity in the Earth: * thermal in origin?

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slide1

Seismological observations Earth’s deep interior, and their geodynamical and mineral physical interpretation

Arwen Deuss, Jennifer Andrews

University of Cambridge, UK

John Woodhouse

University of Oxford, UK

global tomography
Global tomography

Velocity heterogeneity in

the Earth:

* thermal in origin?

* also chemical/compositional

heterogeneity?

* lithosphere/asthenosphere

boundary?

* what happens in the

transition zone?

* where do slabs go?

Ritsema, van Heijst & Woodhouse (1999)

mantle discontinuities
Mantle discontinuities

mineral physics

seismology

Seismology

(Deuss & Woodhouse, GRL, 2002)

two different data types
Two different data types …

* reflected waves

* both continents and oceans

* converted waves

* only beneath stations

transition zone precursors
Transition zone Precursors

SS precursors:

* 410 and 660km

visible in all

PP precursors:

* 410km always

visible

* 660km visible

in some regions

660 km discontinuity precursors
660-km discontinuity Precursors

Clear reflections

from 660 km depth

in PP precursors

(Deuss et al.,

Science, 2006)

660 km discontinuity precursors1
660-km discontinuity Precursors

Long period:

single peaks

Short period:

double peaks

transition zone receiver functions
Transition zone Receiver functions

Single peak at 660

Double peaks at 660

* Receiver functions also show complex structure of 660km,

while 410km discontinuity is simple

* No 520 km discontinuity

mineral physics 660 km discontinuity
Mineral physics: 660 km discontinuity

For pyrolite mantle

composition

(after Hirose, 2001)

application mantle plumes
Application: mantle plumes

Modified from http://www.mantleplumes.org

application mantle plumes1
Application: mantle plumes

Using SS precursors in plume locations from Courtillot et al, 2003

(Deuss, P4, in press, 2007)

Mantle plumes are characterised by deep 410, in combination

with both deep or shallow 660 (dependent on temperature)

520 km discontinuity precursors
520-km discontinuity Precursors

Splitting of 520-km discontinuity

* more complicated than just olivine

* garnet phase change?

trace elements?

(Deuss & Woodhouse, Science, 2001)

splitting observations
Splitting observations

520 km discontinuity

* no correlation with tectonic features

mineral physics 520 km discontinuity
Mineral physics: 520 km discontinuity

Pyrolite phase diagram

b

a

g

* high Fe-content:

no b-g transition

* wet conditions:

b-g much sharper

* low Ca-content:

no gt-CaPv transition

but there is more
But: there is more …

SS precursors

In addition to

transition zone:

* Reflectors at 220,

260 and 320 km in

the upper mantle

* Continuous range

of scatterers in

the lower mantle

Receiver functions

upper mantle clapeyron slopes
Upper mantle Clapeyron slopes

Lehmann discontinuity: mainly negative Clapeyron slopes

(Deuss & Woodhouse, EPSL (2004))

upper mantle mineral physics
Upper mantle Mineral physics

Phase transitions:

* Coesite –Stishovite,

250-300 km depth, dP/dT=2.5-3.1

* Orthoenstatite – High clinoenstatite,

250-300 km depth, dP/dT=1.4

Change in deformation mechanism:

* Dislocation-diffusion creep

dry: 340-380 km depth, dP/dT=-2.4

wet: 240-280 km depth, dP/dT=-2.4 Karato (1993)

lower mantle precursors
Lower mantle Precursors

Stack for North America

220

410

520

660

800

1050

1150

(Deuss & Woodhouse, GRL, 2002)

lower mantle precursors1
Lower mantle Precursors

Stack for Indonesia

220

410

520

660

1050

1150

(Deuss & Woodhouse, GRL, 2002)

lower mantle 800 900km
Lower mantle 800-900km

* in different regions, both continental and oceanic

lower mantle 1000 1200 km
Lower mantle 1000-1200 km

* mainly in subduction zone areas related to slabs?

lower mantle mineral physics
Lower mantle – Mineral physics

Phase transitions

* stishovite -> CaCl2-type (in SiO2) free silica?

* (Mg,Fe)SiO3 perovskite,

orthorhombic -> cubic phase unlikely!

Others

* change in chemical composition?

* change in deformation mechanism?

* MORB heterogeneity, mechanical mixture?

conclusions
Conclusions

* to explain the seismic observations of transition zone discontinuities, we need phase transitions in garnet in addition to the olivine phase transitions (consistent with a pyrolite mantle model )

* lateral variations in minor elements are also required, which will influence slab penetration and upwelling of mantle plumes differently from region to region

* significant amount of seismic scatterers in upper and lower mantle, without a mineral physical explanation in the lower mantle

* focus research towards discoveries in mineral physics, i.e. discontinuities in attenuation, free silica lower mantle, mechanical mixture vs. equilibrium