Transcript arwen_deuss_Chris_Fest

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:
Ritsema, van Heijst & Woodhouse (1999)
* thermal in origin?
* also chemical/compositional
heterogeneity?
* lithosphere/asthenosphere
boundary?
* what happens in the
transition zone?
* where do slabs go?
Mantle discontinuities
mineral physics
seismology
Seismology
(Deuss & Woodhouse, GRL, 2002)
Two different data types …
* reflected waves
* both continents and oceans
* converted waves
* only beneath stations
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
Clear reflections
from 660 km depth
in PP precursors
(Deuss et al.,
Science, 2006)
660-km discontinuity
Precursors
Long period:
single peaks
Short period:
double peaks
Transition zone
Single peak at 660
Receiver functions
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
For pyrolite mantle
composition
(after Hirose, 2001)
Application: mantle plumes
Modified from http://www.mantleplumes.org
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
Splitting of 520-km discontinuity
* more complicated than just olivine
* garnet phase change?
trace elements?
(Deuss & Woodhouse, Science, 2001)
Splitting observations
520 km discontinuity
* no correlation with tectonic features
Mineral physics: 520 km discontinuity
Pyrolite phase diagram
a
b
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 …
SS precursors
In addition to
transition zone:
Receiver functions
* Reflectors at 220,
260 and 320 km in
the upper mantle
* Continuous range
of scatterers in
the lower mantle
Upper mantle
Precursors
Upper mantle
Clapeyron slopes
Lehmann discontinuity: mainly negative Clapeyron slopes
(Deuss & Woodhouse, EPSL (2004))
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
Stack for North America
220
410
520
660
800
1050
1150
(Deuss & Woodhouse, GRL, 2002)
Lower mantle
Precursors
Stack for Indonesia
220
410
520
660
1050
1150
(Deuss & Woodhouse, GRL, 2002)
Lower mantle
800-900km
* in different regions, both continental and oceanic
Lower mantle
1000-1200 km
* mainly in subduction zone areas
related to slabs?
Lower mantle – Mineral physics
Phase transitions
* stishovite -> CaCl2-type (in SiO2)
* (Mg,Fe)SiO3 perovskite,
orthorhombic -> cubic phase
free silica?
unlikely!
Others
* change in chemical composition?
* change in deformation mechanism?
* MORB heterogeneity, mechanical mixture?
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