Download
Download Presentation
Loading in 2 Seconds...
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?
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.
While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Arwen Deuss, Jennifer Andrews
University of Cambridge, UK
John Woodhouse
University of Oxford, UK
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)
* reflected waves
* both continents and oceans
* converted waves
* only beneath stations
SS precursors:
* 410 and 660km
visible in all
PP precursors:
* 410km always
visible
* 660km visible
in some regions
Clear reflections
from 660 km depth
in PP precursors
(Deuss et al.,
Science, 2006)
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
Modified from http://www.mantleplumes.org
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)
Splitting of 520-km discontinuity
* more complicated than just olivine
* garnet phase change?
trace elements?
(Deuss & Woodhouse, Science, 2001)
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
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
Lehmann discontinuity: mainly negative Clapeyron slopes
(Deuss & Woodhouse, EPSL (2004))
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)
* in different regions, both continental and oceanic
* mainly in subduction zone areas related to slabs?
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?
* 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
