Probing Earth’s deep interior using mantle discontinuities
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Transcript of Probing Earth’s deep interior using mantle discontinuities
Probing Earth’s deep interior Probing Earth’s deep interior using mantle discontinuitiesusing mantle discontinuities
Arwen DeussArwen DeussUniversity of Cambridge, UKUniversity of Cambridge, UK
also: Jennifer Andrews, Kit Chambers, Simon Redfern, John Woodhousealso: Jennifer Andrews, Kit Chambers, Simon Redfern, John Woodhouse
Global tomographyGlobal tomography
Velocity heterogeneity inthe 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 discontinuitiesMantle discontinuities
mineral physics
Seismology
seismology
(Deuss & Woodhouse, GRL, 2002)
HistoryHistory
Number of papers per depth, since 1959
QuestionsQuestions
(1) what is the nature of the Lehmann discontinuity(1) what is the nature of the Lehmann discontinuity
at 220 km depth?at 220 km depth?
(2) what happens in the transition zone and(2) what happens in the transition zone and
how much thermal vs. chemical heterogeneity?how much thermal vs. chemical heterogeneity?
(3) are there discontinuities in the lower mantle?(3) are there discontinuities in the lower mantle?
DataData
Data: 7018 traces
* 6.0 < Mw < 7.0* 100 < distance < 160* depth < 75 km
Global data coverageGlobal data coverage
SS-wave bounce points
Complete data setComplete data set
* synthetics for PREM: discontinuities at 220, 400 and 670 km depth
*complete data set: discontinuities at 410, 520 and 660 km depth
Robustness of reflectionsRobustness of reflections
Stack for North America
(Deuss & Woodhouse, GRL, 2002)
220
800
1050
1150
410
520
660
Robustness of reflectionsRobustness of reflectionsStack for Indonesia
(Deuss & Woodhouse, GRL, 2002)
220
10501150
410
660
520
Reflections per depthReflections per depth
* clear reflections from transition* clear reflections from transition
zone discontinuities at 410 andzone discontinuities at 410 and
660 km depth660 km depth
* additional discontinuities in* additional discontinuities in
upper and lower mantle atupper and lower mantle at
220, 260, 310 and 800 km depth220, 260, 310 and 800 km depth
(Deuss & Woodhouse, GRL 2002)(Deuss & Woodhouse, GRL 2002)
QuestionsQuestions
(1) what is the nature of the Lehmann discontinuity(1) what is the nature of the Lehmann discontinuity
at 220 km depth?at 220 km depth?
(2) what happens in the transition zone and(2) what happens in the transition zone and
how much thermal vs. chemical heterogeneity?how much thermal vs. chemical heterogeneity?
(3) are there discontinuities in the lower mantle?(3) are there discontinuities in the lower mantle?
Upper mantle reflectorsUpper mantle reflectors
(Deuss & Woodhouse, GRL, 2002)
Mantle discontinuities - Mineral physicsMantle discontinuities - Mineral physics
(Deuss & Woodhouse, EPSL, 2004)
Seismological observations Clapeyron Slopes
Mineral physical mechanismsMineral physical mechanisms
Phase transitionsPhase transitions::* Coesite –Stishovite, * Coesite –Stishovite, 250-300 km depth, dP/dT=2.5-3.1250-300 km depth, dP/dT=2.5-3.1* Orthoenstatite – High clinoenstatite, * Orthoenstatite – High clinoenstatite, 250-300 km depth, dP/dT=1.4250-300 km depth, dP/dT=1.4
Change in deformation mechanismChange in deformation mechanism::* Dislocation-diffusion creep* Dislocation-diffusion creep dry: 340-380 km depth, dP/dT=-2.4dry: 340-380 km depth, dP/dT=-2.4 wet: 240-280 km depth, dP/dT=-2.4 wet: 240-280 km depth, dP/dT=-2.4 Karato (1993)Karato (1993)
QuestionsQuestions
(1) what is the nature of the Lehmann discontinuity(1) what is the nature of the Lehmann discontinuity
at 220 km depth?at 220 km depth?
(2) what happens in the transition zone and(2) what happens in the transition zone and
how much thermal vs. chemical heterogeneity?how much thermal vs. chemical heterogeneity?
(3) are there discontinuities in the lower mantle?(3) are there discontinuities in the lower mantle?
Transition zone structureTransition zone structure
410km topography410km topography
(Chambers, Deuss & Woodhouse, EPSL, 2005)
520-km discontinuity Observations520-km discontinuity Observations
(Deuss & Woodhouse, Science, 2001)
Splitting of 520-km discontinuity
* more complicated than just olivine* garnet phase change? trace elements?
Splitting observations Splitting observations
520 km discontinuity
* no correlation with tectonic features
Phase transitions: 520 km discontinuityPhase transitions: 520 km discontinuity
Pyrolite phase diagram
* high Fe-content: no transition
* wet conditions: much sharper
* low Ca-content: no gt-CaPv transition
WKBJ syntheticsWKBJ synthetics
Two reflectors can indeed be observed!
Regional stacks Transition zoneRegional stacks Transition zone
SS precursors:
* 410 and 660km visible in all
PP precursors:
* 410km always visible
* 660km visible in some regions
660-km discontinuity Observations660-km discontinuity Observations
Clear reflectionsfrom 660 km depthin PP precursors
(Deuss et al., Science, 2006)
660-km discontinuity Observations660-km discontinuity Observations
Long period:single peaks
Short period:double peaks
Mineral physics: 660 km discontinuityMineral physics: 660 km discontinuity
For pyrolite mantlecomposition(after Hirose, 2001)
Seismic amplitudesSeismic amplitudes
Variations in amplitudes are consistent with the pyrolite model (using Weidner & Wang, 1998)
QuestionsQuestions
(1) what is the nature of the Lehmann discontinuity(1) what is the nature of the Lehmann discontinuity
at 220 km depth?at 220 km depth?
(2) what happens in the transition zone and(2) what happens in the transition zone and
how much thermal vs. chemical heterogeneity?how much thermal vs. chemical heterogeneity?
(3) are there discontinuities in the lower mantle?(3) are there discontinuities in the lower mantle?
Lower mantle DataLower mantle Data
* reflections around 800km and 1000-1200km
Lower mantle 800-900kmLower mantle 800-900km
* in different regions, both continental and oceanic
Lower mantle 1000-1200 kmLower mantle 1000-1200 km
* mainly in subduction zone areas related to slabs?
Lower mantle – Mineral physicsLower mantle – Mineral physics
Phase transitionsPhase transitions
* stishovite -> CaCl2-type (in SiO* stishovite -> CaCl2-type (in SiO22) ) free silica?free silica?
* (Mg,Fe)SiO* (Mg,Fe)SiO33 perovskite, perovskite,
orthorhombic -> cubic phase orthorhombic -> cubic phase unlikely!unlikely!
OthersOthers
* change in chemical composition?* change in chemical composition?
* change in deformation mechanism?* change in deformation mechanism?
What next? Fresnel zonesWhat next? Fresnel zones
Dahlen, 2003
ConclusionsConclusions
* comparison of seismic observations of mantle * comparison of seismic observations of mantle discontinuities with mineral physics implies significant discontinuities with mineral physics implies significant amount of chemical heterogeneityamount of chemical heterogeneity
* important implications for mantle flow* important implications for mantle flow
* we need to expand to other data types and implement * we need to expand to other data types and implement new techniques (such as finite frequency kernels) to new techniques (such as finite frequency kernels) to further understand the level of heterogeneityfurther understand the level of heterogeneity