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