Seismological observations Seismological observations Earth’s deep interior, Earth’s deep interior,

and their geodynamical and and their geodynamical and mineral physical interpretationmineral physical interpretation

Arwen Deuss, Jennifer AndrewsArwen Deuss, Jennifer AndrewsUniversity of Cambridge, UKUniversity of Cambridge, UK

John WoodhouseJohn WoodhouseUniversity of Oxford, UKUniversity of Oxford, UK

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 discontinuitiesmineral 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 PrecursorsTransition zone Precursors

SS precursors:* 410 and 660km visible in all

PP precursors:* 410km always visible* 660km visible in some regions

660-km discontinuity Precursors660-km discontinuity Precursors

Clear reflectionsfrom 660 km depthin PP precursors

(Deuss et al., Science, 2006)

660-km discontinuity Precursors660-km discontinuity Precursors

Long period:single peaks

Short period:double peaks

Transition zone Receiver functionsTransition zone Receiver functions

* Receiver functions also show complex structure of 660km, while 410km discontinuity is simple* No 520 km discontinuity

Single peak at 660 Double peaks at 660

Mineral physics: 660 km discontinuityMineral physics: 660 km discontinuity

For pyrolite mantlecomposition(after Hirose, 2001)

Application: mantle plumesApplication: mantle plumes

Modified from http://www.mantleplumes.org

Application: mantle plumesApplication: mantle plumes

(Deuss, P4, in press, 2007)

Mantle plumes are characterised by deep 410, in combinationwith both deep or shallow 660 (dependent on temperature)

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

520-km discontinuity Precursors520-km discontinuity Precursors

(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

Mineral physics: 520 km discontinuityMineral physics: 520 km discontinuity

Pyrolite phase diagram

* high Fe-content: no transition

* wet conditions: much sharper

* low Ca-content: no gt-CaPv transition

But: there is more …But: there is more …SS precursors

Receiver functions

In addition totransition zone:

* Reflectors at 220, 260 and 320 km in the upper mantle

* Continuous range of scatterers in the lower mantle

Upper mantle PrecursorsUpper mantle Precursors

Upper mantle Clapeyron slopesUpper mantle Clapeyron slopes

(Deuss & Woodhouse, EPSL (2004))

Lehmann discontinuity: mainly negative Clapeyron slopes

Upper mantle Mineral physicsUpper mantle Mineral physics

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)

Lower mantle PrecursorsLower mantle Precursors

Stack for North America

(Deuss & Woodhouse, GRL, 2002)

220

80010501150

410520660

Lower mantle PrecursorsLower mantle PrecursorsStack for Indonesia

(Deuss & Woodhouse, GRL, 2002)

220

10501150

410

660520

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 physicsPhase 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?

* MORB heterogeneity, mechanical mixture?* MORB heterogeneity, mechanical mixture?

ConclusionsConclusions* to explain the seismic observations of transition zone * to explain the seismic observations of transition zone

discontinuities, we need phase transitions in garnet in discontinuities, we need phase transitions in garnet in addition to the olivine phase transitions (consistent with a addition to the olivine phase transitions (consistent with a pyrolite mantle model )pyrolite mantle model )

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

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

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