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? * also chemical/compositional heterogeneity? * lithosphere/asthenosphere boundary? * what happens in the transition zone? * where do slabs go? Ritsema, van Heijst & Woodhouse (1999)

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 Receiver functions * Receiver functions also show complex structure of 660km, while 410km discontinuity is simple * No 520 km discontinuity Single peak at 660Double peaks at 660

Mineral physics: 660 km discontinuity For pyrolite mantle composition (after Hirose, 2001)

Application: mantle plumes Modified from

Application: mantle plumes (Deuss, P 4, in press, 2007) Mantle plumes are characterised by deep 410, in combination with both deep or shallow 660 (dependent on temperature) Using SS precursors in plume locations from Courtillot et al, 2003

520-km discontinuity Precursors (Deuss & Woodhouse, Science, 2001) Splitting of 520-km discontinuity * more complicated than just olivine * garnet phase change? trace elements?

Splitting observations 520 km discontinuity * no correlation with tectonic features

Mineral 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 … SS precursors Receiver functions In addition to transition zone: * 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 (Deuss & Woodhouse, EPSL (2004)) Lehmann discontinuity: mainly negative Clapeyron slopes

Upper mantle Mineral physics Phase transitions: * Coesite –Stishovite, km depth, dP/dT= km depth, dP/dT= * Orthoenstatite – High clinoenstatite, km depth, dP/dT= km depth, dP/dT=1.4 Change in deformation mechanism: * Dislocation-diffusion creep dry: km depth, dP/dT=-2.4 dry: km depth, dP/dT=-2.4 wet: km depth, dP/dT=-2.4 Karato (1993) wet: km depth, dP/dT=-2.4 Karato (1993)

Lower mantle Precursors Stack for North America (Deuss & Woodhouse, GRL, 2002)

Lower mantle Precursors Stack for Indonesia (Deuss & Woodhouse, GRL, 2002)

Lower mantle km * in different regions, both continental and oceanic

Lower mantle km * mainly in subduction zone areas related to slabs?

Lower mantle – Mineral physics Phase transitions * stishovite -> CaCl2-type (in SiO 2 ) free silica? * (Mg,Fe)SiO 3 perovskite, orthorhombic -> cubic phase unlikely! orthorhombic -> cubic phase 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