Constraints on the Constraints on the observation of mantle plumes observation of mantle plumes

using global seismology using global seismology

Arwen DeussArwen DeussUniversity of Cambridge, UKUniversity of Cambridge, UK

Mantle plumes…?Mantle plumes…?

* tomography: narrow plumes?* tomography: narrow plumes? super plumes?super plumes? flattening at thermal boundary layers?flattening at thermal boundary layers? density?density? temperature, melting? temperature, melting?

* discontinuities: plumes in the transition zone?* discontinuities: plumes in the transition zone? structure at the CMB?structure at the CMB?

Seismologists study: velocity, anisotropy, Seismologists study: velocity, anisotropy,

attenuationattenuation

Body wave dataBody wave data

* sensitive to velocity* sensitive to velocity

* small scale structure* small scale structure

* limited global coverage* limited global coverage

* tomography and discontinuities* tomography and discontinuities

temperature and composition?

Normal mode dataNormal mode dataSurface patterns

Radial patterns

* sensitive to velocity and density

* large scale structure

* good global coverage

temperature and composition!

Tomographic modelsTomographic models* mostly: velocity models using P or S body waves* mostly: velocity models using P or S body waves

plumes or not?plumes or not? resolution?resolution? which kernels to use?which kernels to use?

* few: velocity and density models using normal* few: velocity and density models using normal modesmodes

are plumes bouyant and hot?are plumes bouyant and hot? full model space search?full model space search?

Global tomographyGlobal tomography

Shear wave velocity modelS20RTS:

* body waves* surface waves* normal mode splitting functions

Ritsema, van Heijst & Woodhouse (1999)

Tomography S20RTSTomography S20RTS

* complex low shear wave velocity structure beneath Africa extending from CMB

Ritsema et al. 1999S waves and modes

Tomography S20RTSTomography S20RTS

Ritsema et al. 1999

* low velocity anomaly below Iceland is confined to the upper mantle

BUT: are these hot and bouyant?

Tomography modesTomography modesTrampert et al. 2004

* super plumes are dense and compositional in origin …

670-1200 km

1200-2000 km

2000-2891 km

Density models

DataData

Data: 7018 traces

* 6.0 < Mw < 7.0* 100 < distance < 160* depth < 75 km

Robustness of reflectionsRobustness of reflections

Stack for North America

(Deuss & Woodhouse, GRL, 2002)

220

800

1050

1150

410

520

660

Transition zone discontinuitiesTransition zone discontinuities

* discontinuities are caused by olivine phase changes

thin transition zone in hotspot/plume regions

DiscontinuitiesDiscontinuitiesDeuss et al. 2005

* using SS precursors, only large scale structure

* thin transition zone, correlation with hotspots/plumes?

Discontinuities HotspotsDiscontinuities Hotspots

Deuss, 2005

* hotspots with deep or CMB origin (Courtillot et al. 2003, Montelli et al. 2004)

* only Afar, Ascension and Tristan have thinner TZ than average

* Azores, Hawaii and Reunion have thicker TZ than average

Discontinuities Mineral PhysicsDiscontinuities Mineral Physics

BUT:BUT:

* at high temperature, not olivine but garnet has the dominant* at high temperature, not olivine but garnet has the dominant

phase transition at 660 km… phase transition at 660 km… (Weidner & Wang, 1998)(Weidner & Wang, 1998)

(Hirose, 2002)

ConclusionsConclusions

* global seismology is very useful in searching for mantle * global seismology is very useful in searching for mantle plumesplumes

* seismology only provides tomographic velocity models,* seismology only provides tomographic velocity models, and (?) large scale density modelsand (?) large scale density models

* mineral physics is very important for interpreting the * mineral physics is very important for interpreting the resultsresults

* at the moment, there is not enough evidence (yet) to * at the moment, there is not enough evidence (yet) to interpret if mantle plumes are observed in seismologyinterpret if mantle plumes are observed in seismology

Multi-disciplinary approach ….Multi-disciplinary approach ….

van der Hilst, 2004