Constraints on the observation of mantle plumes using global seismology Arwen Deuss University of Cambridge, UK

Mantle 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? structure at the CMB? structure at the CMB? Seismologists study: velocity, anisotropy, attenuation attenuation

Body wave data * sensitive to velocity * small scale structure * limited global coverage * tomography and discontinuities temperature and composition?

Normal mode data Surface patterns Radial patterns * sensitive to velocity and density * large scale structure * good global coverage temperature and composition!

Tomographic models * 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 modes modes are plumes bouyant and hot? are plumes bouyant and hot? full model space search? full model space search?

Global tomography Shear wave velocity model S20RTS: * body waves * surface waves * normal mode splitting functions Ritsema, van Heijst & Woodhouse (1999)

Tomography S20RTS * complex low shear wave velocity structure beneath Africa extending from CMB Ritsema et al. 1999S waves and modes

Tomography S20RTS Ritsema et al * low velocity anomaly below Iceland is confined to the upper mantle BUT: are these hot and bouyant?

Tomography modes Trampert et al * super plumes are dense and compositional in origin … km km km Density models

Data Data: 7018 traces * 6.0 < Mw < 7.0 * 100 < distance < 160 * depth < 75 km

Robustness of reflections Stack for North America (Deuss & Woodhouse, GRL, 2002)

Transition zone discontinuities * discontinuities are caused by olivine phase changes thin transition zone in hotspot/plume regions

Discontinuities Deuss et al * using SS precursors, only large scale structure * thin transition zone, correlation with hotspots/plumes?

Discontinuities 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 Physics BUT: * at high temperature, not olivine but garnet has the dominant phase transition at 660 km… (Weidner & Wang, 1998) phase transition at 660 km… (Weidner & Wang, 1998) (Hirose, 2002)

Conclusions * global seismology is very useful in searching for mantle plumes * seismology only provides tomographic velocity models, and (?) large scale density models and (?) large scale density models * mineral physics is very important for interpreting the results * at the moment, there is not enough evidence (yet) to interpret if mantle plumes are observed in seismology

Multi-disciplinary approach …. van der Hilst, 2004