Mantle plumes as presently imaged by seismic tomography. Barbara Romanowicz 1,2

Transcription

1 Mantle plumes as presently imaged by seismic tomography Barbara Romanowicz 1,2 1 Collège de France, Paris 2 Univ. of California, Berkeley Contributors: V. Lekic, S. French, S. Cottaar, Kaiqing Yuan Collège de France, June 26 th, 2018

2 Hot spots and mantle plumes 76 Ma T. Wilson, 1963 Morgan, 1971

3 Isotopic composition of trace elements (Nd-Sr) Hotspot volcanoes Workman, 2005

4 Griffiths and Campbell, 1990 Depth (km) Temperature ( o C) Courtesy of Cinzia Farnetani

5 Mantle Plumes exist!! As currently resolved by global seismic tomography: Rooted at the core-mantle boundary Clustered within the large low shear velocity provinces of the lower mantle Not purely thermal Thermo-chemical or Partially molten and/or involve a non-newtonian rheology for the lower mantle Their morphology highlights the more vigorous convection in the upper 1000 km of the mantle and a very sluggish lower mantle

6 Imaging mantle plumes: Challenge nr 1 Principle of travel time tomography Hawaii mantle core

7 Imaging mantle plumes: Challenge nr 2 Low velocity regions of relatively small size are hidden from view when considering only first arriving waves 1 2 Low velocity body 2 1 1

8 Iceland Hotspot and Plume R. Allen, Nolet, Morgan et al., 2002 Absolute w/r 4.5 km/s Relative velocities

9 P wave travel time tomography Depth (km) Montelli, Nolet, Dahlen et al., 2004, Science

10 Nolet, Allen and Zhao., 2005 P wave travel time tomography

11 P wave travel time tomography Hawaiian plume (1) (1) (2) (2) mantle core mantle Wolfe, Solomon et al., 2011

12 Long wavelength global shear velocity models Voting map Lekic et al., 2012

13 Shear velocity Depth = 2800 km LLSVP, or Superplumes S362ANI SAW24B16 S20RTS Paleo-pole locations (Besse and Courtillot, 2002)

14 Degree 2 pattern in the mantle Hotspot distribution Shear attenuation in the transition zone Shear velocity at 2800 km depth Dziewonski, Lekic and Romanowicz., 2010

15 S40RTS: Ritsema, Deuss et al, 2011 S40RTS

16 Full Waveform Tomography Data Synthetics 3D Synthetics: seismic wavefield in complex 3D Earth models : now can be computed accurately using the Spectral Element Method (SEM) and directly compared to observed full seismograms UC B e r k e l e y - Challenges: computational time increases as ω 3 - Thin slow layers in crust - Several hundred of events, non-linear (iterations)

17 S40RTS: Ritsema et al, 2011 S40RTS

18 Full waveform tomography SEMUCB_WM1 S40RTS French and Romanowicz, Nature, 2015

19 Whole mantle model SEMUCB_WM1 Pacific superswell region French and Romanowicz, 2015 Model: SEMUCB_WM1

20 Atlantic Plumes French and Romanowicz, 2015 Rickers et al., 2012

21 These broad plumes are found under major hotspots that lie over the LLSVPs SEMUCB-WM1 at 2800 km depth French and Romanowicz, 2015 f r e n c Other hotspots according to Steinberger (2000)

22 Cape Verde These plumes are broad Canary MacDonald St Helena

23 -> Quasi-vertical from the CMB to ~ 1000 km depth -> Often laterally offset from corresponding hotspots Canary Cape Verde Depth= 1000 km MacDonald St Helena

24 - Denser basal layer - Viscosity varies with depth and temperature: - factor of 10 jump at 660 km - hot upwellings 2 orders of magnitude less viscous than background

25 Slabs trapped at 1000 km western Java ±1.2% ±1.2% eastern Java Fukao and Obayashi (2013)

26 ->The plumes are rooted in (mostly) isolated patches of very low shear velocity Cape Verde Canary MacDonald St Helena

27 SEMUCB_WM1 Depth = 2800 km Hawaian plume viewed from South East Hawaii Cottaar and Romanowicz, 2012 ULVZ: Height: ~20-25km Diameter:~910 km Velocity reduction:~20% SEMUCB_WM1

28 Detection of an ultra low velocity zone at the base of the mantle Observation Filter: s Prediction SH diff, degrees azimuth (dg) time around S diff (s) ULVZ: Height: ~25km Diameter:~800 km Velocity reduction:~20% Cottaar and Romanowicz, 2012, EPSL

29 ULVZ at the base of the Iceland Plume Yuan and Romanowicz, 2017, Science

30 (A) Event 1, 12.0km, (B) (C) (D) x2e 06 m/s SH Data 20 x3.6e 06 m/s SH 3D 20 x3.6e 06 m/s SH SEMUCB Azimuth [ ] (B) (B)(C) (D) t [s] x2e 06 m/s SH Data x2e 06 x3.6e 06 m/s m/s SH Data SH 3D x3.6e 06 m/s t [s] SH SEMUCB t [s] Azimuth [ ] Azimuth [ ] t [s] t [s] t [s] t [s] Yuan and Romanowicz, 2017

31 (A) Event 1, 12.0km, (B) (C) (D) x2e 06 m/s SH Data 20 x3.6e 06 m/s SH 3D 20 x3.6e 06 m/s SH SEMUCB Azimuth [ ] ULVZ: 30 Circular base Height: 35 ~15km Diameter:~800 km Velocity 40 reduction:~25% 20 (B) (A) (B)(C) (D) t [s] (B) t [s] (C) t [s] (D) x2e 06 m/s Event 1, 12.0km, SH Data x2e 06 x3.6e 06 m/s m/s SH Data SH 3D x3.6e 06 x2e 06 m/s m/s SH SEMUCB SH Data 20 x3.6e 06 m/s SH 3D 20 x3.6e 06 m/s SH SE Azimuth [ ] Azimuth [ ] Azimuth [ ] t [s] t [s] t [s] t [s] t [s] t [s] t [s] Yuan and Romanowicz, 2017 => Roots of fat plumes likely contain partial melt

32 Yellowstone SEMUCB_WM1 Nelson and Grand,2018 French and Romanowicz, 2015

33 u Mantle plumes do exist!! Conclusions u The major upwelling flow is in the form of ~ 25 broad low shear velocity that are located over the large low shear velocity provinces (LLSVPs) u They extend quasi-vertically from the CMB to ~1000 km depth -> very sluggish circulation in the lower mantle u Horizontally deflected around ~1000 km depth and likely entrained into secondary scale, more vigorous, upper mantle circulation u The roots of at least some of these fat plumes contain large, axi-symmetric ULVZ s, likely dense and containing partial melt u They may be the manifestation of strong topography at the top of an otherwise very thin core-mantle interaction zone of dense partial melt u LLSVP s may not be piles extending to mid-lower mantle depths, but rather bundles of fat plumes in a halo of hotter than average background.