1. Mount Erebus(photo NASA) The role of mantle plumes in the Earth's heat budget Chapman Conference, August 2005 Guust Nolet With thanks to: Raffaella Montelli Shun Karato …. and NSF

2. space upper mantle lower mantle core D” 44 TW (observed) ~8 TW 2+3 TW 44-13=31 TW

3. 8-15 TW 16-23 TW cold hot Fluxing 31 TW through the 670 discontinuity How much of that is carried by plumes?

4. Plume flux from surface observations: Davies, 1998 Buoyancy flux B measured from swell elevation e B =   e  width  v plate =  C p Q c Observed B indicates low plume flux (~3TW) wmwm

5.  V P /V P (%) at 1000 km depth PRI-P05

7.  V S /V S (%) at 1000 km depth PRI-S05

10. Cape Verde to Azores PRI-P05 PRI-S05

11. Easter Island PRI-P05 PRI-S05

12. Hawaii

13. PRI-P05 PRI-S05 Kerguelen

14. PRI-P05 PRI-S05 Tahiti

15. Tahiti: comparisons (  T) (a)PRI-P05 (b)Zhao et al., 2004 (c)PRI-S05 (d)Ritsema et al., 1999

16. Richard Allen PRI-P05 PRI-S05

17. Upper Mantle only CMB origin

18. Bottom line: Plumes are obese (or we would not see them), with  T max =100-300K, Ergo: they contain a lot of calories, Either: they carry an awful lot of heat to the surface, or: they go terribly slow….

19. Can we quantify that qualitative notion? The plume contains: H =  c P  T d 3 x Joules But we do not know how fast it rises to the surface!

20. Excursion, back to textbook physics:

21. Tahiti, 1600 km,  T > 150K actual tomogram  T (>150K) output of resolution test

22. Tahiti: rise velocity underestimated by factor of 4 Tahiti, 1600 km Vz from actual tomogram Vz from resolution test image

23. For wider plume (  T> 110K) v z underestimated by factor 3 Tahiti, 1600 km

24. observed reduction in tomography and this is the resolving error factor If the earth v z shows up here in the tomographic image Then the real earth v z must have been close to here

25. But what parameters to use at depth? Forte & Mitrovica, 2001 Lithgow-Bertelloni & Richards, 1995 6  10 22 Pa s

26. 70 110 150 Tahiti estimated heat flux as function of depth = well resolved values, corrected for bias

27. Tahiti 1500 km 700 km

28. Inferred heat flux Q is too high. Possible solutions (1)The buoyancy flux at surface underestimates Q at depth

29. flux loss factor  B Escape into asthenosphere mantle not adiabatic heat diffusion, entrainment B =  B C p Q c /  delayed or escape at 670?

30. Inferred heat flux Q is too high. Possible solutions (1)The buoyancy flux at surface underestimates Q at depth (2)The reference viscosity 6  10 22 Pas (at 800 km) is too low

31. Inferred heat flux Q is too high. Possible solutions (1)The buoyancy flux at surface underestimates Q at depth (2)The reference viscosity 6  10 22 Pas (at 800 km) is too low (3)Iron enrichment makes the plume heavier (4)H 2 O increases dV/dT, therefore lowers  T

32. Conclusions -High viscosity in lower mantle makes convection there 'sluggish' at best - Large viscosity contrast points to two strongly divided convective regimes in the Earth - Large flux loss may also imply plume resistance at 670 and/or escape into asthenosphere

33. Speculations - Exchange of material between sluggish lower mantle and less viscous upper mantle is limited (most likely periodic). - Plumes may carry all of the upward flow of heat (>16TW) through the 670 km discontinuity. -The next breakthrough (flood basalt?) may be at Cape Verde/Canary Islands, Chatham or Tahiti.

34. Equal mass flux hypothesis: Over time, slabs transport as much mass into the lower mantle as plumes return to the upper mantle. There is no other mass flux through the 670 discontinuity