Mount Erebus(photo NASA)

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

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

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

4. Plume flux from surface observations: rw rm Davies, 1998 Buoyancy flux B measured from swell elevation e B = Dr´ e ´ width ´ vplate = a Cp Qc Observed B indicates low plume flux (~3TW)

5. DVP/VP (%) at 1000 km depth PRI-P05

6. DVP/VP (%) at 1000 km depth PRI-P05

7. DVS/VS (%) at 1000 km depth PRI-S05

8. DVS/VS (%) at 1000 km depth PRI-S05

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

11. PRI-P05 PRI-S05 Easter Island

12. PRI-P05 PRI-S05 Hawaii

13. PRI-P05 PRI-S05 Kerguelen

14. PRI-P05 PRI-S05 Tahiti

15. Tahiti: comparisons (D T) PRI-P05 Zhao et al., 2004 PRI-S05 Ritsema et al., 1999

16. PRI-P05 PRI-S05 Richard Allen

17. CMB origin Upper Mantle only

18. Bottom line: Plumes are obese (or we would not see them), with DTmax =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 =  cPT d3x Joules But we do not know how fast it rises to the surface!

20. Excursion, back to textbook physics:

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

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

23. For wider plume (D T> 110K) vz underestimated by factor 3 Tahiti, 1600 km

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

25. But what parameters to use at depth? 6 ´ 1022Pa s Forte & Mitrovica , 2001 Lithgow-Bertelloni & Richards, 1995

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

27. 700 km Tahiti 1500 km

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

29. delayed or escape at 670? mantle not adiabatic flux loss factor wB Escape into asthenosphere heat diffusion, entrainment B = wB Cp Qc/a

30. Inferred heat flux Q is too high. Possible solutions The buoyancy flux at surface underestimates Q at depth The reference viscosity 6´ 1022 Pas (at 800 km) is too low

31. Inferred heat flux Q is too high. Possible solutions The buoyancy flux at surface underestimates Q at depth The reference viscosity 6´ 1022 Pas (at 800 km) is too low Iron enrichment makes the plume heavier H2O increases dV/dT, therefore lowers DT

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