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

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

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

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

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

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

Cape Verde to Azores PRI-P05 PRI-S05

Easter Island PRI-P05 PRI-S05

Hawaii

PRI-P05 PRI-S05 Kerguelen

PRI-P05 PRI-S05 Tahiti

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

Richard Allen PRI-P05 PRI-S05

Upper Mantle only CMB origin

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

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!

Excursion, back to textbook physics:

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

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

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

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

But what parameters to use at depth? Forte & Mitrovica, 2001 Lithgow-Bertelloni & Richards,  Pa s

Tahiti estimated heat flux as function of depth = well resolved values, corrected for bias

Tahiti 1500 km 700 km

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

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

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

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

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

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.

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