Mount Erebus(photo NASA)

The role of mantle plumes in theEarth'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 TW16-23 TW coldhot

Fluxing 31 TW through the 670 discontinuity

How much of that is carried byplumes?

Plume flux from surface observations:

Davie

s, 1

99

8

Buoyancy flux B measured from swell elevation eB = e width vplate = Cp Qc

Observed B indicates low plume flux (~3TW)

w

m

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

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

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

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

Cape Verde toAzores

PRI-P05 PRI-S05

Easter IslandPRI-P05 PRI-S05

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

Rich

ard

Alle

nPRI-P05 PRI-S05

Upper Mantleonly

CMBorigin

Bottom line:

Plumes are obese (or we wouldnot see them), with Tmax =100-300K,

Ergo: they contain a lot of calories,

Either: they carry an awful lot of heatto the surface,or: they go terribly slow….

Can we quantify that qualitative notion?

The plume contains:

H = cPT d3x Joules

But we do not know how fastit 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) vz underestimated by factor 3

Tahiti, 1600 km

observed

reductionin tomography and this is the

resolving errorfactor

If the earth vz showsup here in thetomographic image

Then the realearth vz must havebeen close to here

But what parameters to use at depth?

Forte & Mitrovica , 2001Lithgow-Bertelloni & Richards, 1995

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

6 1022Pa s

70110

150

Tahiti estimated heat flux as function of depth

= well resolved values,corrected for bias

Tahiti1500 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 Cp Qc/

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 1022 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 1022 Pas (at 800 km) is too low

(3) Iron enrichment makes the plume heavier

(4) H2O 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 muchmass into the lower mantle as plumesreturn to the upper mantle.There is no other mass flux throughthe 670 discontinuity