The role of mantle plumes in the Earth's

The role of mantle plumes in the Earth's

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) 44 TW (observed) ~8 TW

space upper mantle lower mantle D core

2+3 TW 44-13=31 TW Fluxing 31 TW through the 670 discontinuity 8-15 TW 16-23 TW

cold hot How much of that is carried by plumes? Plume flux from surface observations: w

Davies, 1998 m Buoyancy flux B measured from swell elevation e B = e width vplate = Cp Qc Observed B indicates low plume flux (~3TW)

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 to Azores PRI-P05 PRI-S05

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) (b)

(c) (d) PRI-P05 Zhao et al., 2004 PRI-S05 Ritsema et al., 1999

PRI-P05 PRI-S05 Richard Allen Upper Mantle only

CMB origin Bottom line: Plumes are obese (or we would not see them), with Tmax =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. Can we quantify that qualitative notion? The plume contains: H = cPT d3x

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, 1600 km Tahiti: rise velocity underestimated by factor of 4

Vz from actual tomogram Vz from resolution test image For wider plume ( T> 110K) vz underestimated by factor 3 Tahiti, 1600 km

Then the real earth vz must have been close to here observed If the earth vz shows up here in the tomographic image

reduction in tomography and this is the resolving error factor But what parameters to use at depth?

QuickTime and a TIFF (LZW) decompressor are needed to see this picture. 6 1022Pa s Forte & Mitrovica , 2001

Lithgow-Bertelloni & Richards, 1995 Tahiti estimated heat flux as function of depth 70 110 150

= well resolved values, corrected for bias 700 km 1500 km Tahiti

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 delayed or escape at 670?

mantle not adiabatic heat diffusion, entrainment B = B Cp Qc/ 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 much

mass into the lower mantle as plumes return to the upper mantle. There is no other mass flux through the 670 discontinuity

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