(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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