mount erebus(photo nasa) the role of mantle plumes in the earth's heat budget chapman...
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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