Heatflux in
the Earth
Goals of the Project:
To use
the tomographic images obtained with finite-frequency tomography to
estimate the heat flux carried by hot and cold plumes as well as other
geodynamical characteristics.
Principal Investigators:
Guust Nolet
Postdocs:
Raffaella
Montelli (now at Exxon-Mobil Research)
Results so far:
Plume flux is considered to be low (about
3TW) and not contributing much to the Earth's heat budget. However,
despite large uncertainties, when we determine plume fluxes across
well-resolved plume sections from seismic tomography, we find values
that are consistently higher than those determined at the surface from
buoyancy flux - even if we maximize the (rather poorly known) viscosity
of the lower mantle we find flux values about five times the values
determined by Sleep (1991) in his classic paper on buoyancy flux.
Figure 1. A comparison of plume heat flux as determined from surface
buoyancy, with flux calculated for a very viscous lower mantle
(visocity at 800 km depth 6 × 1022 Pa s,
extrapolated using an Arrhenius law) shows higher values from seismic
tomography. If one lowers the mantle viscosity by a factor of two,
tomography flux would go up by a factor of two, etc. This indicates
that the heat brought up by plumes is substantial.
In addition, we
observe a resistance for plumes near the 670 discontinuity for a number
of plumes. This resistance is very much like the one observed for
slabs: while some slabs seem to pass unimpeded into the lower mantle,
others reside - at least for a while but perhaps permanently - in the
transition zone. We show two figure that seem to illustrate the same
behaviour. Figure 2 shows how Tahiti and several other plumes spread out beneath 670 before
breaking through. Figure 3 ambivalent behaviour for four
other plumes (the
Atlantic plume seems to feed the ridge in this way!).
Figure 2.
Plumes that show a clear resistance at 670 and spread out below it
before breaking through.
Figure 3.
Plumes
that show a more ambivalent behaviour.
The
observation of high plume heat flux, combined with the observation of a
resistance at 670 indicates that mass flux through 670 may be effected
by slabs and plumes only. The
nature of the resistance at 670 is not completely clear: the jump in
viscosity, the negative Clapeyron slope, or iron enrichment of the
lower mantle may all play a role. But is seems that slabs and plumes
need the extra buoyancy from ther material above and below them,
respectively, to gain enough force to break through.
This has significant implications for
geodynamics. If only plumes and slabs can cross the 670, mass flux is
limited to the volume subducted into the upper mantle, and may be lower
than that if not all slab material ends up in the lower mantle. Even if
the slabs would sink into the lower mantle at the present rate
(somewhere between 100 and 200 km<sup>3</sup> per year),
this is barely enough to recycle all of the lower mantle over the
history of the Earth - thus providing a chemical reservoir for argon,
helium and a number of trace elements.
Publications:
Nolet, G., S.-i. Karato and R.
Montelli, Plume fluxes from
seismic tomography: Earth. Plan. Sci.
Lett., in press, 2006. PDF preprint
Collaborations:
Shun-ichiro Karato (Yale)
Nick Arndt (Grenoble)
Funding:
NSF
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