[SIO GP Seminars] FRIDAY: 3:15 PM: Don Anderson, Caltech

[rmatoza at ucsd.edu]

Geophysics Seminar Announcement-
Join us on Friday for this week's Geophysics Seminar.

This week, the seminar will be another joint IGPP-GSMCG seminar held at the
usual IGPP time and location:

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Friday, December 1, 3:15 PM
   (refreshments served at 3:00 PM)
   Munk Conference Room

Don Anderson, Caltech

Title: "The Smoking Gun; Continents, Mantle and Midplate Magmatism"

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ABSTRACT
The composition and growth of the continental crust have received recent
attention and the various paradoxes and surprises involved in upper
crustal composition and arc growth rates have been resolved. Rapid
recycling of the lower crust seems implicated. At the same time the
attribution of midplate volcanism to recycled oceanic crust is falling
into disfavor. A new subterranean magmatic cycle, loosely coupled to the
supercontinent cycle and the plate tectonic cycle seems to be indicated.
This represents a novel mode of mantle convection, called the Galileo
thermometer mode. It involves delamination or crustal foundering,
reheating of large fertile blobs and return to the surface on a
relatively short time scale. The scale of mantle heterogenity becomes an
important parameter in petrology and geodynamics. The chemistry and mass
balance work out if so-called melting anomalies are due to recycled
lower continental crust and subducted seamount chains. There is no mass
balance or geochemical requirement for massive return of oceanic crust
to the surface, particularly if deep subduction started in the Late
Archean. The implications of the mantle being a net sink for oceanic
crust–and possibly water and CO2– need to be investigated. The
conclusions of the present study were anticipated by Ringwood and
Green's (1966) orogenic cycle, and Daly's (1933) continental stopingand
to some extent by Arthur Holmes and George Kennedy. I use the phrase
Eclogite Engine to emphasize that the cycle is driven by phase changes,
including the upwelling, not thermal expansion. Eclogite can have low
seismic velocities, even when cold and sinking. Fertile blobs, even when
colder than ambient mantle, can be mistaken for thermal plumes. The
smoking gun for the alternative mechanism appears to have been found.