Geoscience Reference
In-Depth Information
Chapter 5
The eclogite engine
The World's great age begins anew,
The golden years return,
The Earth doth like a snake renew
Her winter weeds outgrown
being recognized that large blocks of eclogite in
the mantle may be an important fertility source.
Delaminated continental crust differs in many
important respects from recycled MORB. It does
not go through subduction zone and seafloor pro-
cessing, it starts out hotter than MORB, it may
occur in bigger blobs, and it is not accompanied
by the same amount, if any, of buoyant infer-
tile harzburgite. Figure 5.1 illustrates the lower
crust delamination cycle. The crust thickens by
tectonic or igneous processes, eventually forming
dense eclogite that detaches and sinks into the
mantle. It reaches a level of neutral buoyancy and
starts to warm up. Eventually it rises and forms a
warm fertile patch in the mantle. If the overlying
continents have moved off, a midplate magmatic
province is the result.
Thermal expansion is the main source of
buoyancy in thermal convection of simple flu-
ids. Phase changes can be more important in
the mantle. When basalt converts to eclogite,
or when it melts, there are changes in density
that far exceed those associated with thermal
expansion. Removal of the dense lower continen-
tal crust is an important element of plate tecton-
ics that complements normal subduction zone
and ridge processes. It causes uplift and magma-
tism and introduces distinctive materials into the
mantle that are dense, fertile and have low melt-
ing points. After delamination, eclogitic lower
crust sinks into the mantle where it has rela-
tively low seismic velocities and melting point
(Figures 5.2 and 5.4) compared with normal man-
tle peridotite. These fertile mafic blobs sink to
various depths where they warm up, melt and
Shelley
The water cycle drives geological processes at
the surface. The fact that water coexists as fluid,
vapor and solid is crucial in shaping the Earth's
surface. The fact that conditions in the upper
mantle can readily convert eclogite to magma to
basalt, and back, with enormous density changes,
is crucial in global magmatism and tectonics.
Phase changes in the mafic components of the
upper mantle are larger than thermal expansion
effects and they drive the
eclogite engine
.
There are several ways to generate massive
melting in the mantle; one is to bring hot mate-
rial adiabatically up from depth until it melts;
the other is to insert low-melting point fertile
material -- delaminated lower arc-crust, for exam-
ple -- into the mantle from above and allow the
mantle to heat it up. Both mechanisms may
be involved in the formation of large igneous
provinces -- LIPs. The timescale for heating and
recycling of lower-crust material is much less
than for subducted oceanic crust because the for-
mer starts out much hotter and does not sink as
deep.
The standard petrological models for magma
genesis involve a homogenous pyrolite mantle,
augmented at times by small-scale pyroxenite
veins or recycled oceanic crust. It is increasingly
