Delaminated lower continental crust is an overlooked
source of mantle heterogeneity. Removal of dense garnet
clinopyroxenite cumulates–eclogite–explains
the sharp cutoff in crustal thickness at 50-km and
a variety of other geological and tectonic phenomena,
including uplift, magmatism and crustal composition.
Eclogite is dense but has lower shear velocities and
melting points than peridotite. Arc eclogites differ
from subducted oceanic crust and equilibrate at different
depths in the mantle; in particular, they can explain
the low-velocity zones (LVZs) found near 200-km and
above the 410 and 650-km discontinuities. Delaminated
eclogite starts out warmer than subducted oceanic
crust and melts sooner; it becomes neutrally buoyant
at depths as shallow as 200-km. The volumes of eclogite
placed in the mantle by this mechanism are comparable
to the total hotspot flux. The oceanic plateaus in
the Atlantic and Indian oceans mostly formed about
1000-km offshore, and about 45 Myr after continental
breakup. These are interpreted as the re-emergence
of delaminated crustal roots. The DUPAL anomaly has
the same origin. This mechanism for fertilizing the
mantle and creating melting point and lithologic heterogeneity
works at normal mantle temperatures and avoids the
various thermal and geochemical paradoxes associated
with thermal upwellings and oceanic crust recycling.
In fact, if current plate tectonics has operated for
1 Gyr, little of the oceanic crust need be recycled;
it can be stored in a layer only 100-km thick in the
transition region. It is more likely that delaminated
continental crust, stored in the upper mantle and
the top of the transition region, is recycled. It
is reheated by ambient mantle temperature and returns
to the crust by underplating or accretion of oceanic
plateaus. Delamination causes crustal uplift, upwelling
and decompression of the asthenosphere, and creates
low-velocity zones in the underlying mantle. The upper
mantle LVZs found under some continental volcanic
provinces may be, in part, cold dense eclogite sinkers
and passive upwelling asthenosphere, rather than hot
deep upwellings. Thus, both ends of the delamination
cycle can be associated with intraplate volcanism,
and are the work of the Eclogite Engine.
MECHANISMS