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   Abstract

2005 AGU Fall meeting abstract

Crustal delamination, mantle heterogeneity and large igneous provinces; the eclogite engine

Don L. Anderson

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.

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