The eclogite engine: Chemical geodynamics as a Galileo thermometer
Published:January 01, 2007
Migrating and incipient ridges and triple junctions sample the heterogeneous mantle created by plate tectonics and crustal stoping. The result is a yo-yo vertical convection mode that fertilizes, cools, and removes heat from the mantle. This mode of mantle convection is similar to the operation of a Galileo thermometer (GT).1 The GT mode of small-scale convection, as applied to the mantle, differs from the Rayleigh-Taylor (RT) instability of a homogeneous fluid in a thermal boundary layer. It involves stoping of over-thickened continental crust and the differences in density and melting behavior of eclogites and peridotites in the mantle. The fates of subducted and delaminated crust, underplated basalt, and peridotite differ because of differences in scale, age, temperature, melting temperature, chemistry, thermal properties, and density. Cold subducted oceanic crust—as eclogite—although denser than ambient mantle at shallow depths, may become less dense or neutrally buoyant somewhere in the upper mantle and transition zone, and may be gravitationally trapped to form mafic eclogite-rich blobs or layers. Detached lower continental crust starts out warmer; it thermally and gravitationally equilibrates at shallower depths than do slabs of cold mature lithosphere. The density jumps at the depths of 400 and 650 km act as barriers. Trapped eclogite is heated by conduction from the surrounding mantle and its own radioactivity. It is displaced, entrained, and melted as it warms up to ambient mantle temperature. Both the foundering and the re-emergence of mafic and ultramafic blobs create midplate magmatism and uplift. Mantle upwellings and partially molten blobs need not be hotter than ambient mantle or from a deep thermal boundary layer. The fertile blobs drift slowly in the opposite direction to plate motions—the counterflow model—thereby maintaining age progressions and small relative motions between hotspots. Large-scale midplate volcanism is due to mantle fertility anomalies, such as large chunks of delaminated crust or subducted seamount chains, or to the release of accumulated underplate when the plate experiences flexure or pre-breakup extension. Eclogite can have lower shear velocities than volatile-free peridotite and will show up in seismic tomograms as low-velocity, or red, regions, even when cold and dense. This model removes the paradoxes associated with deep thermal RT instabilities, propagating cracks and small-scale thermal convection. It explains such observations as relative fixity of melting spots, even though the fertile blobs are shallow.