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The eclogite engine; chemical geodynamics as a Galileo thermometer

Don L. Anderson
The eclogite engine; chemical geodynamics as a Galileo thermometer (in Plates, plumes, and planetary processes, Gillian R. Foulger (editor) and Donna M. Jurdy (editor))
Special Paper - Geological Society of America (2007) 430: 47-64

Abstract

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). 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.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 430
Title: The eclogite engine; chemical geodynamics as a Galileo thermometer
Title: Plates, plumes, and planetary processes
Author(s): Anderson, Don L.
Author(s): Foulger, Gillian R.editor
Author(s): Jurdy, Donna M.editor
Affiliation: California Institute of Technology, Seismological Laboratory, Pasadena, CA, United States
Affiliation: Durham University, Department of Earth Sciences, Durham, United Kingdom
Pages: 47-64
Published: 2007
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 90
Accession Number: 2008-045004
Categories: Solid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With discussion (Don L. Anderson)
Illustration Description: illus.
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200813
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