Mantle Flow at Subduction Zones

Volcanism associated with the subduction of oceanic lithosphere is the primary source of new continental crust and as such is a significant contributor to the chemical differentiation of the crust-mantle system. Several conceptual models have been proposed to descibe melt transport at subduction zones, including diapiric flow of a hydrous or partially molten component, distributed porous flow of magma and magma flow through fractures. Recent geochemical studies of Be isotopes and U-Th-Ra disequilibrium in subduction magmas require extremely rapid magma transport through the mantle (i.e., ~100 km in as little as 30,000 years). This provides a strong constraint on models of melt transport.

I use both laboratory tank models and numerical models to address questions of mantle flow at subduction zones. Below are some images from a series of laboratory experiments I conducted to investigate the diapiric flow model of subduction zone magmatism in light of the time constraints provided by geochemical studies. The experiments demonstrated that the diapiric flow model is capable of producing a wide range of transport times, depending on the flux of buoyant fluid (i.e., partially molten material) into the system at depth and the rate of subduction. For high buoyancy flux/slow subduction regimes, individual diapirs linked up to form a conduit network over which transport was extremely rapid (on the order of 10,000 years, scaled to the mantle).

Cartoon of the subduction zone conceptual model. Pressure dependent dehydration reactions within the subducting plate release water into the overlying mantle wedge, creating a thin layer of low viscosity hydrous material. This layer develops Rayleigh-Taylor instabilities which break off to ascend as diapirs.

A cartoon illustration of the laboratory model. The apparatus pictured is submerged in a tank of corn syrup, which models the behaviour of the mantle. A dyed alcohol solution is then injected into the tank at depth to model the hydrous low viscosity layer.

	Steady diapiric flow: Low buoyancy fluxes Fast subduction rates (Click image to download movie (.mov))
	Unsteady flow & network formation: Intermediate buoyancy fluxes Intermediate subduction rates (Click image to download movie (.mov))
	Networked flow: High buoyancy fluxes Slow subduction rates (Click image to download movie (.mov))

Subduction Zone Papers and Abstracts

Hall, P.S. and C. Kincaid, Diapiric flow at subduction zones: A recipe for rapid transport, Science, 292, 2472-2475, 2001.

Kincaid, C. and P.S. Hall, The role of back-arc spreading in circulation and melting at subduction zones, (J. Geophys. Res., 108, 2003).

Hall, P.S. and C. Kincaid, Melt transport at subduction zones by dia piric flow, EOS Trans. AGU, 79(17), 348, 1998.

Kincaid, C., P.S. Hall and D. Coleman, On melting, back-arc spreadin g and mantle dynamics in subduction zones, EOS Trans. AGU, 79(45), 991, 1998.

Hall, P.S. and C. Kincaid, An experimental investigation of plate- a nd buoyancy-driven flow at subduction zones, EOS Trans. AGU, 78(17 ), 323, 1997.

Subduction Zone Links

Margins (US program to study subduction related processes)

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email: phall@gso.uri.edu
Last updated: 21 July 2003