HR: 08:40h
AN: V31A-03    [PDF]
TI: Modeling Mantle Melting During Continental Breakup: Implications for Anomalous Volcanic Productivity at Rifted Margins
AU: * Hopper, J R
EM: jhopper@geomar.de
AF: GEOMAR, Wischhofstrasse 1-3, Geb. 8, Kiel, D-24148 Germany
AU: Nielsen, T K
EM: thomas.kofoed@mail.dk
AF: Danish Lithosphere Center, Oster Voldgade 10, Copenhagen, DK-1350 Denmark
AB: Mid-ocean ridge accretion generates remarkably uniform crust, placing a powerful constraint on the state of the upper mantle. Models invoked to explain melt anomalies associated with volcanic rifted margins and other large igneous provinces must be consistent with what is known based on mid-ocean ridge studies. A successful model of volcanic margin formation should be able to produce a single transient pulse of excess magmatism followed by steady-state plate driven oceanic crustal accretion. To assess under what conditions buoyantly driven upwelling or small-scale convection at rifted plate boundaries is important, we employ a fluid-dynamical model with non-Newtonian viscosity that includes the feedback from melting on the physical properties of the mantle. Melting affects both the buoyancy and viscosity of the mantle, and may facilitate a transition to other creep mechanisms by enhancing grain boundary sliding. The model shows that some small-scale convection can occur. However, models that evolve into steady-state oceanic accretion show only modest excess melt productivity . In general, models that generate high magmatic productivity at breakup require a viscosity and density structure that also leads to excessive time dependence and/or anomalously high productivity long after breakup. Assuming an abrupt change in lithospheric thickness to enhance convective instabilities does not fundamentally alter this conclusion. In models that include a dehydration induced viscosity increase, the problem is exacerbated by suppressing buoyant upwelling above the depth to the dry solidus thereby restricting shallow flow to plate drive upwelling. The model can be used to quantify the spatial and temporal scales of anomalous volcanism associated with continental breakup under a variety of different assumptions about the viscosity and density structure of the upper mantle. To explain the large (factor of two greater than steady-state), short lived (~10 m.y.) pulse of breakup volcanism observed along SE Greenland, an exhaustible layer of anomalous material beneath the lithosphere seems required. Determining the source of this layer and establishing whether it is a chemical or thermal anomaly are needed to fully understand the North Atlantic igneous province.
DE: 1025 Composition of the mantle
DE: 8109 Continental tectonics--extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8162 Rheology--mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting