HR: 0800h
AN: V21B-0612    [Abstracts]
TI: Layer Formation in Convective Magma Chambers
AU: * H\"{o}ink, T
EM: hoeink@uni-muenster.de
AF: Institut f\"{u}r Geophysik, Corrensstr. 24, M\"{u}nster, 48149 Germany
AU: Schmalzl, J
EM: joergs@earth.uni-muenster.de
AF: Institut f\"{u}r Geophysik, Corrensstr. 24, M\"{u}nster, 48149 Germany
AU: Hansen, U
EM: hansen@earth.uni-muenster.de
AF: Institut f\"{u}r Geophysik, Corrensstr. 24, M\"{u}nster, 48149 Germany
AB: The dynamics of a convective magma chamber is crucially influenced by the competetion between sedimentation and convective suspension of crystals. Crystal settling combined with the crystal's density contribution is a possible mechanism leading to differentiation and layer formation. Here we address the question whether crystals can remain suspended or whether they are able to dynamically form a layered structure within the convective lifetime of a magma chamber. We employ an existing numerical method that, by means of a finite volume scheme, discretizes the equations for thermally driven convection in an infinite Prandtl-number Boussinesq fluid in Cartesian geometry. We implement a newly developed settling algorithm for the numerical study of finite-sized-particle settling in a non-dilute convective suspension. Our approach considers a consistent settling velocity and the density contribution due to particle mass. The buoyancy ratio $B$, which is the ratio of the density variation due to crystal mass to the thermal density variation, is varied for five different Rayleigh numbers, covering a range of four orders of magnitude. We find $B$ to be a critical parameter and its critical value to depend on the Rayleigh number. For subcritical values we observe that the presence of a crystal phase reduces convective vigor and most crystals stay suspended. When a critical buoyancy ratio is exceeded, the presence of crystals can significantly alter convective motion. For all investigated Rayleigh numbers we find a critical buoyancy ratio, above which layering can be achieved from an initially unstratified fluid. Most of the crystal mass collects in the dynamically created bottom layer, even for cases where the average settling velocity is three orders of magnitude smaller than the root mean square convective velocity. The time it takes a crystal to travel across the height of the cell with the full settling velocity in the absence of a thermal gradient defines the settling timescale. Layer formation in all observed layering cases occurs on this time scale, even though the average settling velocity is reduced by at least one order of magnitude due to hindered settling. In many cases (e.g. basaltic magma chambers) the settling time is short compared to the time that magma chambers take to solidify. We conclude that dynamical layer formation that is connected to crystal settling and the crystals' density contribution is a likely mechanism for creating layered structures within the convective lifetime of a magma chamber.
DE: 8439 Physics and chemistry of magma bodies
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8125 Evolution of the Earth
DE: 8145 Physics of magma and magma bodies
DE: 8147 Planetary interiors (5430, 5724)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting