HR: 14:30h
AN: U13C-04    [Abstracts]
TI: Magma dynamics with the Enthalpy Method
AU: * Katz, R F
EM: rfk22@cam.ac.uk
AF: Institute for Theoretical Geophysics, Department of Applied Mathematics & Theoretical Physics, CMS Wilberforce Road, Cambridge, CB3 0WA, United Kingdom
AB: Forward models of magma genesis and transport through the mantle are an important tool for studying the dynamics of plate boundaries because they have the potential to track geochemical signals in magma from depth to the surface or, at least, to the Moho. To accomplish this, however, requires a self-consistent thermodynamic closure for the conservation equations for mass, momentum, composition and enthalpy of the magma/mantle system (e.g. McKenzie '84). Such a closure should simultaneously account for all modes of melting and freezing, allowing magma to interact with a cold thermal boundary layer, the lithosphere. I have implemented a model in which I assume a two-component system that is in local thermodynamic equilibrium everywhere within the domain. I thus close the system of equations by prescribing a binary phase diagram that should approximate the mantle. This approach is known as the Enthalpy method (e.g. Alexiades and Solomon '93). It differs from previous treatments of magma dynamics in that it captures polybaric, polythermal, two-component melting and freezing with one parameterization: the phase diagram. I have implemented this system in 2D and configured it to model a mid-ocean ridge. Melting occurs by adiabatic decompression at a rate determined self-consistently through conservation of energy and the phase diagram. Melt segregates from the mantle matrix and rises buoyantly. The existence of a freezing boundary, where rising magma reaches its solidus temperature, results in lateral melt transport towards the ridge axis (Sparks and Parmentier '94). Similar effects are expected in arc models, and should be accessible with the same general approach. A disadvantage of the Enthalpy method is that it requires the system to be in equilibrium everywhere, which makes it computationally difficult to solve, and is at odds with observations of disequilibrium of major elements in primitive basalts from ridges. An advantage is that both melting and freezing are treated according to a standard parameterization: the two-component phase diagram. While still fairly simple to model, two components allow for univariant melting, which is not the case with a single component (Sramek et. al '06). In the ridge model, I have chosen to use a binary loop phase diagram, however this choice is not due to any constraint from the method.
UR: http://www.damtp.cam.ac.uk/user/rfk22
DE: 1037 Magma genesis and partial melting (3619)
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 4255 Numerical modeling (0545, 0560)
DE: 8178 Tectonics and magmatism
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
SC: Union [U]
MN: 2007 Fall Meeting