HR: 16:00h
AN: V24A-01 INVITED     [Abstracts]
TI: Asthenospheric Melt Segregation and Channelization: The Influence of Differential Yielding and Disaggregation on Fluid Flow in Ductile Rocks
AU: * Connolly, J A
EM: james.connolly@erdw.ethz.ch
AF: Swiss Federal Institute of Technology, Sonneggstr. 5, Zurich, 8092 Switzerland
AU: Podladchikov, Y Y
EM: y.y.podladchikov@fys.uio.no
AF: Uinversity of Oslo, Blindern 99, Oslo, 0517 Norway
AB: Asthenospheric melting processes produce small quantities of melt, yet geochemical evidence suggests this melt must be collected from its source and transported to the surface on extraordinarily short time scales. We propose that a mechanical flow channeling instability, which arises because of viscoplastic behavior of the rock matrix, may resolve this dilemma. To characterize differential yielding we employ a model in which the ratio of the matrix viscosity during decompaction to the viscosity for compaction is treated as a free parameter, {\it R}. Numerical solutions of the compaction equations for porous flow in a matrix with differential yielding reveal that solitary dike-like porosity waves initiate from vanishingly small perturbations to a uniform background porosity. The waves grow and accelerate as they propagate by drawing fluid from the background porosity. For {\it R} $<<$ 1, the wave evolution is self-similar in time such that the growth rate of the waves is $\sim {\it R}^{-3/8}/\tau$, where $\tau$ is the viscous compaction time scale. This result implies an initial melt fraction of $10^{-3}$ could be amplified to the conditions for matrix disaggregation in $\sim 10^{4}$ y for {\it R} = $10^{-8}$, a value expected for the case that decompaction is limited by the melt viscosity. From such an initial melt fraction, wave velocities (10 m/y) may be too low for the waves to be the primary mechanism for rapid melt transport, however the waves offer a mechanism by which by melt may be localized to form a magmatic suspension from which dikes initiate. For flow originating from a high volume source, as in metamorphic devolatilization or some lithospheric melting processes, differential yielding causes flow to be channelled into the tails of dike-like porosity waves with a characteristic spacing corresponding to the viscous compaction length $\delta$ and widths $\sim \delta {\it R}^{1/2}$. The numerical results can be understood in the context of an analytical solution of the compaction equations that is completely general with respect to the constitutive relations used to define the matrix rheology and permeability. This solution combines the porosity dependence of the rheology and permeability in a single hydromechanical potential, which can be used to construct phase diagrams depicting the conditions for smooth pervasive flow, wave propagated melt extraction and matrix disaggregation (dike formation).
DE: 8434 Magma migration
DE: 8045 Role of fluids
DE: 8145 Physics of magma and magma bodies
DE: 5114 Permeability and porosity
DE: 3600 MINERALOGY AND PETROLOGY (replaces
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting