Journal of Conference Abstracts

Coupled Flow and Reaction in Porous Media: Implications for Melt Extraction from the Mantle and the Lower Crust

Einat Aharonov (einat@ldeo.columbia.edu)¹, Marc Spiegelman (mspieg@ldeo.columbia.edu)¹ & Peter Kelemen (peterk@cliff.whoi.edu)²

¹LDEO, Rt 9W, Palisades, NY 10964, U.S.A.

²WHOI, Woods Whole, MA, U.S.A.

In order to understand melt extraction from the mantle one needs to understand the processes which melt undergoes as it upwells, and one of the most basic is coupled fluid flow and fluid/solid reaction in porous media. We present numerical and theoretical studies of flow and reaction in porous media, in both a rigid and a compacting matrix, and discuss implications for the mantle.

Our studies show that when a fluid dissolves the porous matrix through which it flows, flow may become increasingly channelized in the direction parallel to the flow, with accompanying formation of long-range correlations and anisotropies in the porosity field. Since melt upwelling adiabatically beneath mid-ocean ridges dissolves the pyroxene through which it is flowing, we expect such melt focusing and anisotropic permeability fields in regions beneath mid-ocean ridges. Simulations from rigid and compacting matrixes indicate that the structure of the focusing depends on the rigidity of the matrix, where compaction actually enhances focusing effects. Simulation results are compared with field observations of dunites (believed to be such frozen channels) found in ophiolites.

On the other hand, simulations and theory show that flow coupled with precipitation results in reduction of correlations in the porosity along the flow path, and diffusion and dispersion of even initially focused flow. Periodic fracturing and fluid extraction may accompany precipitation. Such a scenario may operate at the base of the crust, where upwelling melt is cooling and crystallizing, and forming a permeability barrier. We present a simple model for periodic extraction of melt by magma fractures generating from sills at the base of the crust. Observations from ophiolites suggest that layered gabbros may be remnants of such sills.