HR: 1330h
AN: V12C-0612    [PDF]
TI: Channelling of Melt Above Plumes and Beneath MORs
AU: * Mueller, K
EM: kdg@gmx.de
AF: IMGF, Feldbergstrasse 47, Frankfurt, 60323 Germany
AU: Schmeling, H
EM: schmeling@geophysik.uni-frankfurt.de
AF: IMGF, Feldbergstrasse 47, Frankfurt, 60323 Germany
AB: We investigate melt transportation in partially molten rocks under different stress fields above the head of a mantle plume or beneath a spreading mid-oceanic ridge under hydrous and anhydrous conditions. We model such aggregates with the 2D-FD code FDCON [1] by means of a porous deformable matrix with melt under the influence of a given stress field to clarify the following key questions: Could channeling occur in a matrix containing a random melt distribution under a given stress field? Which orientation does it take? Is it possible to achieve a focusing of melt towards a MOR (dykes)? Does applying simple or pure shear to the matrix result in a difference in the formation and orientation of channels? How does the channel instability evolve during finite simple shear? In a deforming partially molten aggregate, weakening of the solid matrix due to the presence of melt creates an instability in which melt is localized by the following mechanism: regions of initially high melt fraction are areas of low viscosity and pressure, so that melt is drawn into these regions from higher pressure surroundings. This further enhances the melt weakening, producing a self-excited localization mechanism [2]. The channeling developing in models with a random melt distribution of 3.5 $\pm$ 0.5% shows that melt is accumulated preferably in inclined channels. For both, simple as well as pure shear, the growth rate is highest for an orientation parallel to the direction of the maximum compressive stress and proportional to applied stress and the reverse of the Melt Retention Number. This also confirms the theoretical growth rate found by Stevenson [2]. In our isothermal models we found that the influence of water reduces the growth rate, in contrast to non-isothermal models of Hall [3]. Under simple shear melt channels evolve from an irregular melt distribution at angles of 45 degrees to the direction of shear. Upon further straining they rotate out of the orientation of maximum growth rate and partly disrupt. At later stages the mean channel orientation deviates from the compressive stress orientation. We further seek to clarify which physical quantities may affect the orientation of channels and their wavelengths. Possible factors are the compaction length and the nature of the stress field. The latter assumption will be tested combining pure and simple shear. References [1] H. Schmeling. Partial melting and melt segregation in a Convecting mantle. Physics and chemistry of partially molten rocks; N. Bagdassarov and D. Laporte and A. B. Thompson, Kluwer Academic Publishers, 141-178, 2000 [2] D.J. Stevenson. Spontaneous small-scale melt segregation in partial melts undergoing deformation. Geophys. Res. Lett., 16(9):1067-1070, 1989 [3] C.E. Hall and E.M. Parmentier. Spontaneous melt localization in a deforming solid with viscosity variations due to water weakening. Geophys. Res. Lett., 27:9 -12, 2000
DE: 3210 Modeling
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting