Anisotropic Permeability of Experimental Partially Molten Dunite Deformed In Shear
Abstract
Melt migration beneath mid ocean ridges exerts important controls on the physical and chemical processes of the upper mantle and crust. Experimental and field observations suggest that melt transport in the upper mantle beneath mid-ocean ridges is strongly channelized. With increasing shear strain, initially homogeneously distributed melt can segregate into an array of melt-rich bands, flanked by melt-depleted regions. Using a digital rock physics approach, we quantify here the transport properties of sheared partially molten rocks. We imaged using synchrotron microtomography an olivine-basalt aggregate that was sheared to a total strain of 13.3 at 1200°C and 300MPa in a torsion apparatus by Qi et al., 2013. The sample is composed of 10% basaltic melt and 90% solid olivine. The resolution of the three-dimensional images was 0.16 micron per voxel. We processed the image data to associate voxels either with the melt or the olivine. We then quantified the melt fraction and permeability in the melt rich and melt poor regions of the sample. Our results show that melt distribution is bimodal: in the melt poor regions, melt fractions range from 0.07-0.10, with an average of 0.08 whereas in the melt rich regions, melt fractions range from 0.10 to 0.18, with an average of 0.13. Permeability in melt rich regions is highly anisotropic. Along melt bands (kx and kz), permeability is at least a factor of 2-3 higher than perpendicular to the bands (ky).The relationship between kx and melt fraction in banded regions can be captured by a power-law relationship with an exponent n~4-5, considerably higher than the n=2.6 appropriate for isotropic partially molten rocks (Miller et al., 2014). In most subvolumes, we find no connectivity across the melt bands, resulting in vanishingly small permeability ky. In melt poor regions, we find little or no connectivity in any direction. Thin melt films could be present below the detection limit of the imaging technique. Nevertheless, the permeability of the melt poor regions appears much less than that of the melt rich bands. These data show that sheared partially molten aggregates have an anisotropic permeability allowing melt flow essentially only parallel to melt bands. This implies that melt band formation can exert a dominant control on melt migration and melt focusing at mid ocean ridges.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V11F0156B
- Keywords:
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- 1033 Intra-plate processes;
- GEOCHEMISTRY;
- 7208 Mantle;
- SEISMOLOGY;
- 8137 Hotspots;
- large igneous provinces;
- and flood basalt volcanism;
- TECTONOPHYSICS;
- 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY