DI007-0005
Strongly Focused Flow along Shear-Induced Melt Bands in Experimental Olivine-Basalt Aggregate
Wednesday, 9 December 2020
Poster
James A Bader1, Wenlu Zhu2, Laurent Montesi2, Benoit Cordonnier3, Chao Qi4 and David L Kohlstedt5, (1)University of Maryland College Park, College Park, MD, United States, (2)University of Maryland College Park, Department of Geology, College Park, MD, United States, (3)Independent Scientist, Oslo, Norway, (4)University of Minnesota, Minneapolis, MN, United States, (5)University of Minnesota Twin Cities, Minneapolis, MN, United States
Abstract:
Melt distribution and permeability of partially molten rock beneath mid-ocean ridges exert first-order controls on melt transport. Experimental and field evidence show these properties may be heterogeneous, with most melt flow occurring in highly permeable, melt rich channels. In particular, olivine-melt aggregates deformed in shear develop melt-rich bands separated by melt poor regions. To test how effective melt bands may be in redirecting melt flow towards mid-ocean ridge axes, we analyze 3-dimensional (3D) melt distribution and perform digital flow simulations using X-ray micro-tomography experiments. A sample consisting of 90% olivine and 10% basaltic melt was sheared to a total strain of 13.3 at 1200°C and 300 MPa in a torsion apparatus (Qi et al., 2018). We imaged a portion (roughly a 328 µm cube) of the deformed sample containing several melt bands. Grain size is about 3.0 µm and the 3D image resolution is 0.16 µm.
Melt distribution in the sample is highly heterogeneous. Inside melt bands, melt fraction Φ ranges from 0.116 to 0.178, and melt is connected, with the connected melt fraction Φc = 1.2Φ - 0.06. Outside melt bands, melt fraction ranges from 0.079 to 0.100 and melt is disconnected. The lack of connectivity at relatively high melt fractions contrasts with results from undeformed olivine-basalt aggregates (Zhu et al., 2011) and may indicate no texture equilibrium outside melt bands.
Permeability subparallel to the shear direction inside a melt band is related to melt fraction by a power-law relationship with an exponent of 3.2±0.8. The power-law exponent is marginally higher than in undeformed samples (Miller et al., 2014). This could reflect preferential connection in the shear direction, although the relationship may also be affected by the limited range of melt fraction. Permeability across the melt band is a factor of 2-3 smaller than along the band. Importantly, the melt poor regions between bands prevent large-scale flow in a direction perpendicular to the bands. These results imply melt flow at the plate boundary scale should be efficiently redirected to the melt band direction.
