As a two-phase, solid-melt material flows around rigid particles, melt-depleted and melt-enriched regions (i.e., pressure shadows) develop due to the coupled fluxes of melt and solid driven by pressure gradients around the particles. To study this compaction-decompaction process, samples composed of fine-grained San Carlos olivine plus mid-ocean ridge basalt containing dispersed sub-millimeter-sized, single crystal beads of olivine were deformed in torsion at a temperature of 1473 K and a confining pressure of 300 MPa. Indicated by melt distribution maps obtained from reflected-light optical and backscattered electron microscopy, melt-enriched and melt-depleted regions around the beads became observable at a local shear strain of γ ≈ 1 in samples with an initially homogeneously distributed melt fraction of φ ≈ 0.05. The melt-enriched regions (φ-high≈0.06 to 0.10) and the melt-depleted regions (φ-low≈0.02 to 0.04), extending as far as one radius of the bead, were symmetrically distributed around the bead. The flow field of the olivine matrix determined from crystallographic preferred orientations agrees with theoretical predictions based on two-phase flow analysis. These experiments are the first to produce pressure shadows in partially molten rocks. One implication of this study is that it will be possible to constrain the ratio of bulk to shear viscosity, which is inferred from the distribution of melt using a combination of experimental observations and numerical simulations.
Bibliographical noteFunding Information:
We are grateful to Mark Zimmerman, Dan King and Lars Hansen for their help with our experiments. We thank to Richard Katz and Ben Holtzman for stimulating discussions. This work was supported by NSF OCE 1060983 (to D.L.K.) and NSFC grants 41274094 and 40874043 (to Y.-H.Z.). Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. Electron microprobe analyses were carried out at the Electron Microprobe Laboratory, Department of Earth Sciences, University of Minnesota-Twin Cities.
- Partial melts
- Pressure shadow
- Two-phase flow