Field geological observation showed that, pressure shadows with two regions filled with a low-viscosity phase located around a large single crystal or hard inclusion are widely observed in metamorphic rock. To quantitatively study the forming conditions of pressure shadow, samples which composed of fine-grained San Carlos olivine plus mid-ocean ridge basalt (MORB) containing dispersed sub-millimeter-sized single crystal beads of olivine or zirconia were deformed in torsion at high-pressure and high-temperature. The melt initially homogeneously distributed with a melt fraction of 0. 05 before deformation. Specimen is size of D8. 9mm × L5. 5mm containing 8 particles size of Dlmm. The torsion experiment was done at a temperature of 1473K, confining pressure of 3OOMPa, shear strain rate of γ ≈l × 10-4s-1 and the maximum shear strain is 4. Indicted by the experimental results, melt-enriched and melt-depleted regions around the beads became observable and symmetrically distributed round the rigid olivine bead (i. e. pressure shadow structure) when the local shear strain reaches 1 and rocks were deformed by torsional force. Inhomogeneous distribution of melt formed due to the pressure gradient, melt-enrich region fraction varies from 0. 1 to 0.3 and the melt-depleted regions fraction varies from 0.01 to 0.02. Due to the disturbance area of the pressure distribution around the rigid ball is about the scale of the rigid ball scope, therefore, melt distribution tends to be homogeneous at a certain distance away from the rigid ball.
|Number of pages
|Yanshi Xuebao/Acta Petrologica Sinica
|Published - 2016
- High temperature and high pressure
- Partial melt
- Pressure shadow
- Torsion experiment