Influence of deformation on melt topology in peridotites

Melt microstructures in partially molten fine-grained olivine-basalt aggregates have been characterized using image processing methods to quantify the effects of deformation on the geometry of the melt network. Olivine-basalt aggregates with melt fractions of 0.018 to 0.165 and mean grain size between 10 and 25 um were deformed at differential stresses between 0 and 250 MPa. For samples with grain sizes of ∼ 10 to 25 μm, deformation at differential stresses greater than ∼ 100 MPa results in a preferred orientation of melt such that a larger fraction of melt is in melt pockets which have their long axis at 15-20° to σ₁. The melt preferred orientation becomes more well developed with increasing differential stress and is related to the minimum in mean pressure that occurs at the ends of melt pockets that are inclined to σ₁. As a result of this minimum in mean pressure, melt flows to the ends of melt pockets causing them to extend. Hydrostatic annealing of deformed samples results in the redistribution of melt into melt pockets oriented more perpendicular to the original σ₁; this melt distribution is consistent with the preferred wetting of (010) planes of olivine which are oriented subperpendicular to σ₁ after deformation. The development of an anisotropic melt distribution in response to stress during deformation could be important for the focusing of melt to mid-ocean ridges.