MR022-0018
A closer look at melt network geometry in deformation experiments: Implications for seismic anisotropy

Wednesday, 16 December 2020
Poster
Cassandra Seltzer, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, Matej Pec, Massachusetts Institute of Technology, EAPS, Cambridge, MA, United States, Mark E Zimmerman, Univ Minnesota, Minneapolis, MN, United States, Benjamin K Holtzman, Lamont Doherty Earth Observatory, Columbia Univ., Palisades, NY, United States and David L Kohlstedt, University of Minnesota Twin Cities, Minneapolis, MN, United States
Abstract:
Partial melting alters the mechanical and transport properties of rocks in ways detectable by remote sensing. Under stress, melt aligns on the grain scale, imparting anisotropy in the deforming aggregate on longer length scales. Here, we report the results of experiments on San Carlos olivine with 2 wt% of mid-ocean ridge basalt (MORB) added to examine the influence of deformation on melt network geometry.

General shear experiments were performed in a Paterson gas medium deformation apparatus at T=1250°C and Pc=300 MPa, reaching finite strains of 0.3-2.3 at shear stresses ranging from 55 – 175 MPa. The melt network geometry was analyzed on 2D polished surfaces using high-resolution SEM images. In the undeformed material, melt pockets reside on triple junctions and along grain boundaries, and have a median equivalent diameter dequ ≈1µm. The axial ratios (α = b/a) of individual pockets follow a Gaussian distribution with a mean of 0.5. The undeformed material shows a random melt preferred orientation (MPO) and a bulk axial ratio of 0.96, indicating that the melt network is nearly isotropic on longer length scales. The starting equivalent diameter for solid olivine particles is dequ≈5 µm.

After deformation, median melt pocket sizes decrease to ~0.5 µm, axial ratios decrease to ~0.3, and aspect ratios develop along a log normal distribution. On the sample scale, a moderate MPO develops sub-parallel to σ1, with a bulk axial ratio of α ≈ 0.7. Olivine grain equivalent diameter decreases significantly to dequ ≈2.5µm after deformation.

Our preliminary data show that deformation aligns melt pockets along grain boundaries and preferentially wets grain boundaries perpendicular to the σ3 orientation. Little correlation between melt network geometries and stress and strain was observed, indicating that the MPO is set up early in the experiment and is not highly sensitive to the magnitude of the stress or strain during deformation. These observed changes to melt network geometry during deformation can aid the interpretation of seismic anisotropy in the Earth’s upper mantle.

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