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3.10 g. The multi-anvil press as a deformation apparatus, revisited (W. Durham and D.C. Rubie)

This project has developed out of attempts to quantify the amount of non-hydrostatic deformation that occurs in samples during the course of high-pressure experiments in the multi-anvil apparatus. If significant amounts of deformation (i.e. shortening) can be imposed on sample assemblies, for example, by the selection of certain rigid materials for pistons acting on the ends of a cylindrical sample, then it might be possible to carry out rheological tests on materials at mantle pressures, something that has never been attempted until recently. Researchers working with multi-anvil devices have long suspected that some deformation may be occurring when a sample assembly is held at elevated pressure and temperature, say, as gasketing material slowly creeps away from the high-pressure center of the assembly, but no one has attempted to quantify how much, if any, deformation is actually occurring. Our task has been to measure deformation in various kinds of 10/5, 14/8, and 18/11 (octahedron edge length/truncation edge length, in mm) multi-anvil assemblies. What we find is that there is a transient period of apparent movement of the outer guide blocks towards one another, suggestive of creeping gaskets, that lasts 6-12 hours depending on temperature in the cell. Thereafter, the guide blocks do not appear to move at all.

Measurement of octahedron size after depressurization, however, suggests that even the transient deformation does not really reflect deformation in the octahedra - all octahedra tend to have nearly the same size, independent of time and confining pressure. (There is a strong dependence of octahedron size at low pressures where compaction of the porous octahedron occurs). Octahedron size does seem to have a weak inverse dependence on run temperature. The final size also depends on octahedron material. There is hope for those wanting to do large strain experiments in the multi-anvil apparatus that certain "soft" gasket/octahedron materials will behave differently from conventional materials. Finding the right combination of materials is the focus of ongoing research.

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