engleski

Towards a 3-D view of the melt geometry in partially molten dunite

Models of the melt geometry in partially molten rocks generally are based on observations from 2-D polished surfaces. The three-dimensional shape of the melt pockets, and hence also the degree of interconnectedness, are inferred from approximations of the complex shape of the interstitial melt pockets in two dimensions. To resolve existing controversies, in particular whether two-grain boundaries are wetted by melt, we are conducting a program of serial sectioning and high resolution imaging on a partially molten sample with a moderate amount of melt (~ 4%). The sample was run in a piston cylinder at 1350&ordm ; C and 1 GPa for 432 hours. During this time the grain size increased from ~ 1 micron to 33 microns, resulting in a steady state grain size distribution (Faul and Scott, Contrib. Mineral. Petrol., 2006). The sample is polished to a final polish with colloidal silica to enable high resolution imaging with a Field Emission SEM. The amount of material removed at each polishing step is determined interferometrically by measuring the depth of an initially ~ 30 micron deep laser pit. The average thickness of the removed layers is about 1.5 micron. Initial results show that larger pockets in individual 2-D sections change relatively rapidly with depth, while thin, elongate inclusions, separating two neighboring grains, persist over a depth corresponding to the size of the grains. This confirms that wetted two-grain boundaries exist under conditions of steady state grain growth in partially molten upper mantle rocks. This will affect seismic properties, as well as permeability, particularly for volatile-rich melts in subduction zones.

model; melt; permeability; imaging; boundary; dunite

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