Abstract

Melting reactions can create melt overpressure that may induce microcracking. To determine whether such microcracking can enhance rock permeability and melt extraction, we have studied the partial melting of a muscovite-bearing metaquartzite at 800 MPa and 950–1126 K. Melting begins at muscovite-quartz grain boundaries and results in progressive replacement of muscovite by melt pools containing mullite and biotite. The volume change for the reaction (0.021 m3 per m3 of original rock) generates randomly oriented microcracks that emanate from melting sites. The mean crack length in two-dimensional sections is 151 ± 5 µm and reflects the spacing between melting sites. Experiments in which quartz sand was loaded with the metaquartzite to act as a drain verified that the microcracks, together with the melt pools, form a connected network. The estimated network permeability is 10−14 ± 1 m2, at least four orders of magnitude greater than permeabilities characteristic of regional metamorphic environments. For reaction-induced microcracking to occur, the reaction must take place on a time scale such that creep cannot accommodate the associated volume change. Our analysis suggests that that requirement can be met on regional metamorphic time scales and that reaction-induced microcracking is a feasible mechanism of permeability enhancement during partial melting and devolatilization.

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