REACTION-INDUCED MICROCRACKING - AN EXPERIMENTAL INVESTIGATION OF A MECHANISM FOR ENHANCING ANATECTIC MELT EXTRACTION

Melting reactions can create melt overpressure that may induce microcr acking. To determine whether such microcracking can enhance rock perme ability and melt extraction, we have studied the partial melting of a muscovite bearing metaquartzite at 800 MPa and 950-1126 K. Melting beg ins at muscovite-quartz grain boundaries and results in progressive re placement of muscovite by melt pools containing mullite and biotite. T he volume change for the reaction (0.021 m(3) per m(3) of original roc k) generates randomly oriented microcracks that emanate from melting s ites. The mean crack length in two-dimensional sections is 151 +/- 5 m u m and reflects the spacing between melting sites. Experiments in whi ch quartz sand was loaded with the metaquartzite to act as a drain ver ified that the microcracks, together with the melt pools, form a conne cted network. The estimated network permeability is 10(-14 +/- 1) m(2) , at least four orders of magnitude greater than permeabilities charac teristic of regional metamorphic environments. For reaction-induced mi crocracking to occur, the reaction must take place on a time scale suc h that creep cannot accommodate the associated volume change. Our anal ysis suggests that that requirement can be met on regional metamorphic time scales and that reaction-induced microcracking is a feasible mec hanism of permeability enhancement during partial melting and devolati lization.