Testing the influence of high-voltage mineral liberation on grain size, shape and yield, and on fission track and 40Ar/39Ar dating

Mineral liberation and separation are fundamental for modern thermochronology. The fragmentation of rocks by high-voltage electrical pulses, called electrical fragmentation, is a new technology, only available since the development of a lab-sized machine a few years ago. The proposed advantages (e.g. disintegration along grain boundaries) compete with suspected negative effects of the locally occurring high temperatures (up to 104 K) on the physical properties of the liberated minerals. Comparison of electrical fragmentation with conventional mechanical fragmentation (jaw crusher) revealed similar amounts of liberated apatite and zircon from the same quantity of granite, but 45% production of fines (< 80 μm) for mechanical fragmentation versus 5% for electrical fragmentation. Electrical fragmentation yielded larger-sized apatite grains (180–250 μm) compared to 80–125 μm for mechanical liberation, but no differences in shape factors (elongation, roundness, compactness). The liberation of idiomorphic crystals from coarse-grained rocks is ameliorated by the new method, as shown by the liberation of mm-sized idiomorphic biotite crystals from granite. Degassing curves of 40Ar from electrically and mechanically liberated biotites from the same sample are nearly identical (maximum difference: six percentage points); 40Ar/39Ar ages of biotites from three samples (two mechanically, one electrically fragmented) from the same intrusion are identical within 2σ error. Length measurements of apatite fission tracks gave identical results within 2σ error regardless of fragmentation method used. In summary, the positive aspects of electrical fragmentation have been confirmed in this study, while effects negative for thermochronology are not observed.