PHENOCRYST SIZE DISTRIBUTIONS OF LATE-ERUPTED BISHOP TUFF

In this study, we present crystal size distributions of late-erupted Bishop Tuff obtained using x-ray tomography. Crystal size distributions are important in that they bear on our understanding of crystal nucleation, growth, and fragmentation history. We compare crystal size distributions in late-erupted pumice clasts with those previously determined for early-erupted pumice clasts.

X-ray tomography provides 3D images of whole pumice clasts, including fragmented but still articulated crystals. Five pumice clasts of late-erupted Bishop Tuff were studied and their bulk densities range from 0.42-1.18 g/cm³. Because spatial resolution increases as cylinder size decreases in our tomographic datasets, five cylinders of varying diameter were cut to allow determination of distributions over a broad range of crystal sizes (from 8.75-1120 μm). Crystal sizes were determined by image processing using Blob3D. For each mineral or mineral group in a clast, one crystal size distribution was computed by combining data from all five cylinders. Our data comprise crystal size distributions for magnetite, biotite + pyroxene, and quartz + sanidine.

Overall, despite a range in clast bulk density and crystal contents, crystal size distributions for all five clasts are strikingly similar to each other, particularly for magnetite. In comparison with early-erupted crystal size distributions, however, late-erupted distributions show larger population densities. In all late-erupted distributions, semi-logarithmic plots show a concave-upward trend. These plots can be well-approximated by power law (fractal) distributions, which are consistent with fragmentation. It is surprising to see this relationship for late-erupted magnetite as early-erupted clasts and our tomographic images show unfragmented crystals. This suggests that fractal size distributions may not only be formed by fragmentation and can be magmatic in origin.