Crystallization Stages of the BishopTuff Magma Body Recorded in Crystal Textures in Pumice Clasts

The BishopTuff is a giant silicic ignimbrite erupted at 0·76Ma in eastern California, USA. Five pumice clasts from the late-erupted Bishop Tuff (Aeolian Buttes) were studied in an effort to better understand the preand syn-eruptive history of the Bishop magma body and place constraints on the timescales of its existence. This study complements and expands on a previous study that focused on early-erupted BishopTuff pumice clasts. Bulk densities of pumice clasts were measured using an immersion method, and phenocryst crystal contents were determined using a sieving and winnowing procedure. X-ray tomography was used to obtain qualitative and quantitative textural information, particularly crystal size distributions (CSDs).We have determined CSDs for crystals ranging in size from 10 to 1000 mm for three groups of mineral phases: magnetite ( ilmenite), pyroxeneþ biotite, quartzþ feldspar. Similar to early-erupted pumice, late-erupted pumice bulk density and crystal contents are positively correlated, and comparison of crystal fraction vs size trends suggests that the proportion of large crystals is the primary control on crystallinity. Porosity is negatively correlated with crystal content, which is difficult to reconcile with closed-system crystallization. Magnetite and pyroxeneþ biotite size distributions are fractal in nature, often attributed to fragmentation; however, crystals are mostly whole and euhedral, such that an alternative mechanism is necessary to explain these distributions. Quartzþ feldspar size distributions are kinked, with a shallowsloped log^linear section describing large crystals (4140 mm) and a steep-sloped log^linear section describing small crystals (5140 mm). We interpret these two crystal populations as resulting from a shift in crystallization regime.We suggest that the shallow-sloped section describes a pre-eruptive quartzþ feldspar growth-dominated regime, whereas the steep-sloped section represents a population that grew during a nucleation-dominated regime that began as a result of decompression at the onset of eruption.Timescales of quartz growth calculated from the slopes of these two segments of the size distributions indicate that the pre-eruptive crystal population grew on timescales on the order of millennia and may describe the timescale of crystallization of the Bishop magma body.The syn-eruptive population gives timescales of51^2 years (but possibly much less) and probably marks the onset of eruptive decompression.