NOMADIC CRYSTALS: RECORDS OF MAGMA DYNAMICS AT MT ST HELENS AND MT SHASTA

Crystal ages can provide unique insights about timescales of storage and mixing of liquid and crystal populations within a system, particularly in cases where mixing is difficult to recognize because successive magma batches and crystal populations are chemically similar. We present two case studies where we reevaluate previously-published U-series, and specifically Ra-Th, ages for mineral separates in mafic and intermediate lavas from Mt St. Helens (MSH) and Mt. Shasta by incorporating new constraints on the quantity of Ra initially present in the crystals. Several observations emerge from these revised ages: 1) plagioclase crystals are on average hundreds to thousands of years older than the ages of eruption; 2) four of the nine samples yield discordant pairs of U-series ages (Ra-Th and U-Th) for the same mineral separates, which can be explained by mixtures of crystal populations (or zones) of different ages; and 3) plagioclase separates in the 1982 dacite (MSH) and the Black Butte dacite (Shasta) have anomalously high Ra contents compared to what would theoretically be in equilibrium with liquids like those erupted recently, which may reflect rapid crystal growth (possibly as rims on older crystals) during final ascent and degassing of magmas. One implication of these observations is that recycling of crystals within the same reservoir and related multiple episodes of crystal growth appear to be common (though not ubiquitous) in arc systems. Recycled crystals may have been stored at relatively low temperatures, perhaps in a crystal mush, based on the diffusive behavior of Ba compared to the Ba zoning preserved in individual MSH crystals. In this respect, arc volcanoes may be like slow-spreading midocean ridge systems, where liquid reservoirs are small, ephemeral, or nonexistent. The residence times of crystals within these systems appears to be intermediate between the days to hundreds of years observed in mafic systems (e.g., Kilauea, Hawaii, Ardoukoba, Africa) and the tens to hundreds of ka found in many silicic systems (e.g., Long Valley, Toba caldera).