UNDERSTANDING THE MOUNT ST. HELENS MAGMATIC SYSTEM USING ZIRCON SURFACE GEOCHRONOLOGY AND GEOCHEMISTRY, 3D X-RAY TOMOGRAPHY, AND GLASS COMPOSITIONS

Zircon U-Series and U-Pb geochronology from 25 samples spanning the eruptive history of Mount St. Helens volcano (MSH) reveals complex and wide-ranging age populations, suggesting that some magmas intruded beneath MSH are stored for up to a few 100 kyr before being rejuvenated, recycled into younger hot magmas, and erupted (Claiborne et al., 2010). The zircon compositions record thermal and chemical transitions in the magmatic system independent of the eruptive behavior and hint at different composition magmas in the system crystallizing simultaneously. We analyzed the outer rims of zircon from 6 eruptive units by pressing grains into indium and analyzing their surfaces. In 3 samples, zircon ages within error of the established eruption age were identified; In 3 samples, no eruption age-surfaces were identified. Each sample exhibited variation in surface ages and compositions among grains, including some more than 10 kyr older than eruption. These variations indicate that the individual zircons within a single sample were in different environments shortly before eruption. In order to identify these variable environments (e.g. inclusion in other minerals, dissolution, extraction from different magmas), it was necessary to put the zircons in compositional and textural context. Since zircons are scarce in MSH rocks and rarely visible in thin section, we used differential absorption x-ray tomography to create 3D Zr maps for pumices for which we have surface data. We characterized the zircons in 2 pumices, described their textural relations, and were able to identify multiple populations of zircons in individual samples (i.e. zircons in glass and zircons included in other minerals). This may account for the variability in surface ages and compositions, but does not rule out growth in different magmas. We plan to analyze glass compositions for zircon-bearing samples using SEM, which may help us better understand melt compositions during zircon growth. These data, combined with our existing ‘conventional’ zircon age and compositional data, reveal the complexity of the Mount St. Helens plumbing system, where zircons are stored for up to 100 kyr prior to eruption, crystallizing and dissolving as magmas move through or stall in the open system, before being entrained by hot, young, rapidly ascending magmas and erupted.