Crystal Mush History at the Unzen Volcanic Complex of Southwestern Japan, determined using U-series Zircon Geochronology

Abstract:

Crystal-rich (40-50 vol.-%) intermediate to silicic lava has been the primary eruptive product of several recent hazardous eruptions: Mt. Pinatubo, Philippines (1991), Soufriere Hills, Montserrat (1995-present), and Unzen, Japan (1990-1995). Despite this association with such devastating eruptions, the formation, timing, and evacuation of such magmas is not well understood: do such eruptions tap a long-lived, multi-cycle crystal mush, or, is it generated in a single magmatic cycle prior to eruption? We are exploring these questions at the Unzen Volcanic Complex, Nagasaki-ken, Japan where a 500 ka history of crystal-rich dacitic dome eruptions has built the Unzen Volcanic Complex. Previous studies (e.g. Nakamura, 1995) have demonstrated the role of mafic recharge in rejuvenating crystal mush a few months prior to the most recent eruptions, but the history of the mush is unknown. Preliminary U-Th/U-Pb in zircon geochronology from the most recent eruption (Heisei-Shinzan; 1990-1995) and the 1792 dome collapse lava (Tenguyama; erupted 4.1±0.8 ka) reveal a complex history of individual zircons. Unpolished rims from the 1990-1995 eruption range in age from ~24.6 to 180 ka, while interior ages (on polished grains) are older, ranging from 60 to 200 ka. Zircons from Tenguyama record unpolished rim model ages of ca. 21 to 90 ka and interior ages of 37 to 225 ka. Rim-interior age differences range from 24 to 122 ka and 6 to 180 ka for Heisei-Shinzan and Tenguyama, respectively. No model ages younger than 20 ka have so far been determined. This leads to the hypothesis that eruptions at Unzen are tapping a long-lived, multi-cycle mush of significant longevity. Acquisition of additional U-Th/U-Pb data, including trace elements from other Unzen domes ranging in age back to 440 ka, is underway to test this hypothesis and more tightly link volcanism with subsurface processes.