HOW DOES DEGASSING IN AN ACTIVE CALDERA DIFFER FROM OTHER SYSTEMS? U-SERIES ISOTOPES CONSTRAIN THE TIME SCALES OF GAS TRANSFER AT RABAUL CALDERA, PNG

While it is acknowledged that gas transfer and volatile accumulation play an important role in triggering volcanic eruptions, constraining the time scales over which these processes occur is problematic. U-series isotopes provide a method for assessing the time scales of degassing because intermediary ²²²Rn is formed. Fifteen volcanic bombs were measured for (²¹⁰Pb/²²⁶Ra) disequilibrium from the 1994-2001 eruption at Rabaul Caldera. Both ²¹⁰Pb excesses and deficits were measured. Three samples with ²¹⁰Pb deficits (0.85 – 0.93) indicate open system gas loss from a magma batch emplaced in 1992. Four samples display significant ²¹⁰Pb excess (1.23- 1.92) that can be attributed to internal gas transfer or introduction of volatiles from an underlying injected magma batch. Internal differentiation, sublimate contamination or plagioclase accumulation cannot explain the development of ²¹⁰Pb excesses. A comparison of the temporal variations at Rabaul Caldera with stratovolcano eruptions at Arenal and dome growth at Mount St Helens shows fundamental differences. In particular, at Arenal and Mount St Helens most samples lie within secular equilibrium except during short periods surrounding major eruptive activity. However, at Rabaul ²¹⁰Pb excesses are prevalent during the latter part of the eruptive sequence.