(~(210)Pb/~(226)Ra) variations during the 1994-2001 intracaldera volcanism at Rabaul Caldera

摘要

Determining the timing and source of gas transfer during intermittent intracaldera volcanism can aid in our understanding of degassing in these large systems. Using (~(210)Pb/~(226)Ra) ratios, (parentheses denote activity ratios) as a time-sensitive tracer, injections of ~(222)Rn and the subsequent time scales of gas accumulation and loss can be determined. Variations in (~(210)Pb/~(226)Ra) have been measured for 15 volcanic products erupted at Rabaul Caldera over the period 1994 to 2001. In addition, one basaltic enclave from the 1937 eruption was also analyzed. Water and carbon dioxide contents determined from olivine hosted melt inclusions erupted in 1997 are < 1% and suggest extensive shallow-level degassing. Both ~(210)Pb excesses and deficits are found in andesites and dadtes, whereas the basaltic enclave displays an (~(210)Pb/~(226)Ra)_0 ratio of 7. Between 1994 and 1997 three samples with (~(210)Pb/~(226)Ra) deficits were erupted which indicate open system gas loss since 1992 and 1994. No correlation exists between (~(210)Pb/~(226)Ra) and lava chemistry, eruptive style or date. ~(210)Pb excesses are more common than deficits in Rabaul samples but cannot be explained by plagioclase feldspar accumulation, Pb sublimate accumulation or differentiation. Instead, a model of intra-magma ~(222)Rn transfer can produce 210Pb excesses of the appropriate magnitude if gas transfer occurs over 1-5 years from an underlying body of magma that is 2-10 times larger than the volume of erupted material and that is consistent with geophysical estimates. Although intermittent gas transfer events can be inferred by the development of ~(210)Pb excess, there is no evidence at Rabaul for a direct link between eruptive style, gas flux and (~(210)Pb/~(226)Ra).