U-series crystal ages in historic eruptions of Volcan Quizapu, Chile

The time scales of generation, homogenization, and storage of large volumes of intermediate to silicic magmas have been the subject of much recent debate. Crystal ages determined by U/Th, Rb/Sr and U/Pb dating can help delimit these time scales, and for many of these systems are on the order of several hundred thousand years. Voluminous eruptions with SiO2 contents of 65-70 wt.% that are recent enough to employ 226Ra-230Th dating are rare. We present preliminary U-Th-Ra crystal ages for historic eruptive products of Volcan Quizapu in Central Chile, a satellite vent of Cerro Azul. Quizapu erupted 4-5 km3 of dacite in 1846/47 and again in 1932, of almost identical bulk composition (67-68 wt.-%). The 1932 dacite was accompanied by volumetrically minor rhyodacite and by an initial and terminal andesite, which most likely was not stored in the main magmatic reservoir (Hildreth and Drake, 1992, Bull. Volc. 54:93-125). Plagioclase and glass separates for the 1932 rhyodacite and the 1846 dacite show 230Th-232Th ratios indistinguishable from each other and within error of 230Th-238U radioactive equilibrium. When an amphibole separate is included, the data for the rhyodacite yield a zero age isochron indicating crystallization within few thousand years of eruption. In addition, preliminary 226Ra-230Th data for plagioclase in the 1932 rhyodacite and the 1846 dacite show 226Ra excess. Taking differential partitioning of Ra and Ba into account and assuming that plagioclase crystallized from the entraining glass separates yield crystal ages of less than ~3000 years and less than ~5600 years, respectively, consistent with the 230Th-238U isochron age for the rhyodacite sample. Additional analyses are in progress, including plagioclase and amphibole separates from the 1932 dacite. Based on the interpretation from previous studies that the main magmatic system remained almost compositionally homogenous over a timescale of 80 years and that the crystallization and removal of major phases was minor, it is likely that plagioclase phenocrysts stayed dispersed in the magma between the two major eruptions. We therefore predict that crystal ages in the two dacites will be similar and young. This would imply that homogenous intermediate magma bodies on the order of ~10 km3 may evolve and establish within a few ten thousand years and may not need long timescales as might be inferred from older magmatic systems.