Dacite formation at Ilopango Caldera, El Salvador: U-series disequilibrium and implications for petrogenetic processes and magma storage time

[1] Ilopango Caldera was formed ∼1810 years ago by eruption of the Tierra Blanca Joven (TBJ) dacite (70 km³) in central El Salvador. A subsequent eruption in 1880 produced a cluster of dacite domes in the center of Lago Ilopango that contain olive-bearing enclaves of basaltic andesite. The purpose of this study is to use trace element, isotope, and U-series data from the TBJ and 1880 eruptions to assess petrogenesis and the timescale of magma storage. We find that although the range of trace element data in the TBJ dacite can be reproduced by simple crystal fractionation of a plagioclase- and amphibole-rich mineral assemblage, the⁸⁷Sr/⁸⁶Sr and ²⁰⁷Pb/²⁰⁴Pb data suggest that the 1880 basaltic andesite enclave has a different source than the dacites. This is consistent with U-series data that show the TBJ dacites have lower (²³⁰Th/²³²Th) than the 1880 basaltic andesite enclave (1.5 versus 1.6, respectively). All Ilopango rocks have ²³⁰Th excesses, and the range in (²³⁸U/²³²Th) of the TBJ dacites can be modeled by crystal fractionation of a mineral assemblage including accessory zircon and allanite from a magma that is similar in composition to the enclave. Mineral isochrons yield crystallization ages of <10,000 years, and disequilibrium (²²⁶Ra/²³⁰Th) values suggest a similarly short residence time. Our data suggest that the large volumes of erupted dacite at Ilopango Caldera are generated very rapidly.