Closed- to open-system differentiation at Arenal volcano (1968–2003)

Arenal volcano, located in northern Costa Rica, has been continuously erupting since 1968. Magmas during the first half of the eruption by volume (Stage 1: 1968–1971) were related by largely closed-system crystal fractionation that had produced a compositionally zoned magma chamber prior to 1968. It erupted downward from the most differentiated magma in 1968 to the most mafic by early 1971. In contrast, the second half of the eruption has been dominated by recharge and compositions have become more evolved with time (Stage 2: 1971–current). We base these conclusions on new major and trace element plus Sr–Nd–Hf–Pb isotope analyses of 56 whole rocks from throughout the eruption. Differentiates are enriched in incompatible elements in both stages, but compatible element concentrations drop much more during Stage 1 than 2. Changes during Stage 1 were successfully modeled using least squares and MELTS models despite the mineral complexity of the rocks. About 19% fractional crystallization of phenocryst phases (plagioclase > orthopyroxene > clinopyroxene > magnetite) is required, consistent with crystallization at 4 kb and from 1145 to 1088 °C of a melt initially containing 2.5 wt.% H2O at quite oxidizing conditions (QFM + 2). An implication is that most phenocrysts formed during decompression and degassing. Changes during the second stage were successfully modeled using EC-E′RAχFC with the ratio of recharge to crystallization decreasing from 17 to 5 over ∼ 30 years. Crystallization rates (dFS / dt) increase from 0.05 to 0.4%/a from closed- to open-system behavior and are even faster than inferred from U-series disequilibria. The recharging magma results from a smaller degree of flux melting of a mostly similar source than for the resident magma prior to the eruption, with the two events separated by ∼ 450 years. The most recent compositions have no precedent at Arenal.