U–Th–Ra disequilibria and the time scale of fluid transfer and andesite differentiation at Arenal volcano, Costa Rica (1968–2003)

To evaluate the time scale of fluid transfer and andesite differentiation at Arenal volcano in Costa Rica, we have measured trace-element concentrations and U-series disequilibria in whole rocks and mineral separates (pyroxene, plagioclase, magnetite) from lavas of the current eruption (1968 to 2003) by HR-ICP-MS, TIMS and PIMMS techniques. Whole rock and mineral separate analyses show a small but measurable variation in (230Th)/(232Th) (1.10 to 1.18). In contrast, (230Th)/(238U) range from 0.91 to 1.04 reflecting the moderate spread in Th/U. Stage 1 (1968–1971) whole rocks and mineral separates have both higher (230Th)/(232Th) and (238U)/(232Th) than to younger stage 2 lavas (1971 to present), which have lower, nearly constant (230Th)/(232Th) and lower, slightly variable (238U)/(232Th). 226Ra excesses exist in both whole rocks and mineral separates with (226Ra)/(230Th) ranging between 0.94 and 4.8. Whole rock (226Ra)/(230Th) are largest early in the eruption and decrease in the later lavas, which are influenced by newer recharge material. 30Th whole rock and mineral data produce an inclined array on an equiline diagram, which we interpret to represent progressive melting of a variably fluxed mantle wedge and a Nicaraguan sediment component, and subsequent mixing. 238U–230Th internal isochrons suggest that minerals grew instantaneously with respect to the half-life of 230Th. Whole rock and mineral separate (226Ra)/(230Th) data indicate that melts were produced, transported, differentiated and erupted in < 8 kyr. Mineral (226Ra)/Ba–(230Th)/Ba model ages are calculated and corrected for melt inclusions and glass adherents in the mineral fractions, and for the differential partitioning of Ra and Ba. Plagioclase model ages and 238U–230Th isochron ages suggest that plagioclase could be as young as a few years or as old as several centuries upon eruption.