A simple model of 222Rn accumulation leading to 210Pb excesses in volcanic rocks

While most rocks from active volcanoes display (210Pb/226Ra) activity ratios lower than or close to the equilibrium value of 1, several other have ratios much higher than 1. Transfer of 222Rn by magmatic volatiles has often been advocated to explain both 210Pb deficits and excesses. We develop here a model to account for 210Pb excesses through 222Rn accumulation. We show that large 210Pb excesses can be readily obtained, even for moderate ratios of degassing magma over accumulating magma, in a closed-system model where the 222Rn atoms produced by decay of 226Ra in the degassing magma are continuously extracted by a gas phase. For a given duration of volatile transfer, relative 210Pb excesses are expected to be much larger than 210Pb deficits. This model is applied to samples from Santorini (Aegean arc) and Surtsey (Iceland) both of which show high (210Pb/226Ra) ratios of 6.7 and 2.3 respectively at the time of eruption. The agreement with field data suggests that 222Rn accumulation leading to 210Pb excesses might be a rather common process in zoned magma chambers as well as in individual lava flows, and that it can be described by a closed-system model.