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

• Published in: Earth and planetary science letters 2010-05, Vol.293 (3-4), p.331-338
• Main Authors: Condomines, Michel, Sigmarsson, Olgeir, Gauthier, Pierre-Jean
• Format: Article
• Language: English
• Subjects: Earth Sciences; Environmental Sciences; Geochemistry; Global Changes; Sciences of the Universe
• ISSN: 0012-821X
• DOI: 10.1016/j.epsl.2010.02.048

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