PO-210-PB-210 DATING OF RECENT VOLCANIC-ERUPTIONS ON THE SEA-FLOOR

THE globe-encircling mid-ocean ridge system accounts for most of the Earth's volcanism and influences its heat budget, morphology and the composition of the oceans. It is therefore important to constrain the size, frequency and compositional variability of ridge eruptions, but this is made difficult by the remoteness of much of the sea floor and the paucity of applicable radioactive chronometers. Remote monitoring has provided indirect evidence for active ridge volcanism(1-3), which in some cases has been strengthened by submersible observations(4-5), but it has not been possible to date these eruptions directly. Here we present a new chronometer based on Po-210-Pb-210 radioactive disequilibrium, which allows us to date glassy eruption products within a few years of their eruption. Our first results, on lavas from 9 degrees 50' N on the East Pacific Rise, confirm that the ridge erupted only months before sample collection, and indicate that the eruptions extended over at least one year. The chronometer begins with Po volatilization during eruption (all glasses analysed were 75-100% degassed). A crystalline pillow interior was <20% degassed, however, implying that the flow surface may control Po degassing during deep submarine eruptions. Although the Po-210-Pb-210 chronometer is useful for only a short time-frame, it should prove valuable for dating recent but unobserved submarine eruptions and determining details of eruptive sequences.