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 E arth's volcanism and influences its heat budget, morphology and the co mposition of the oceans. It is therefore important to constrain the si ze, frequency and compositional variability of ridge eruptions, but th is is made difficult by the remoteness of much of the sea floor and th e paucity of applicable radioactive chronometers. Remote monitoring ha s provided indirect evidence for active ridge volcanism(1-3), which in some cases has been strengthened by submersible observations(4-5), bu t it has not been possible to date these eruptions directly. Here we p resent a new chronometer based on Po-210-Pb-210 radioactive disequilib rium, which allows us to date glassy eruption products within a few ye ars 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 ov er at least one year. The chronometer begins with Po volatilization du ring eruption (all glasses analysed were 75-100% degassed). A crystall ine pillow interior was <20% degassed, however, implying that the flow surface may control Po degassing during deep submarine eruptions. Alt hough the Po-210-Pb-210 chronometer is useful for only a short time-fr ame, it should prove valuable for dating recent but unobserved submari ne eruptions and determining details of eruptive sequences.