Uranium-series chronology of Gorda Ridge volcanism: new evidence from the 1996 eruption

We present new uranium-series plagioclase and glass data for the 1996 eruption of the North Gorda Ridge. The glass data provide a more accurate estimate of ‘zero-age’ disequilibria for use in external isochron dating than was previously available. Furthermore, plagioclase–glass 226Ra–230Th disequilibria delimit the degree of initial fractionation of radium from barium during crystal growth, with effective DRa/DBa ∼0.25–0.5. These data are inconsistent with the common assumption that DRa=DBa but are qualitatively consistent with theoretical model predictions that radium and barium should be fractionated during crystallization, with DRa/DBa∼0.2. In more detail, differences between model predictions and data could be explained by an extreme combination of model and data uncertainties, but more likely suggest suppression of efficient fractionation during rapid crystallization. We also assess the extent to which use of barium as an analog for radium would result in underestimating 226Ra–230Th disequilibria produced during melting. Effects of a lower value of DRa on most melting models are small, with a slight increase in the porosity or melt fraction for a given value of 226Ra/230Th. The new plagioclase data also indicate that plagioclase accumulation and assimilation in the crustal reservoir would have only a negligible effect on mantle-derived 226Ra–230Th disequilibria. External isochron ages from U–Th, U–Pa, and Th–Ra data calculated using initial disequilibria from the 1996 sample are concordant, with one exception. Anomalously young ages for off-axis samples most likely reflect volcanism up to 1 km off-axis. Radium ages for near-axis dredge samples for other areas of the North Gorda Ridge generally range from 2000 to 4000 yr, similar to calculated steady-state eruptive periodicity of 3000 yr for this ridge segment. However, comparison of radium excesses for the 1996 eruption and a nearby older lava suggests that recent volcanism at this site occurs with relatively short-lived (<few hundred year) episodicity.