Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge

We measured significant activities of short-lived radium isotopes, Ra-223 (half-life= 11 days) and Ra-224 (half-life=3.7 days), around the margins of the Hawaiian Islands to water depths of 3500 m. These measurements suggest fluid inputs from the basalt to the surrounding ocean. In general Ra-223 activities were considerably greater than Ra-224 in spite of the expected higher production rate of Ra-224 activity in basalt. The Ra-223 was not supported by dissolved Ac-227. The highest enrichments of Ra-223 were measured over the Puna Ridge (2100 m depth) east of Hawaii. Here Ra-223 activities reached 19 dpm/m(3), similar to activities measured near sites of active submarine groundwater discharge in the South Atlantic Bight. To explain the high activities of 223Ra unaccompanied by Ra-224, we postulate that thermally-driven circulation of seawater through the Puna Ridge deposits Pa-231 on basalt surfaces. With time the Pa-231 produces Ac-227 and Ra-223; and Ra-223 desorbs into the circulating fluids. These fluids then transport Ra-223 into the overlying ocean. Based on the inventory of 223 Ra above the Puna Ridge, we estimate the flow of fluids through the ridge to be on the order of 20-60 cm(3) cm(-2) day(-1). In less than 1000 years the incoming seawater could provide enough Pa-231 to basalt surfaces to balance the inventory of Ra-223 above the ridge if only 8% of the Ra-223 was transported to the overlying water. These observations on the flanks of a volcanically-active ocean island have significant implications for quantifying fluid fluxes from the flanks of the mid-ocean ridge system. By mapping Ra-223 inventories in the ocean above ridge flanks and measuring the activity of Ra-223 in the emerging fluids, the fluid flux can be obtained. (c) 2007 Elsevier B.V All rights reserved.