Near-field hydrothermal plume dynamics at Brothers Volcano (Kermadec Arc): A short-lived radium isotope study

The naturally occurring radium (Ra) isotope quartet was measured for the first time at an intraoceanic arc hydrothermal setting along the Kermadec Island Arc (SW Pacific). Ra-224 (3.7 d half life) and Ra-223 (11.4 d half life) are assumed to be tracers solely originating from hydrothermal discharge and have been used to resolve near-field hydrothermal plume dynamics by applying both advection and eddy diffusion-based dispersion models. Assuming dominant advective processes, we identified different horizontal plume age structures at Brothers volcano, with a slow moving plume inside the caldera (0.060 cm/s) and a faster moving plume outside the caldera (0.323 cm/s), indicating a longer plume residence time within the caldera structure. The decreasing trend in Ra activity also allows the calculation of horizontal eddy diffusion coefficients inside and outside of the caldera, with 0.164 m(2)/s and 3.545 m(2)/s for Ra-224, respectively. These trends are in accordance with the radial horizontal eddy diffusion model. Our results reveal that net advection might not necessarily play a major role at Brothers, as radial horizontal eddy diffusion reproduces the observed Ra activity distribution to the same degree. Additional model simulations based on a combination of advection and diffusion, favor slow net advection to explain the Ra activity distribution. Within the rising non-buoyant plume, Ra isotopes suggested constant dilution along the first 170 m of the vertical water column and a vertical transport velocity on the order of 0.156 cm/s for altitudes up to 150 m above the vent and around 0.004 cm/s for altitudes above 150 m. When a common source of diffuse and focused venting is assumed and only small amounts of Ra are added to the ascending plume, due to recoil originating from high water to rock ratios, fluids from diffuse vents show a slower migration rate through the subsurface than fluids from focused vents. Our data imply that the fluids from diffuse vents needed approximately five days longer to percolate through the underlying subsurface to reach the water column plume, compared to the fluids from direct vents which emanate through chimney structures. This study demonstrates the potential of short-lived Ra isotopes as tracers of near-field plume dynamics, which will improve our understanding of hydrothermal vents as sources for various trace elements to the global ocean.