HELIUM, RADON AND MANGANESE AT THE TAG AND SNAKE-PIT HYDROTHERMAL VENT FIELDS, 26-DEGREES AND 23-DEGREES-N, MID-ATLANTIC RIDGE

He-3, He-4 and Rn-222 have been measured in samples of vent fluids from the TAG and Snakepit hydrothermal vent fields and in samples from the neutrally buoyant plume at TAG. The hydrothermal end-member He-3/He-4 at TAG, normalised to the ratio in air is R/R(a) = 7.5 +/- 0.9, Rn-222 = 187 dpm kg-1 and Ra-226 = 13.5 +/- 0.4 dpm kg-1. At Snakepit, R/R(a) = 7.86 +/- 1.85, Rn-222 = 82 dpm kg-1 and Ra-226 = 10.3 +/- 0.2 dpm kg-1. Helium isotope-tracer systematics have been described which allow hydrothermal end-member properties to be estimated from plume data or non-conservative behaviour in the buoyant plume to be identified. Comparison of plume Mn concentrations and He isotope data shows that Mn behaves conservatively in the plume system over the time scale considered. In contrast, Rn-222 is highly non-conservative and there is a large Rn-222 excess in the neutrally buoyant plume (the hydrothermal end-member accounts for less than 2.5% of the excess Rn-222 enrichment at the top of the buoyant plume) attributed to entrainment of Rn-222-enriched fluids during buoyant plume rise. Rn-222/He-3 ages of the TAG plume give values of around 1-2 days close to the vent field and up to 10 days in adjacent waters. Rn-222/Mn ages are similar, except for a region of anomalous water at the level of neutral buoyancy. which is enriched in Rn-222 and Si but not He-3 and Mn, and which may be derived from the walls of the axial valley at the depth of the neutrally buoyant plume.