Particle recycling in the TAG hydrothermal plume
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Analysis of Th isotope distributions in the TAG hydrothermal plume yields evidence for pronounced Th-234/Th-230 isotope fractionation. This fractionation can be attributed to a combination of four processes: (1) particle ageing; (2) differential scavenging of Th-234 and Th-230; (3) differential focusing of Th-234 and Th-230 and (4) recycling of old particles. We present arguments for the importance of the last of these processes, particle recycling. Particles in the TAG hydrothermal plume exhibit grain sizes which fall in the range 1-10 mum. Applying the range of densities reported for amorphous iron oxyhydroxide and iron oxide mineral phases, Stokes’ Law predicts that these particles should achieve terminal settling velocities ranging between 4 and 8 m d 1 for coarse-grained (5 mum radius) particles and 0.2-0.3 m d-1 for fine-grained particles (1 mum radius). These velocities have been used in a physical model of re-entrainment and recycling in neutrally buoyant plumes. Our results indicate that particle recycling and re-entrainment in hydrothermal plumes may be important over length scales of 1-10 km. The model predicts that the proportion of particles re-entrained is independent of particle size but increases with plume strength. For plumes with volumetric fluxes greater than 10(3) m3 s-1, the proportion of particles re-entrained exceeds 50%, in agreement with previous calculations based upon Th-230 distributions. Sampling strategies in the past have typically concentrated upon processes operating within less-than-or-equal-to 1 km of active vent fields and will need to be revised accordingly in the future.