A chemical model of the buoyant and neutrally buoyant plume above the TAG vent field, 26 degrees N, Mid-Atlantic Ridge

Abstract

The kinetics of iron particle formation in the neutrally buoyant plume above the TAG vent field (26 degrees N, Mid-Atlantic Ridge) have been calculated from submersible-collected CTD data within the initial 150 m of plume rise. Results show that particles form by a two-stage process: about half the iron in the high temperature vent fluid is removed as sulfides within a few seconds of venting and the remainder is removed by Fe 2+ oxidation. The pseudo-first-order rate constant for the second process has been calculated ( k 1 = 0.329min −1, similar to literature values for seawater) and gives a halflife time for Fe 2+ in solution of 2.1 minutes. The kinetics of iron particle formation have been used in a conceptual model of the chemistry of the TAG plume. The average dilution at which iron oxyhydroxide particles form, E¯ Fe, is ∼ 570 from which element/Fe ratios of particles at the top of the buoyant plume have been predicted. Oxyanion/Fe ratios can be chiefly accounted for by coprecipitation for Cr (71%), V (67%), As (45%) and P (42%) but Mo (0.1%) and U (0.02%) show anomalously low coprecipitation. Th/Fe and REE/Fe ratios are greater than can be accounted for by coprecipitation, demonstrating that scavenging occurs in the buoyant plume for these elements. 98% of the Th uptake and 15–75% of the REE uptake is by scavenging. Scavenging rate constants are 3.1 * 10 −6 (nmol/kg) −1 s −1 for Th and 1.4–33* 10 −8 (nmol/kg) −1 s −1 for the REE. A scavenging model has been fitted to trace metal data previously reported for neutrally buoyant hydrothermal plume particulate samples collected above the TAG vent field. The model is based on the assumption that there is a characteristic t 1/2 for the dilution of the neutrally buoyant plume and this value has been calculated, by comparing 228Th and 230Th with 234Th isotope data, as fourty-one days (λ p = 0.0170 day −1). Scavenging rate constants are 2* 10 −9 (nmol/kg) −1 s −1 for Th and 3.5–16*10 −11 (nmol/kg) −1 s −1 for the REE. This shows preferential removal of the intermediate REE from sea water, a pattern which has also been found in those TAG sediments formed from neutrally buoyant plume fallout. Chalcophile elements (Cu, Zn, Co, Pb, Sn) all appear to undergo release from plume particles at the same rate ( t 1/2 ≈ 42days) suggesting alteration or removal of a common substrate. Model ages for particles above the TAG vent field are <50 days within 500 m of the vent field and 50–100 days at greater distances or below the height of the neutral plume. Quantitative removal of vent fluid derived REE, with the possible exception of Eu, during buoyant plume rise means that hydrothermal activity has no direct impact on the seawater chemistry of the REE. If coprecipitation and scavenging within the TAG hydrothermal plume are typical, such processes during plume rise (the buoyant plume) and dispersion (the neutrally buoyant plume) play a significant role in the removal of reactive trace metals and oxyanions from seawater, at rates of the same order as those of river input to the oceans.