A CHEMICAL-MODEL OF THE BUOYANT AND NEUTRALLY BUOYANT PLUME ABOVE THETAG VENT FIELD, 26 DEGREES-N, MID-ATLANTIC RIDGE

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 proc ess: about half the iron in the high temperature vent fluid is removed as sulfides within a few seconds of venting, and the remainder is rem oved by Fe2+ oxidation. The pseudo-first-order rate constant for the s econd process has been calculated (k(1) = 0.329 min-1, similar to lite rature values for seawater) and gives a half-life time for Fe2+ in sol ution of 2.1 minutes. The kinetics of iron particle formation have bee n used in a conceptual model of the chemistry of the TAG plume. The av erage dilution at which iron oxyhydroxide particles form, E(Fe)bar is approximately 570 from which element/Fe ratios of particles at the top of the buoyant plume have been predicted. Oxyanion/Fe ratios can be c hiefly 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 coprec ipitation. Th / Fe and REE / Fe ratios are greater than can be account ed 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 1 0(-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 previ ously reported for neutrally buoyant hydrothermal plume particulate sa mples collected above the TAG vent field. The model is based on the as sumption that there is a characteristic t1/2 for the dilution of the n eutrally buoyant plume, and this value has been calculated, by compari ng Th-228 and Th-230 with Th-234 isotope data, as fourty-one days (lam bda(p) = 0.0170 day-1). Scavenging rate constants are 210(-9) (nmol/k g)-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 n eutrally buoyant plume fallout. Chalcophile elements (Cu, Zn, Co, Pb, Sn) all appear to undergo release from plume particles at the same rat e (t1/2 almost-equal-to 42 days) suggesting alteration or removal of a common substrate. Model ages for particles above the TAG vent field a re <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 remov al of vent fluid derived REE, with the possible exception of Eu, durin g buoyant plume rise means that hydrothermal activity has no direct im pact on the seawater chemistry of the REE. If coprecipitation and scav enging within the TAG hydrothermal plume are typical, such processes d uring plume rise (the buoyant plume) and dispersion (the neutrally buo yant plume) play a significant role in the removal of reactive trace m etals and oxyanions from seawater, at rates of the same order as those of river input to the oceans.