Lead behaviour at the TAG hydrothermal vent field, 26°N, Mid-Atlantic Ridge

Abstract

Samples of basaltic glass, hydrothermal vent fluids, hydrothermal plume particles, mound deposits and metalliferous sediments from the TAG hydrothermal field have been analysed. Local MORB has an isotopic composition of 208Pb/ 204Pb = 37.568, 207Pb/ 204Pb = 15.450, 206Pb/ 204Pb = 18.124 . Directly upon emission vent fluid contains 90 nmol kg −1 Pb with a molar Pb/Fe ratio of 1.5 × 10 −5. The high-temperature vent fluid has an isotopic composition of 208Pb/ 204Pb = 37.734, 207Pb/ 204Pb = 15.476, 206Pb/ 204Pb = 18.304 which is more radiogenic than MORB. Chalcopyrite, atacamite and oxides from the TAG mound also exhibit a slightly more radiogenic Pb isotopic composition than MORB which is suggestive of some seawater or sediment influence. Dissolved Pb concentrations decrease to ∼1 nmol kg −1 immediately above the vent because of sulfide precipitation. Particulate Pb behaves conservatively in the buoyant hydrothermal plume over dilutions of ∼ 1-0.1% of the endmember fluids with ambient seawater and Pb/Fe ratios are maintained. Since about 50% of the Fe is thought to be removed from the buoyant plume by fall-out of sulfides during this mixing, a similar proportion of Pb is similarly removed. The buoyant plume particles have non-radiogenic isotopic compositions ( 208Pb/ 204Pb = 37.734–37.943, 207Pb/ 204Pb = 15.497–15.525, 206Pb/ 204Pb = 18.176–18.463 ), intermediate between vent fluid and basalt, consistent with a dominant hydrothermal source and appear to be mainly associated with sulfides, although there is some evidence from Pb/Fe and 210Pb/Pb ratios that some uptake of Pb from seawater occurs. Sulfides are seen with Fe-oxide particles from the neutrally-buoyant plume. Pb isotope data for these particles fall within a field defined by local vent fluids, modern terrigenous Pb, and what is inferred to be local seawater. Hydrothermal sediments from TAG have a mean Pb/Fe ratio of 10.5 × 10 −5, which is within the range observed in neutrally-buoyant plume particles (9.0–29 × 10 −5) and TAG mound materials (1.3–26 × 10 −5) suggestive of a dual origin. Pb isotopic compositions can be explained by mixing between MORB and Quaternary terrigenous Pb, with some evidence of seawater alteration of mound material.