LEAD BEHAVIOR AT THE TAG HYDROTHERMAL VENT FIELD, 26-DEGREES-N, MID-ATLANTIC RIDGE

Samples of basaltic glass, hydrothermal vent fluids, hydrothermal plum e particles, mound deposits and metalliferous sediments from the TAG h ydrothermal field have been analysed. Local MORB has an isotopic compo sition of Pb-208/Pb-204 = 37.568, Pb-207/Pb-204 = 15.450, Pb-206/Pb-20 4 = 18.124. Directly upon emission vent fluid contains 90 nmol kg-1 Pb with a molar Pb/Fe ratio of 1.5 x 10(-5). The high-temperature vent f luid has an isotopic composition of Pb-208/Pb-204 = 37.734, Pb-207/Pb- 204 = 15.476, Pb-206/Pb-204 = 18.304 which is more radiogenic than MOR B. 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 concen trations decrease to approximately 1 nmol kg-1 immediately above the v ent because of sulfide precipitation. Particulate Pb behaves conservat ively in the buoyant hydrothermal plume over dilutions of approximatel y 1-0.1% of the end-member fluids with ambient seawater and Pb/Fe rati os 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 s imilar proportion of Pb is similarly removed. The buoyant plume partic les have non-radiogenic isotopic compositions (Pb-208/Pb-204 = 37.734- 37.943, Pb-207/Pb-204 = 15.497-15.525, Pb-206/Pb-204 = 18.176-18.463), intermediate between vent fluid and basalt, consistent with a dominan t hydrothermal source and appear to be mainly associated with sulfides , although there is some evidence from Pb/Fe and Pb-210/Pb ratios that some uptake of Pb from seawater occurs. Sulfides are seen with Fe-oxi de particles from the neutrally-buoyant plume. Pb isotope data for the se particles fall within a field defined by local vent fluids, modem t errigenous Pb, and what is inferred to be local seawater. Hydrothermal sediments from TAG have a mean Pb/Fe ratio of 10.5 x 10(-5), which is within the range observed in neutrally-buoyant plume particles (9.0-2 9 x 10(-5)) and TAG mound materials (1.3-26 x 10(-5)) suggestive of a dual origin. Pb isotopic compositions can be explained by mixing betwe en MORB and Quaternary terrigenous Pb, with some evidence of seawater alteration of mound material.