The U-Th-Pb systematics in hot springs on the East Pacific Rise at 21 °N and Guaymas Basin

Abstract

We have determined the concentrations and isotopic compositions of U, Th and Pb in hydrothermal fluids from 21°N, East Pacific Rise and Guaymas Basin, Gulf of California. The purest hydrothermal end members (96%) have 0.06–0.18 ppb U, < 0.1–4.3 ppt Th and 40–67 ppb Pb. Several samples show a ^(234)U enrichment relative to the equilibrium value. This indicates that U was quantitatively removed from seawater and deposited to the crust during the hydrothermal circulation. The 21°N fluids with intermediate Mg content show that U and Mg are coherently removed from seawater, but Pb is not, during mixing of the hot hydrothermal fluid and cold ambient seawater. Both the end-member and intermediate hydrothermal fluids at 21°N have similar Pb isotopic compositions and limited ranges in ^(206)Pb/^(204)Pb (18.444–18.503), ^(207)Pb/^(204)Pb (15.471–15.514), and ^(208)Pb/^(204)Pb (37.832–37.966). These ratios are within the range of values of MORB and are distinctly less radiogenic than the ambient seawater. This means that a significant amount of Pb was removed from the basalts by the hot springs. In contrast, Th does not appear to be significantly removed from the basalts. Some of this Pb was incorporated into the metalliferous sediments in a wide area straddling the EPR. The Pb isotopic composition of a hydrothermal sample from the Guaymas Basin is more radiogenic than at 21°N and resembles that of sediments from the Gulf of California. This is consistent with the uptake of Pb from heated sediments having a substantial component of the volcanogenic detritus. Using the present data and the literature values of the natural river fluxes, the global hydrothermal system removes ~25% U from the total natural U flux to the ocean and contributes < 0.02% Th and ~2% Pb to the ocean. The U/Pb ratio in the hydrothermally altered oceanic crust may be increased by from 40% to a factor of 2, and the Th/U ratio decreased by from 10 to 25%. The ridge crest-hydrothermal system represents only a part of a continuous geochemical process which modifies the U-Th-Pb systematics of the oceanic upper crust or ultimately the source of some oceanic island basalts.

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