A geochemical study of metalliferous sediment from the TAG Hydrothermal Mound, 26°08′N, Mid-Atlantic Ridge

An Alvin push core collected near the base of the TAG hydrothermal mound, 26°08′N, Mid-Atlantic Ridge, sampled a 7 cm-thick layer of Fe-rich red-brown mud (20–40% Fe) overlying a 4 cm-thick layer of carbonate ooze (5–70% CaCO₃) which also contains up to 32%Fe. On the basis of chemical compositions, two separate layers can be identified within the red-brown mud. The core position, X ray diffractometry, Pb isotope analyses, and elemental abundances all indicate that these two layers were derived from mass wasting of oxidized, originally sulphidic material from the slopes of the TAG hydrothermal mound. High concentrations (11–19 ppm) of seawater-derived U occur in both Fe-rich layers. These enrichments may derive from uptake of U linked to the oxidation of sulphide phases. Metal concentrations in the rich layer are lower than in the upper part of the core but higher than in open ocean pelagic sediments. The ²³⁰Th_xs/Fe, ²³¹Pa_xs/Fe, and rare earth element/Fe distributions indicate that the hydrothermal input to the carbonate-rich layer is dominated by settling of suspended particulate material from the overlying hydrothermal plume. Such CaCO₃-rich sediments may record an important component of the flux of hydrothermal material to sediments of the Mid-Atlantic Ridge rift valley.