The origin of clay minerals in active and relict hydrothermal deposits

Samples of Fe-oxide-rich hydrothermal sediments were collected from active and inactive portions of the TransAtlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge. Clays separated from TAG metalliferous sediments in this study all consist of Al-poor nontronite. Oxygen isotope thermometry of the clays yields formation temperatures of 54-67°C for samples from the inactive Alvin mound compared with 81-96°C for samples from the active TAG site. The latter are the highest recorded temperatures for authigenic hydrothermal clays. Sr isotope analysis of the clays from the active mound suggests that they precipitated from seawater-dominated fluids, containing less than 15% hydrothermal end-member fluid. In contrast, nontronite from the inactive Alvin mound has ⁸⁷Sr/⁸⁶Sr values that closely resemble that of detrital North Atlantic clays, suggesting a dominantly continental source for the Sr. Rare earth element data are consistent with a significant detrital input to the inactive site but also demonstrate the extent of hydrothermal input to the low temperature fluid. Crystallographic fractionation of the trivalent REE is apparent in the heavy REE enrichments for all nontronite samples. The inferred formation-mechanism for nontronite-rich Fe-oxyhydroxide deposits at the surface of the active mound is by direct precipitation from low temperature fluids. At the inactive Alvin site, in contrast, the deposits form during alteration of pelagic sediments by diffuse fluids and replacement of biogenic carbonate with nontronite and Fe-oxyhydroxide. These two modes of formation are both important in seafloor hydrothermal settings where clay minerals are a significant component of the hydrothermal deposit.