Abstract
Manganese oxide crusts were recovered from Baby Bare seamount in order to investigate the history of off-axis hydrothermal venting. Baby Bare is a small basement high protruding from a regional sediment cover on the eastern flank of the Juan de Fuca Ridge that acts as a focus for discharging crustal fluids. Stratabound Mn-oxide crusts were collected where warm venting has been observed near the seamount summit. Mn-oxide crusts are composed primarily of 10 Å manganate ± pyrolusite, with minor nontronite, saponite, and/or barite. These assemblage and chemical characteristics such as high Mn/Fe ratios and low trace metal and REE concentrations are indicative of a hydrothermal origin. Minimum ages for these deposits, calculated using growth rates (324 to ∼ 1800 mm/Ma) and estimated thicknesses of manganese outcrops, show that Baby Bare has been hydrothermally active for at least 0.5 Myr, and possibly since its formation (1.7–2.7 Ma). Hydrothermal manganese oxide crusts such as these from Baby Bare record interactions between the hydrothermal fluids and seawater and are important tools for estimating the longevity of off-axis hydrothermal activity.
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