Rare earth elements and yttrium (REY) variability with water depth in hydrogenetic ferromanganese crusts

The origins of ferromanganese (Fe–Mn) crusts that form in seawater and elemental partitioning into these crusts have previously been studied using samples from water depths shallower than 4 km. Here we report geochemical analyses of Fe–Mn crusts sampled from petit-spot volcanoes within the Pacific Ocean at water depths of 5.2–6.0 km. The Fe–Mn crusts have low Mn/Fe ratios (0.63–1.5), high contents of Co, Ni, and rare earth elements and Y (REY), and positive Ce anomalies, indicating a hydrogenetic origin. The Fe–Mn crusts are characterized by lower shale-normalized La/Pr ratios (La_SN/Pr_SN) than typical hydrogenetic Fe–Mn crusts from the Pacific Ocean. Despite negative La_SN/Pr_SN and Nd_SN/Sm_SN and positive Eu_SN/Gd_SN and Dy_SN/Ho_SN correlations with water depth, the distribution coefficients of the REY are constant. This indicates that the observed REY variability with water depth in these hydrogenetic Fe–Mn crusts reflects that of seawater. In particular, a strong correlation between La_SN/Pr_SN and water depth has potential for use as a (paleo)ocean hydro-barometer. The variety of REY compositions of Fe–Mn crusts should be interpreted on the basis of distribution coefficients calculated for each water depth.