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 (LaSN/PrSN) than typical hydrogenetic Fe-Mn crusts from the Pacific Ocean. Despite negative LaSN/PrSN and NdSN/SmSN and positive EuSN/GdSN and DySN/HoSN 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 LaSN/PrSN 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.