Anomalous phase association of REE in ferromanganese crusts from Indian mid-oceanic ridges: Evidence for large scale dispersion of hydrothermal iron

Abstract Ferromanganese crusts collected at selected locations along the ~7000 km long stretch of Carlsberg, Central Indian and Southwest Indian ridges are studied for bulk geochemical composition and distribution of rare earths and yttrium in major geochemical phases; exchangeable/carbonate, Mn-oxide, Fe-oxyhydroxide and residual silicate. The crusts are dominated by vernadite with traces of quartz and calcite. A high concentrations of major (Fe, Mn), trace (Co, Ni, Cu & Zn) and rare earth elements indicates that these crusts are hydrogenous in nature. However, the negative or no cerium anomalies in the shale normalized REE pattern indicates that some of these crusts are of mixed type (hydrogenous and hydrothermal). Irrespective of their nature (mixed or hydrogenous), the phase distribution of REE-Y shows 70–83% of total REE-Y are associated with Fe-oxyhydroxide phase, whereas only about 17–30% are associated with Mn-oxide phase. Such phase-distribution is expected in mixed type crusts because it is a combination of hydrothermal (where 90% of the total REE-Y is associated with only Fe-oxyhydroxide phase) and hydrogenous processes (where 50% of the total REE-Y is associated with only Fe-oxyhydroxide phase). Conversely, the hydrogenous crusts also shows similar phase-distributions (70–83% association with Fe-oxyhydroxide phase) which is certainly different from typical hydrogenous crusts where both Fe-oxyhydroxide and Mn-oxide phases host equal amounts (50% each). The large scale dispersion of hydrothermal particulate iron along the Indian Ocean ridges is likely to contribute an additional Fe-bound REE-Y pool to these crusts. The positive yttrium anomaly in Mn-oxide phase of all these crusts substantiates the hydrothermal contribution.