Selective leaching studies of sediments from a seamount flank in the Central Indian Basin: Resolving hydrothermal, volcanogenic and terrigenous sources using major, trace and rare-earth elements

Abstract

We have investigated the sediments from a site where a newly discovered submarine hydrothermal alteration of recent origin is located to identify the source components and estimate their relative contribution. The site is situated on the flank of a seamount in the Central Indian Basin along the 76° 30′ E fracture zone which traces the movement of the Rodriguez triple junction. Detailed analyses of major, trace and rare earth elements (REE) on the bulk, leach, residue and clay fraction, and infra red spectroscopy studies of the clay fraction helped characterize the hydrothermal nature of the sediments. Selective dissolution carried out using HCl has allowed the discrimination between a leach phase (leachable Fe–Mn oxide) and a residue phase (refractory). The sediments are iron-rich, majority of Fe, Mn is in leachable fraction and are compositionally comparable to ridge-crest metalliferous sediments. Elemental ratios and geochemical discrimination plots of major element data reveal that the sediments are a mixture of Fe-rich mineral phases on one hand apparently from hydrothermal sources and aluminosilicates such as illite, zeolites, micaceous clays and smectites contributed from the Bengal fan, weathering of basinal basalts and authigenic processes. Ternary mixing calculations of bulk sediment major element data show that 35–45% metalliferous sediments could reproduce Fe and Mn values of our sediments. The shale normalized REE patterns of the bulk sediments are remarkably similar to those of the HCl soluble leach fraction, implying that the REEs in the HCl soluble leach fraction dominate the bulk fraction. HCl leacheates exhibit characteristics of vent fluids as well as seawater with pronounced Eu anomaly with HREE enrichment and negative Ce anomaly. The residue on the other hand has a high concentration of Fe, V, Zn, and Pb indicating a sulfidic input to the sediments. Ternary mixing calculations of REEs carried out on the residual fraction indicate that the sulfide-like material dominates (64–72%) over terrigenous (10–20%) and MORB derived (16–18%) components. This study helped us identify and quantify two types of hydrothermal material in the sediments, dominated by Fe–Mn phases (ferruginous oxides and silicates; Mn-oxides) in leachable fraction and sulfide-like material in residual phases. The clay fraction is mainly composed of dioctahedral smectites and has a composition similar to Alvin nontronite. The geochemical and mineralogical characteristics collectively suggest that these sediment-hosted precipitates formed at relatively low temperatures ranging between 50 and 100 °C from a mixture of seawater and hydrothermal fluids. The presence of hydrothermal sediments in the vicinity of a fracture zone suggests the occurrence of localized hydrothermal activity associated with intra plate volcanism. This is probably induced by reactivation of tectonic forces along the fracture zones.