Hydrothermal signatures along the Carlsberg ridge using magnetic and geochemical investigations

The Carlsberg ridge (CR) is a slow spreading mid-ocean ridge (MOR), which separates the Indian and Somalian tectonic plates in the northwest Indian Ocean. The CR has a total length of 1500 km with only four sites having hydrothermal activities being identified. Hydrothermal activities associated with the MORs can leave its signatures into the sediments. Eight sediment spade cores (SCs) from the ridge valley and flank regions of CR have been analysed for rock magnetic and geochemistry to find/fingerprint the presence of hydrothermal activity. Magnetic susceptibility (clf) ranges between 1.3 and 37.1 x10^-8 m³ with relatively higher values in the ridge valley sediment cores. High clf and coarser magnetic grainsize observed in area A and two cores from area B can be probable new sites with active/ extinct hydrothermal activity in the vicinity . S – ratio and X-ray Diffraction (XRD) confirms the presence of magnetite as the main magnetic mineral in the sediment. Calcium carbonate (CaCO₃) ranges from (42.8–89.6%) and organic carbon (Corg) varies between 0.3% and 2.9% in all the spade cores analysed. High concentration of CaCO₃ is present in ridge flank sediment may be related to high surface water column productivity. The sediment core 77/4 records clear enrichment of chalcophile elements Cu, Cd, Pb, Co, Zn, which may be due to rapid loss from the neutrally buoyant plume either by preferential settling, as such the above elements concentration is relatable with the concentration of another core 77/6 from area B except Cu and Cd. The low concentration of Cu and Cd may be linked to the oxidative dissolution of sulphides. Downcore similarity in variations in two sediment cores from the valley and flank, confirms common processes controls their variations, that may be present under the plume trajectory. The rare earth element (REE) concentration for all the cores varies from 84.1 ppm to 206.6 ppm with middle rare earth element (MREE) enrichment, and highest concentration of REEs for the flank core. Magnetics, trace elements and REE suggest hydrothermal activity in the study area.