Evidence for recent hydrothermal activity in the Central Indian Basin

This study documents the first actual proof of recent intraplate volcanic-hydrothermal activity in the Central Indian Basin (CIB). Twenty-six surface sediments and a spade core (37 cm long) from the CIB were examined for the presence of volcanogenic-hydrothermal materials (vhm). High concentrations of vhm were discovered in a grab and the core top—both located at the base of an intraplate seamount. The vhm consist of ochrous metalliferous sediments, volcanic spherules and glass shards. The radiolaria associated with the vhm suggest a ∼ 10 ka age for the hydrothermal episode. The metalliferous sediments are semi-indurated, yellow to orange colored FeSi oxyhydroxides with FeO and Si02 contents between 54–73% and 16–30%, respectively, and have been derived as a result of hydrothermal precipitation. Incipient formation of nontronite is noted to co-occur with these sediments. The CIB metalliferous sediments have close similarities to those reported from the intraplate regions of the Pacific Ocean. The volcanic spherules occur in various shapes and sizes and are dominantly composed of magnetite and lesser amounts of ilmenite, hematite and maghemite. Electron microscopy shows the arrangement of magnetite crystals in various textural forms. Inclusions within the spherules are of olivine, pyroxene and feldspar. The spherules have formed by a process of liquid immiscibility of a silicic-basic magma, dependent on oxygen fugacity. Rhyolitic glass shards are ubiquitous at 1–2 cm depth in the core and constitute 55% of the coarse fraction. Microprobe analyses of the CIB shards show clear differences in Ti- and SiAl ratios that, together with the vast differences in age of eruption, preclude their derivation from Toba (Indonesia).