A New Find of Anorthoclase Crystals-lined Vug in a Deep-sea Basalt, Central Indian Ocean Basin

ABSTRACT We detail a new and first find of a vug in a deep-sea basalt that is lined with well-formed anorthoclase crystals. The specimen was dredged from the seafloor (water depth 4,440 m) near a seamount in the Central Indian Ocean Basin. The basalt has a thin veneer of glass covered with ferromanganese (FeMn) oxides while the interior is quite fresh. The geochemical analysis of the basalt depicts different degrees of alteration that vary from the outermost layer with FeMn oxides to the interior of the specimen. In general, the TiO2 (<1.5 wt%) and MgO (~5 wt%) contents are low while CaO, Na2O, K2O and P2O5 contents change within the specimen. The FeMn oxides have Fe2O3 (21 wt%) and MnO (8–27 wt%) and appear as botryoids and occur near and/or over the anorthoclase crystals. The trace and rare earth elements do not show significant variations from the outer to the interior of the specimen. Microscopy (binocular and scanning electron) and energy dispersive spectrometry of the vug revealed a dominance of drusy anorthoclase crystals. The formation of the anorthoclase crystals is ascribed to precipitation from hydrothermal solutions that seeped into the basalt through cracks. This is attested by the co-existing crystals of phillipsite and feldspar and FeMn oxides.