Recovery of hydrothermal wustite-magnetite spherules from the Central Indian Ridge, Indian Ocean

A sediment sample with high abundance (19 spherules in ~ 85 g) of spherules was recovered from Central Indian Ridge (CIR) segment S2 (70° 54′ E, 25° 14′ S to 70° 50′ E, 24° 41′ S), ~ 85 km north of Rodrigues triple junction (RTJ). On the external surface of the spherules, magnetite appears as crystals, whereas wustite mostly appears as a homogenous glass phase. These spherules are composed of wustite and magnetite hosting Mn, unlike micrometeorites which essentially host Ni. Mn is more heterogeneously distributed with a relatively higher concentration in the wustite phase than the magnetite, suggesting hydrothermal origin. Furthermore, the presence of sulfide nano-particles in the wustite phase and a minor quantity of Pb and S in the ferrihydrite matrix points to the fact that CIR spherules are of hydrothermal origin. The CIR spherules could have formed either by the interaction of the reduced hydrothermal fluids with the ultramafic/basaltic rocks or silica-undersaturated magmatic melts, or by alteration of original particles (such as cosmic spherules, volcanic spherules, or even foraminifera) via Mn-bearing hydrothermal fluids, such as released during the serpentinization of ultramafic rocks. The finding of Mn hosting wustite-magnetite assemblage suggests an hydrothermal system in the near vicinity and can be considered as an additional proxy for indication of hydrothermal activity.