Contrasting geophysical and geochemical signatures of a volcano at the axis of the Wharton fossil ridge (N-E Indian Ocean)

The Styx volcano is a prominent seamount (20-30 km diameter, 2500 m high) l ocated at the axis of the Wharton fossil spreading center (N-E Indian Ocean ), where seafloor spreading stopped 40 Ma ago. Gravity modeling shows that it was emplaced on a weak lithosphere, in agreement with an on axis origin when seafloor spreading was active. The rocks dredged at the summit of the volcano are extreme typical alkaline basalts, indicating a deep enriched ma ntle source. We show here that most of this seamount was emplaced during th e final stage of the seafloor spreading in the Wharton Basin. We then discu ss how alkaline magmatism can be emplaced at the summit of the Styx.