Highly siderophile elements reveal ancient subarc mantle beneath the South West Indian Ridge at Marion Rise

The ultra-slow spreading South West Indian Ridge (SWIR) near Marion Rise comprises segments of thin crust and abundant abyssal peridotites exposed near the ridge axis. Zhou and Dick (2013) proposed that buoyant depleted mantle beneath the SWIR may represent Proterozoic melting residues recycled from Gondwana lithosphere, resulting in excess buoyancy at Marion Rise. In order to test this hypothesis, serpentinized harzburgites, dunites and scarce lherzolites were dredged and sampled by remotely operated vehicle along SWIR (36°54.0’ E to 39°13.5’ E) during expedition SO 273 in 2020. The major and trace element compositions of the peridotites (Mg# = 0.90-0.92) are similar to other depleted abyssal peridotites and mostly lack visible clinopyroxene. Here, we present new highly siderophile element (HSE) mass fraction and Os isotopes, that constrain the time of melt extraction, style of mantle melting and melt transport. Low HSE abundances (Ru = 0.07-0.3x primitive mantle) coupled with strongly fractionated HSE patterns (high Ru/Os, Ir/Os, Ru/Re) in studied peridotites exposed at normal faults suggest higher melting degrees than in most abyssal peridotites elsewhere. The HSE patterns are similar to those in harzburgites and dunites from ophiolites, interpreted to reflect sub-arc mantle lithosphere that was depleted by flux melting above subduction zones. Measured 187Os/188Os range between 0.119 and 0.142, possibly reflecting melt depletion at 1.4 Ga and interaction with sulfide-undersaturated melts or fluids with suprachondritic 187Os/188Os. The data provide evidence for locally strong depletion of mantle rocks at Marion Rise and the occurrence of ancient subarc mantle underneath a modern oceanic ridge.