Anomalously hot mantle source beneath the Dragon Flag Supersegment of the Southwest Indian Ridge: New evidence from crystallisation temperatures of mid-ocean ridge basalts

Abstract Discontinuities in the magmatic activity along a spreading ridge are usually observed in ultraslow-spreading ridges with a thick crust. However, the reasons for such melting anomaly are unknown. The Dragon Flag Supersegment between the Indomed Fault Zone (FZ) and the Gallieni FZ on the Southwest Indian Ridge (SWIR) is characterised by ultra-thick oceanic crust (up to ~10 km), and it provides a unique opportunity for studying magmatic crustal accretion at an ultraslow-spreading ridge. We determined the compositions of olivine–spinel pairs in mid-ocean ridge basalts (MORBs) collected from Segments 25 and 28 of the SWIR to estimate the temperatures of crystallisation and investigate the source lithologies. The Al-in-olivine thermometer provided crystallisation temperatures that ranged from 1197 ± 37 to 1393 ± 21 °C. The 95th percentile calculated temperature (1345 °C) for the primitive olivine is ~75 °C hotter than for olivines in MORBs from normal ridges but comparable to the data for plume-related oceanic island basalts. The olivines have Fe/Mn ratios and Ni, Ca, and Mn contents that are similar to those of olivines in MORBs from normal ridges, which suggests their derivation from a peridotite-dominant source with little contribution from fertile mantle components. Our results suggest a hot mantle reservoir is present beneath the Dragon Flag Supersegment, and that it is probably related to a mantle plume.