Os isotopic compositions of MORBs from the ultra-slow spreading Southwest Indian Ridge: Constraints on the assimilation and fractional crystallization (AFC) processes

•Some N-MORBs from the SWIR have radiogenic Os isotopic compositions.•Os isotopes of some MORBs were modified by AFC processes.•MORBs from the SWIR and MAR have higher 187Os/188Os ratios than those from the EPR.•Os variations are related to spreading rates and varied structure of oceanic crust.

Normal mid-ocean ridge basalts (MORBs) from the Southwest Indian Ridge (SWIR) (48°E–64°E) have larger ranges of 187Os/188Os ratios from 0.1295 to 0.3023 than previously reported N-MORBs. 187Os/188Os ratios show positive correlations with 1/Os, suggesting possible contamination from a component with higher 187Os/188Os ratios. Mixing modeling between an unradiogenic MORB magma and seawater cannot reproduce the observed Os–Sr isotopic variations, suggesting that seawater alteration is not responsible for such radiogenic Os isotopic compositions. One sample plots between the isochrons of 400 ka and 300 ka, showing a possible radiogenic origin of 187Os after magma eruption. A group of MORBs have 187Os/188Os ratios negatively correlated with MgO. Such correlations can be modeled with starting 187Os/188Os ratio of 0.1357, bulk DOs of 15.3 and assimilation degree from 3% to 11% with a crystallization to assimilation ratio of 0.8. Os isotopic compositions of some MORBs from the SWIR reflect a significant influence of assimilation and fractional crystallization processes, mostly obscured the Os isotopic characteristics of the mantle sources. N-MORBs from the ultra-slow spreading SWIR and slow-spreading Mid-Atlantic Ridge show larger ranges of 187Os/188Os ratios than those from fast-spreading East Pacific Rise, reflecting different structures and chemical compositions of oceanic crusts due to different spreading rates of the ridges.