Impact of geological structure on mantle partial melting beneath SWIR: Evidence from spinel composition in abyssal peridotite

Slow-ultraslow spreading Southwest Indian Ridge (SWIR) is an ideal place to study the relationship between geological structure with mantle partial melting due to the presence of abundant transform faults and segmenting the ocean ridge. Studies on abyssal peridotites from 6 dredges in the ridge segment between the Gallieni transform fault (TF) and Gazelle TF in SWIR reveal that the Cr# of spinel ranges from 0.194 to 0.329, corresponding to the degree of mantle partial melting from 7.6% to 13.0% which is very low in the global oceanic ridge. The degree of mantle partial melting is controlled by transform fault and the enhanced melt extraction in the segment center, which is shown as greater degree of mantle partial melting in the ridge segment center than near the transform fault. Comparison of the studied area with other ridges shows that both the Cr# values of the spinel and the degree of mantle partial melting decrease with the decrease of ridge spreading rate. The relationship between mantle partial melting and ridge spreading rate will be estimated more accurately after the correction of the transform fault effect.