Deep versus shallow melt stagnation in an ultra-slow / ultra-cold ridge segment: the Andrew Bain southern RTI (SWIR)

The Andrew Bain Fracture Zone (ABFZ) represents one of the largest transform faults in the ridge system spanning 750 km in length with a characteristic lens-shape structure. The southern Ridge-Transform Intersection represents the deepest sector of the whole South West Indian Ridge system. During the Italian-Russian expedition S23-AB06, the seafloor in the Southern Ridge Transform Intersection (RTI) has been sampled recovering only ultramafic material in the majority of the dredging sites. The sampled spinel and plagioclase peridotites show hybrid textures, characterized either by deep spinel-field impregnation assemblages (sp+cpx±opx±ol) or by plagioclase-field equilibrated patches and mineral trails (pl+cpx±ol) marked by both crystallization of newly formed plagioclase-field equilibrated trails and formation of plagioclase coronas around spinel. The ones collected from ridge axis show also late gabbroic pockets and veins, variably enriched in clinopyroxene. Overall textures account for important melt percolation/stagnation events occurred in the plagioclase and spinel field. Major and trace element distribution in pyroxenes and spinels from spinel-bearing peridotites overall follow a general melting trend accompanied by a progressive re-equilibration to lower P/T facies at all scales. However, only few samples can be linked to near fractional melting, while the majority of them shows REE pattern and trace element concentrations that cannot be reproduced by fractional melting process. Open-system melting (OSM) better reproduces measured REE patterns. Modeling melting in an open system scenario requires high residual porosity to be accounted for along with generally enriched melts to influx the melting parcel at depth. Melting at high residual porosity suggests a near-batch regime in which enriched melts stagnate in the spinel field. Inhibition of melt segregation during melt/rock interaction asks for a permeability barrier to develop in the region where the mantle potential temperature is suggested to still be high enough to allow partial melting. Energy consumption during garnet breakdown and porosity decrease due to reaction with silica-saturated melt could play a key role in the formation of short scale permeability barriers beneath ABFZ. Alternatively an anomalously thick conductive layer can be responsible of deep inhibition of melting and melt accumulation at depth.