The deep melting structure in ultraslow spreading regions: insights from SWIR-Smoothseafloor peridotites

Ultraslow spreading ridges are characterised by peculiar morphologies and reduced melt supply (Cannat et al., Geology, 2006; G3 2008). Because of the low mantle potential temperatures and melt production, they are key to unravel the deep processes occurring at the base of the melting region. I discuss compositional data from spinel field-equilibrated peridotites collected during the Smoothseafloor cruise along the eastern SWIR (Sauter et al., Nature 2013). Major and trace element mineral data reveal that the (low) observed mantle depletion is partially inherited. Melting in the spinel field occurs under open-system conditions driven by the widespread presence of relatively enriched melts. Influxing/extraction relationships can be modelled revealing near-batch conditions with influx of grt-field-derived melts. At the same time the presence of minor harzburgitic and dunitic rocks attests for local focused melt channelling. Melt focussing possibly occurs beneath the mean melting region resulting in the transport of grt-derived melts at depths significantly shallower than the grt/sp transition without substantial modifications. The melting region appears therefore vertically discontinuous and characterized by progressive reaction zones with low melt extraction. Global correlations reveal that these compositional patterns are widespread also in slow and intermediate spreading ridges.