Brunelli D., Maia M., Ligi M., Bonatti E., Briais A., Campos T., Ceuleneer G., Cipriani A., Cuffaro M., Gregory E., Hamelin C., Jbara R., Kaczmarek M.-A., Lombardi F., Moreira S., Mougel B., Petracchini L., Puzenat V., Revillon S., Seyler M., Soltanmohammadi A., Verhoest L., Trivellato T. & Wang Z. - Cold spots at Mid Ocean Ridges help revealing mantle heterogeneity: a summary of the SMARTIES cruise in the Equatorial Atlantic

The heterogeneity of the suboceanic mantle source is usually defined based on MORB lateral geochemical variability. However, tackling short-scale variability is partially hampered by the large extent of mixing occurring during melt extraction and magma chamber processes. We identified a singular region of the Mid Atlantic Ridge at the eastern Romanche Ridge-Transform Intersection (Equatorial Atlantic) characterized by an extreme lateral thermal gradient. Cooling of the upwelling mantle is here induced by heat transfer to a ca. 50 Ma old plate fronting the ridge-transform intersection. Cooling of the plate edge affects the upper mantle for the whole range where melting usually occurs (30-80 km bsl). The resulting ridge axis consequently shows a lateral gradient in magma productivity and composition (Bonatti et al., 2001 and references therein). MORBs erupted at the ridge centre give rise to a "normal" 5 km thick magmatic crust. Moving to the axial tip magmatism diminishes in volume till zero and progressively increases its alkaline character along with the compositional variability revealing the short-scale heterogeneity of the mantle source (Ligi et al., 2005). This thermal and tectonic configuration represents a regional scale natural experiment where to test the effect of varying mantle potential temperature on a given mantle source. Melting a heterogeneous mantle has profound implications on the km-scale heat transfer because of the differential onset of melting of variably fertile lithologies (Brunelli et al., 2018). Here, we will present the results of the upcoming French-Italian SMARTIES expedition, taking place in July-August 2019. We will present a new high-resolution bathymetric coverage of the RTI and peculiar tectonic features where magma is nearly absent accompanied by direct submarine observations and sampling from Nautile dives.