Hot and juicy - Melt-assisted deformation controls the rheology of the oceanic crust beneath the detachment fault footwall at (ultra)slow-spreading ridges

At (ultra)slow-spreading ridges, oceanic core complexes are commonly interpreted to form through dislocation creep in the lower crust, followed by strain localisation into evolved, oxide-rich domains along detachment faults due to fluid-assisted weakening. New observations from the Atlantis Bank (Southwest Indian Ridge) suggest that this framework underestimates the role of melt-present deformation in controlling lower-crustal rheology.We investigate gabbroic samples from IODP Hole U1473A (Expedition 360), which penetrated ~800 m of the footwall of the Atlantis Bank detachment system. We developed a semi-quantitative microscale strain intensity classification (grades 0–IV, from undeformed to ultramylonite) and compared it with the IODP shipboard macroscopic fabric classification. We furthermore integrate electron backscatter diffraction, scanning electron microscopy, full thin-section chemical mapping, and in situ major and trace element analyses by laser ablation, allowing deformation, melt–rock interaction, and chemical evolution to be assessed from the macro- to the microscale.Intracrystalline deformation is pervasive across all samples, including those classified as undeformed at the shipboard macroscopic scale, with no systematic relationship between strain intensity and bulk compositional evolution. Whole thin-section chemical maps reveal strong asymmetric zoning in porphyroclasts, with rims and neoblasts consistently enriched in more evolved compositions and preserving microstructural evidence of melt–rock interaction across all lithologies and strain classifications. In addition to neoblasts and overgrowths, evidence for the former melt presence is manifested by locally elevated modal proportions of secondary phases (pargasite, oxides, and enstatite), low apparent dihedral angles (