Petrographic and geochemical features of mantle peridotite – gabbro contacts from IODP Hole U1601A (Atlantis Massif, Mid-Atlantic Ridge)

Oceanic lithosphere formation occurs at mid-ocean ridges, where melts migrate through the lithosphere and crystallize to form the lower oceanic crust. At slow-spreading ridges, crustal accretion is characterized by strong lithological and structural heterogeneity, and gabbroic intrusions commonly occur within serpentinized mantle peridotites. IODP Expedition 399 drilled Hole U1601C at the Atlantis Massif, Mid-Atlantic Ridge (30°N), which displays these features. A total of 22 thin sections recovered from Hole U1601C cores, representing different types of gabbro–mantle contacts and contact-related lithologies were investigated. The studied samples include several different contacts and contact-related lithologies consisting of: contacts between coarse-grained gabbro and serpentinized peridotite with either magmatic percolation in the mantle or olivine occurrence in gabbro; thin (< 1 cm) olivine-rich troctolite, and thick (5-60 cm) olivine-rich troctolites. Petrographic observations performed in Università di Pavia were integrated with major-element analyses of magmatic silicates and sulphides by electron probe micro-analysis (EPMA) during a two months Erasmus+ Traineeship in Institut des Sciences de la Terre, in Orléans (France). LA-ICP-MS technique has been carried out in CNR-GEO Pavia for trace-element analyses of clinopyroxene. The investigated contacts display strong textural and compositional variability indicative of mantle-rock reaction. Olivine along the contacts commonly occurs with resorbed textures or as rounded crystals enclosed within plagioclase and clinopyroxene, exhibiting reaction with the melt. Geochemical data further confirm the occurrence of the mantle-melt reaction. High-Mg# clinopyroxene occurs within contact-related troctolites and displays compositions and textural relationships consistent with crystallization during melt–rock reaction rather than simple fractional crystallization. Olivine compositions also vary according to the extent of melt infiltration and re-equilibration, with lower Fo# values associated with lithologies affected by higher melt/rock ratios that re-equilibrate with the percolating melt. A fundamental feature of the contact-related lithologies is magmatic sulphides abundancy. The observed phases are pyrrhotite, pentlandite, and chalcopyrite, which occurrence indicates strong enrichment in sulphur and chalcophile elements during mantle–melt interaction. Thus, we can conclude that the S budget of the section is entirely dominated by mantle-melt reactions, which govern sulphur saturation in the melt and the availability of chalcophile elements that constitute sulphide aggregates. These relationships, together with the occurrence of magmatic phases penetrating the mantle, indicate that the contact-related rocks record open-system interaction between percolating MORB-type melts and a pre-existing olivine-rich mantle matrix. The results indicate that the studied serpentinized peridotite–gabbro contacts represent hybrid zones generated by the interaction between mantle rocks and migrating melts, that control mineral assemblages and element distributions.