Altered peridotites are commonly exhumed along detachment faults at slow-spreading ridges. This is the case at the Atlantis Massif (Mid-Atlantic Ridge, 30°N), where serpentinized and metasomatised harzburgites and dunites, associated with gabbros and dolerites, have recently been drilled by the IODP Expedition 357 (Nov. 2015, RRS James Cook). The recovered peridotites belong to seven shallow (max. 16.4 mbsf) holes spread among five sites forming an east-west transect on the southern wall of the massif, in the vicinity of the Lost City hydrothermal field. This spreading-parallel transect allows examination of 100m- to km-scale heterogeneities in the crust, linked to hydrothermal alteration.
This petrological study primarily focuses on the serpentinization history, with a characterization of the successive textures and mineralogical assemblages, and in situ major and trace elements in serpentine minerals. The similarities and differences between the sites allow to propose a conceptual model for alteration heterogeneities that includes a transition between a pervasive and localized serpentinization and the influence of gabbroic perturbations during exhumation.
The last part of this presentation will briefly introduce the project that will be initiated at ANU: oxygen isotope ratios in serpentine minerals as markers of evolving temperatures and/or fluid-rock ratios during exhumation.