Antigorite crystallization during oceanic retrograde serpentinization of abyssal peridotites

We report on the presence of the serpentine-type antigorite in abyssal-serpentinized peridotite. At mid-ocean spreading ridges, antigorite crystallizes under retrograde metamorphic conditions during tectonic exhumation of the newly formed oceanic lithosphere. Using optical microscopy and micro-Raman spectroscopy, we identified antigorite in 49 samples drilled at the Hess Deep (East Pacific Rise) and the Atlantis Massif (Mid-Atlantic Ridge, 30°N), and dredged along the Southwest Indian Ridge (62°–65°E). Overall, antigorite is common, but occurs in limited modal amounts. SEM and TEM investigations reveal its frequent crystallization after lizardite and chrysotile via dissolution–recrystallization processes and a local association with olivine or talc. We explain antigorite crystallization by the interaction with seawater-derived hydrothermal fluids moderately enriched in silica (metasomatism). The origin of silica is attributed to alteration of mafic intrusions or pyroxenes. Antigorite can, therefore, be considered a marker of preferential fluid pathways under rock-dominated conditions during exhumation of a portion of the oceanic lithosphere. We also measured the in-situ major and trace-element composition of antigorite and the predating and postdating phases. Most of the elements are immobile during the mineralogical transitions. Other elements (Ni, Ca, Al, and Ti) evolve within the serpentine textures, including antigorite, as a result of chemical exchanges accompanying the development of the sequence of serpentine textures. A further category includes elements that are specifically enriched (Mn, Sn) or depleted (Fluid-Mobile Elements: B, Sr, As, U, Sb, and Cl) in antigorite compared to lizardite and chrysotile. These enrichments and depletions possibly reflect a change of the fluid physicochemical characteristics allowing a change in element mobility during the dissolution–recrystallization accommodating the lizardite/chrysotile-to-antigorite transition. Such depletion in FME is comparable to depletions described in studies of serpentinization and antigorite formation in subduction zone setting, which suggests that the origin of antigorite in some subducted samples could be reevaluated.