Petrogenesis and metamorphism of mafic rocks, Atlantis Massif, IODP Expedition 357: implications of two episodes of magmatism

The petrogenesis and metamorphism of International Ocean Discovery Program (IODP) Expedition 357 (Atlantis Massif, Mid-Atlantic Ridge) mafic rocks is the focus of this paper. At least two phases of magmatism are identified. Later Phase I metamorphosed mafic intrusive rocks (MMIs) are moderately to extremely chloritized, recording loss on ignition (LOI) of 3.33–9.99 wt%, and latest Phase II basalts are exceptionally fresh, with LOI of 0.09–1.03 wt%. Phase I MMIs have Fe-chromite identical to Atlantis Massif serpentinites that have undergone melt-impregnation and Phase II basalt Cr-spinels are analogous to those of Atlantis Massif peridotites that have not undergone secondary melt-impregnation. Whole-rock Cr/Y relations of the Phase II basalt record melt fractions (F) of c. 16–25% and melt–liquid equilibria established for Phase II basalt olivine yield crystallization temperatures of 1173–1219°C similar to mid-ocean ridge basalt (c. 1125–1290°C). Trivariant plots of Na₂O–CaO–MgO and XRD analysis confirm extreme chloritization of the Phase I MMIs. Phase I MMI spinel is predominantly Fe-chromite that formed under amphibolite-facies conditions (c. 500°C). Subsequent to peridotite exhumation, serpentinization and Phase I magmatism, an episode (or episodes) of asthenospheric upwelling resulted in contact metamorphism caused by Phase II magmatism.