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

The petrogenesis and metamorphism of IODP Expedition 357 (Atlantis Massif, MAR) mafic rocks is the focus of this communication. At least two phases of magmatism are identified. Later Phase I metamorphosed mafic intrusives (MMI) are moderately to extremely chloritized recording 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 MMI have Fe-chromite identical to Atlantis Massif serpentinite that have undergone melt-impregnation and Phase II basalt Cr-spinel are analogous to those of Atlantis Massif peridotite that have not undergone secondary melt-impregnation. Whole rock Cr/Y relations that the Phase II basalt records melt fractions (F) of ∼16-25% and melt-liquid equilibria established for Phase II basalt olivine yield crystallization temperatures of 1173-1219 °C similar to MORB (∼1125-1290 °C). Trivariant plots of Na₂O-CaO-MgO and XRD analysis confirm extreme chloritization of the Phase I MMI. Phase I MMI spinel are predominantly Fe-chromite that formed under amphibolite facies conditions (∼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.