University of Wyoming, United States

Atlantis Massif exposes serpentinized peridotite and lesser gabbro in a domal high at the inside corner of the eastern intersection of the Mid-Atlantic Ridge (MAR) at 30 degrees N and the Atlantis Fracture Zone. The goals for Integrated Ocean Drilling Program (IODP) drilling during Expeditions 304 and 305 are to characterize variations in rock type, structure, and alteration with depth at this ultramafic oceanic core complex and to obtain core of essentially fresh, in situ peridotite. Drilling, coring, and wireline logging will document the composition, microstructure, and evidence for melt production/migration and the relationships between these factors, as well as syntectonic alteration. This study of lithospheric formation and the causes of variation in crustal structure at slow-spreading centers addresses the Solid Earth Cycles and Geodynamics portion of the IODP Initial Science Plan. Domal massifs capped by corrugated detachment faults have been mapped at several locations on the seafloor. Formation of these large, shallow seafloor features appears to be an episodic manifestation of plate rifting and accretion at slow-spreading ridges. At Atlantis Massif we have the opportunity to core and log the detachment zone at both exposed (footwall) and unexposed (beneath the adjacent hanging wall) sites to address the characteristics of strain localization and the effects of fluid flow. Genetic relationships between the geochemistry of hanging wall basalt and footwall lithologies will be studied, as will patterns of block rotation within and between the footwall and the hanging wall. At the footwall site, the nature of an alteration front within oceanic lithosphere will be studied. Seawater alteration profoundly affects the geophysical properties of the lithosphere, particularly during serpentinization of peridotite. Mantle seismic velocities have been determined at depths as shallow as several hundred meters below seafloor beneath the central dome of Atlantis Massif. Drilling therefore offers an unprecedented opportunity to document relationships between the degree and chemistry of alteration and changes in geophysical properties and to describe the petrologic sequence that occurs across the Mohorovicic discontinuity (Moho) at this location. The potential for recovering unaltered peridotite at Atlantis Massif presents excellent opportunities for advances in understanding residual modes and microstructures associated with mantle flow and melt extraction within the oceanic mantle.