Oxide Gabbros from IODP Hole 1309D: Recorders of High-Temperature Strain and Alteration of the Atlantis Massif, An Oceanic Core Complex on the Mid-Atlantic Ridge

The Atlantis Massif (AM) is an oceanic core complex along the Mid-Atlantic Ridge, at ~30°N. Integrated Ocean Drilling Program (IODP) Hole 1309D reached ~1.4 km below the domal surface of the massif. Averaging 64-75% recovery of predominantly undeformed gabbroic rocks, Hole 1309D contrasts with the highly deformed, serpentinite- and schist- dominated south wall of the AM. Shear zones in Hole 1309D are dominantly granulite facies, and amphibolite through greenschist alteration is generally not associated with shear zones, except for local cataclastic structures. Gabbros with >2% oxide-mineral grains are efficient recorders of this tectonic-magmatic-hydrothermal history. Sampled in several intervals throughout the IODP core, the oxide-mineral grains (imaged with high-resolution, x-ray computerized tomography) form networks with a slight shape anisotropy. This microstructure reflects late (Fe-Ti) melt-transport through the mostly crystallized gabbro bodies. Dynamically recrystallized within local shear zones, the oxide-mineral grains define dominantly monoclinic fabrics, indicative of the strain conditions during the initial phases of exhumation. High-temperature, noncoaxial deformation ceased before magnetite-ilmenite (M-I) exsolution. M-I exsolution lamellae equilibrated over a range from T~350-650°C (Ghiorso-Sack model, modified by Ghiorso and Evans, 2008; QUILF calculations are systematically ~100°C higher). Oxygen fugacity during exsolution was generally buffered by a delta-log(fO2) of 2 to 4 above FMQ, relatively oxidizing conditions for this setting. One deep-seated shear zone records a greater range of T-fO2 values than other sections of the hole. In general, however, variations in T and fO2 are more dependent on depth than on location relative to shear zones. A remaining puzzle is how hydrothermal fluid flow passed through relatively undeformed, and hence impermeable, gabbros across such a wide range of thermal conditions.