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Integrated Ocean Drilling Program (IODP) Expedition 304/305 at Atlantis Massif, 30°N on the Mid-Atlantic Ridge (MAR) (Fig. F1) comprised a coordinated, dual-expedition drilling program aimed at investigating oceanic core complex (OCC) formation and the exposure of ultramafic rocks in young oceanic lithosphere. One of the scientific objectives of this drilling program is to investigate the role of magmatism in the development of OCCs. Whereas precruise submersible and geophysical surveys suggested the potential of recovering substantial amounts of serpentinized peridotite and possibly fresh residual mantle, coring on the central dome of the massif returned a 1.4 km thick section of plutonic mafic rock with only a thin (<150 m) interval of ultramafic or near-ultramafic composition rocks of indeterminate origin. Strikingly, despite the abundance of serpentinized peridotite in dredge hauls and submersible surveys from OCCs, in each instance where scientific ocean drilling has penetrated one of these core complexes (Mid-Atlantic Ridge Kane Fracture Zone, Atlantis Bank, 15°20′N on the MAR, and now Atlantis Massif) virtually the only rock type recovered is gabbro (Ildefonse et al., 2006). Therefore, unraveling the magmatic and alteration history of these plutonic sequences is essential to understanding OCC formation.

Herein we report the results of a new study that was envisioned to develop a method for characterization of the primary sulfide mineral assemblage present in the gabbroic and ultramafic rocks from Atlantis Massif. The abundance and composition of primary sulfide minerals is an integral part of a larger scope collaborative effort aimed at using the entire sulfide mineral assemblage in these gabbroic rocks to estimate sulfur and oxygen fugacity in the magma during crystallization. These estimates can be compared with oxygen fugacities calculated from oxide mineral compositions and ultimately provide constraints on oxide-silicate relationships used to determine temperatures and pressures of equilibration during the crystallization of the gabbros.

Characterization of sulfide mineral assemblages has historically been accomplished through reflected light microscopic techniques. More recently, researchers have adopted geochemical analysis by electron microprobe as a common analytical technique. As a result of the combination of utilization of the electron microprobe as an analytical tool and the fact that petrographic analysis of silicate phases does not require reflected light, the skills required to master sulfide mineral identification are commonly underdeveloped in students. We envisioned a project that would attempt to utilize the transmission electron microscopy (TEM) skills of a researcher who lacked experience in standard petrographic techniques to characterize sulfide minerals. TEM images samples at very high resolution (typically several angstroms), providing crystal structure details through electron diffraction. TEM can also provide chemical composition using X-ray energy dispersive spectroscopy (EDS).