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This project was initiated as part of a collaborative investigation among shipboard science party members from Expedition 304/305. The overall objective of the research plan is to use primary and secondary sulfide minerals to map changes in sulfur and oxygen fugacity during hydrothermal alteration of the plutonic foundation of the ocean crust. Specifically, our group set out to document the composition and diversity of primary sulfide minerals providing the starting end-member for studies evaluating the composition and evolutions of fluids migrating through the massif and contributing to alteration of the rocks. For example, Miller (2007) shows that the sulfide mineral assemblage at ODP Site 1268 (millerite + polydymite in near-surface rocks, progressing in deeper rocks to assemblages including pyrrhotite and then pentlandite) records fluids seeping down from the seafloor, becoming progressively less oxygenated and less sulfur-rich as they react with the host rock and precipitate sulfide minerals.

Classically trained sulfide petrologists can, through years of practice and likely an inherently acute sense of subtle differences in color, reflectivity, and hardness, recognize sulfide species in reflected light. It is rare to find undergraduate or junior graduate students with this talent fully developed. In our case, we were intrigued by the hypothesis that someone without the benefits of traditional reflected light petrography education and practice might be able identify sulfide minerals from these ocean crust rocks using TEM. We hoped to take advantage of the advanced facilities in the Texas A&M University Microscopy and Imaging Center to test this hypothesis, employing a graduate student with TEM experience but no previous opportunity to work with natural sulfide-bearing materials.

Obviously, we had no doubt that TEM could be used to identify sulfide minerals, our purpose was to assess the process to see how sulfide mineral characterization with the TEM compared and contrasted with more traditional techniques.

In summary, by taking advantage of TEM analyses, we are satisfied that the method represents a robust approach to documenting the composition and diversity of sulfide minerals in gabbroic and ultramafic rocks, particularly when lacking experience in more traditional analytical techniques. As demonstrated in this project, with appropriate sample preparation, TEM analysis provides details regarding sulfide mineral crystal structure using electron diffraction and chemical composition from EDS. TEM analysis of sulfide minerals is broadly applicable to any host rock and may have specific interest for geologists and geochemists interested in serpentinization and/or hydrothermal mineralization. Although TEM operation and data interpretation require extensive experience, we surmise that this method might be used as an adjunct to standard petrographic and geochemical analysis techniques for sulfide minerals in oceanic crustal rocks.