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Characterization of Sulfide Minerals from Gabbroic and Ultramafic Rocks by Electron Microscopy1 D.J

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Characterization of Sulfide Minerals from Gabbroic and Ultramafic Rocks by Electron Microscopy1 D.J Blackman, D.K., Ildefonse, B., John, B.E., Ohara, Y., Miller, D.J., MacLeod, C.J., and the Expedition 304/305 Scientists Proceedings of the Integrated Ocean Drilling Program, Volume 304/305 Data report: characterization of sulfide minerals from gabbroic and ultramafic rocks by electron microscopy1 D.J. Miller,2 T. Eisenbach,2 and Z.P. Luo3 Chapter contents Abstract Abstract . 1 This objective of this research was to investigate the potential of using transmission electron microscopy (TEM) to determine sul- Introduction . 1 fide mineral speciation in gabbroic rocks from Atlantis Massif, the Geologic background . 2 site of Integrated Ocean Drilling Program Expedition 304/305. Method development . 2 The method takes advantage of TEM analysis techniques and Results . 2 proved successful in sulfide mineral identification. TEM can pro- Discussion . 3 vide imaging of the sample morphology, crystal structure through Acknowledgments. 4 the electron diffraction, and chemical compositional information References . 4 through X-ray energy dispersive spectroscopy. Electron probe mi- Figures . 5 croanalysis was also used to determine the chemical composition. Table . 11 Introduction 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 po- tential of recovering substantial amounts of serpentinized perido- tite 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. Strik- ingly, despite the abundance of serpentinized peridotite in dredge 1 Miller, D.J., Eisenbach, T., and Luo, Z.P., 2009. hauls and submersible surveys from OCCs, in each instance Data report: characterization of sulfide minerals where scientific ocean drilling has penetrated one of these core from gabbroic and ultramafic rocks by electron microscopy. In Blackman, D.K., Ildefonse, B., John, complexes (Mid-Atlantic Ridge Kane Fracture Zone, Atlantis Bank, B.E., Ohara, Y., Miller, D.J., MacLeod, C.J., and the 15°20′N on the MAR, and now Atlantis Massif) virtually the only Expedition 304/305 Scientists, Proc. IODP, 304/ rock type recovered is gabbro (Ildefonse et al., 2006). Therefore, 305: College Station, TX (Integrated Ocean unraveling the magmatic and alteration history of these plutonic Drilling Program Management International, Inc.). sequences is essential to understanding OCC formation. doi:10.2204/iodp.proc.304305.203.2009 2 Integrated Ocean Drilling Program, Texas A&M Herein we report the results of a new study that was envisioned to University, 1000 Discovery Drive, College Station develop a method for characterization of the primary sulfide min- TX 77845-9547, USA. Correspondence author: eral assemblage present in the gabbroic and ultramafic rocks from [email protected] Atlantis Massif. The abundance and composition of primary sul- 3 Microscopy and Imaging Center, Biological Sciences Building West, Texas A&M University, fide minerals is an integral part of a larger scope collaborative ef- College Station TX 77843-2257, USA. fort aimed at using the entire sulfide mineral assemblage in these Proc. IODP | Volume 304/305 doi:10.2204/iodp.proc.304305.203.2009 D.J. Miller et al. Data report: characterization of sulfide minerals gabbroic rocks to estimate sulfur and oxygen fugac- of relatively oxygenated and sulfur-rich conditions. ity in the magma during crystallization. These esti- These phases occur as armored inclusions in silicate mates can be compared with oxygen fugacities calcu- phases and as intergranular crystals. Secondary sul- lated from oxide mineral compositions and fides, which occur in veins and as inclusions in alter- ultimately provide constraints on oxide-silicate rela- ation phases, include pyrite and tentatively identi- tionships used to determine temperatures and pres- fied tochilinite and awaruite, indicative of highly sures of equilibration during the crystallization of oxygen and sulfur depleted conditions. In some the gabbros. cases, both primary and secondary assemblages oc- Characterization of sulfide mineral assemblages has cur in the same thin section, attesting to the com- historically been accomplished through reflected plex history of sulfide mineralization in the section. light microscopic techniques. More recently, re- searchers have adopted geochemical analysis by elec- tron microprobe as a common analytical technique. Method development As a result of the combination of utilization of the Attempts at TEM analysis of standard petrographic electron microprobe as an analytical tool and the thin sections proved unfruitful, as the sulfide min- fact that petrographic analysis of silicate phases does eral grains are typically small (commonly <50 µm) not require reflected light, the skills required to mas- and the total abundance of sulfide mineral is com- ter sulfide mineral identification are commonly un- monly <1 mod%. Eventually, we settled on grain derdeveloped in students. We envisioned a project mount preparations derived from pulverizing the that would attempt to utilize the transmission elec- samples to <1 mm grain size and then deriving a tron microscopy (TEM) skills of a researcher who heavy mineral separate using a dense sodium poly- lacked experience in standard petrographic tech- tungstate solution. niques to characterize sulfide minerals. TEM images After the sulfides were separated from the bulk mate- samples at very high resolution (typically several rial, they were mixed in ethanol solutions and de- angstroms), providing crystal structure details posited on Cu grids, which were previously glow dis- through electron diffraction. TEM can also provide charged in order to achieve better dispersions. TEM chemical composition using X-ray energy dispersive measurements were carried out using a JEOL 2010 spectroscopy (EDS). TEM system at a working voltage of 200 kV. All imag- ing magnifications were calibrated using the stan- dards of SiC lattice fringes (Luo, 2006) for high mag- Geologic background nifications and commercial cross-line grating replica Drilling in Hole U1309D penetrated >1.4 km with for low magnifications. EDS, which was used to de- >75% recovery, comparable to recovery from the pre- termine chemical composition, was done using an vious most successful penetration into oceanic litho- Oxford Instruments EDS detector with an INCA en- sphere in Ocean Drilling Program (ODP) Leg 118 ergy platform. Electron micrographs were digitalized Hole 735B on the Southwest Indian Ridge. Despite using an Epson (Long Beach, CA) Projection 3200 high seismic velocities interpreted to possibly repre- scanner at 1200 dpi. sent residual mantle at relatively shallow depth, the Reflected light petrography and electron microprobe 1.4 km section appears to be wholly igneous crust analyses were performed on representative samples with little evidence of ductile deformation and brit- of the various major lithologies for comparison with tle deformation confined to a few intervals less than TEM results. Analyses of sulfide minerals were per- a couple of meters thick. formed using the CAMECA SX-50 electron micro- Major plutonic lithologies recovered are, in decreas- probe. Analyses were performed using a 50 nA beam ing abundance, gabbro, olivine gabbro, troctolite, current, an accelerating voltage of 15 keV, and a 1 oxide-bearing gabbro, oxide gabbro, and oxide- and µm beam. Count times range from 20 to 40 s. Stan- olivine-bearing gabbro. The gabbroic rocks have dards used included pyrite (S and Fe); sphalerite (Zn); compositions that are among the most primitive and pure Co, Ni, and Cu wire. sampled along the MAR, as reflected in bulk Mg numbers ranging from ~67 to 90 (see the “Expedi- tion 304/305 summary” chapter). At least two gen- Results erations of sulfide mineralization have been identi- Figure F2A is a TEM image of sulfide mineral grains. fied during shipboard and postcruise reconnaissance These particles have sharp, regular faces and are sev- examination. Based on optical properties, the pri- eral micrometers in breadth. The selected area elec- mary sulfide mineral assemblage includes at least tron diffraction (SAED) patterns (Fig. F2B, F2C) from pyrrhotite, pentlandite, and chalcopyrite, indicative these particles, along two different zone axes of [011] Proc. IODP | Volume 304/305 2 D.J. Miller et al. Data report: characterization of sulfide minerals and [012], show that these grains are crystalline with gabbros have a consistent sulfide mineral assemblage a primitive cubic structure (a = 0.54166 nm) and a of pyrrhotite > chalcopyrite > pentlandite, and pyrite space group of Pa3 (#205). Higher magnification im- is ubiquitous but in variable abundance and is likely ages are shown in Figure F3A, which is further mag- mostly secondary. Pentlandite containing as much nified in Figure F3B from the framed area in Figure as 2 wt% Co occurs as flamelike exsolution features F3A, where lattice fringes (also potentially useful for and as granular to subrounded grains near the mar- mineral identification) are visible.
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