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doi:10.2204/iodp.proc.304305.203.2009

Results

Figure F2A is a TEM image of sulfide mineral grains. These particles have sharp, regular faces and are several micrometers in breadth. The selected area electron diffraction (SAED) patterns (Fig. F2B, F2C) from these particles, along two different zone axes of [011] and [012], show that these grains are crystalline with a primitive cubic structure (a = 0.54166 nm) and a space group of Pa3 (#205). Higher magnification images are shown in Figure F3A, which is further magnified in Figure F3B from the framed area in Figure F3A, where lattice fringes (also potentially useful for mineral identification) are visible. The structure and space group are consistent with the mineral pyrite.

Chalcopyrite grains are shown in a TEM image in Figure F4A. The particles are micrometer scale, with regular shape and sharp crystal faces where unbroken. The SAED patterns from these particles (Fig. F4B, F4C) reveal that they have a tetragonal structure (a = 0.5289 nm, c = 1.0423 nm) and a space group of I4 2d (#122). Figure F4B is along the [001] zone axis of the tetragonal structure, where the reflection spots are in a regular square shape consistent with tetragonal structure along this axis. Figure F5A is an image in higher magnification, the framed area of which is further magnified in Figure F5B, imaging lattice fringes.

In order to verify the chemical composition of grains, we performed EDS on the samples. Figure F6A shows an EDS spectrum from the pyrite sample. The sample contains S and Fe; C is from the support carbon film of the grid and the low Cu peak is from the Cu grids. Future studies could employ Au grids to alleviate the Cu signal. A quantitative analysis shows the composition of Fe:S = 31.2:68.8 in atomic ratio, consistent with the stoichiometry of FeS₂. Alternatively, the EDS spectra from the chalcopyrite (Fig. F5B) clearly shows higher Cu content in addition to Fe and S. The quantitative analysis of the spectra reveals that the composition of these grains is Cu:Fe:S = 30.2:32.3:37.5. Although the stoichiometry of CuFeS₂ is Cu:Fe = 1:1, we assign the discrepancy to less than ideal accuracy of S determination EDS with TEM.

Although a detailed analysis has not been part of this trial, the following observations of primary sulfide mineralogy hold based on examination of 24 thin sections and sulfide mineral analysis of 9 thin sections (Table T1). Two troctolites were examined, one from fairly shallow in the hole (Sample 304-U1309D-60R-3, 59–62 cm) and one from deep in the hole (Sample 305-U1309D-233R-2, 137–140 cm). Despite similar primary mineralogies (75% olivine, 25% plagioclase), the sample from the upper ultramafic horizon is significantly more altered (80% versus 20%), and the sulfide minerals are heazelwoodite and polydimite in the upper sample (Section 304-U1309D-60R-3) and pyrrhotite and mackinawite in the sample from the deeper interval (Section 305-U1309D-233R-2). The olivine gabbros and olivine-bearing gabbros have a consistent sulfide mineral assemblage of pyrrhotite > chalcopyrite > pentlandite, and pyrite is ubiquitous but in variable abundance and is likely mostly secondary. Pentlandite containing as much as 2 wt% Co occurs as flamelike exsolution features and as granular to subrounded grains near the margins of pyrrhotite. Chalcopyrite is most commonly manifested as blade-shaped inclusions in pyrrhotite. Gabbro samples have a similar pyrrhotite > chalcopyrite > pentlandite sulfide mineral assemblage, but several samples also include rare sphalerite. The only sulfide minerals found in oxide-bearing gabbros were pyrrhotite and chalcopyrite, but only a few samples of this lithology were examined so the absence of pentlandite might not be significant. Oxide gabbros contain pyrrhotite > chalcopyrite > pentlandite. The pentlandite in the oxide gabbros contains as much as 12 wt% Co.

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