In the first-pass petrographic study, 85 polished thin sections were prepared and examined from samples selected aboard the JOIDES Resolution with an eye toward locating intervals of sulfide concentration or abundance coincident with other petrological markers, specifically indicators of primitive magmas, that are the most likely host of primary PGE-bearing minerals. Many thin sections were effectively barren, despite observation of sulfide in the sample slab; commonly, only one or two grains were present. A subset of 25 samples that had the most abundant igneous textured sulfides was selected. In addition, a review of pathfinder elements in bulk-rock chemical analyses suggested that another 11 samples had higher than average Cu and Ni compositions. Most of the oxide gabbros (from more evolved magmas) contained a mineral assemblage wherein pyrite was the major, if not the only sulfide phase present. Only the oxide-bearing gabbros with elevated pathfinder element bulk-rock compositions were included in further study. A total of 36 samples were pulverized and sent to Commonwealth Scientific and Industrial Research Organization (CSIRO) Exploration and Mining, North Ryde New South Wales, Australia, for PGE analysis by inductively coupled plasma-mass spectrometry (ICP-MS). Pt, Pd, Ru, and Ir were calibrated by isotope dilution. Rh and Au were determined by external calibration. Analytical methods are detailed in Evans et al. (1993). Petrographic descriptions of these samples are summarized in Table T1.
Sulfide mineral chemistry was determined by electron microprobe on the Texas A&M Department of Geology and Geophysics Cameca SX-50 Superprobe using a combination of glass, mineral, and pure metal standards. Since this instrument is equipped with an ultrathin rather than a beryllium window, we were able to acquire high resolution on all peaks of interest using an accelerating voltage of 15 KeV. Count times were 20 to 30 s with a 1-µm-wide beam.
Bulk rock chemistries were determined by LiBO2 fusion followed by inductively coupled plasma (ICP) spectrometry. Major element oxides were determined by ICP-atomic emission spectrometry and trace elements by ICP-MS. These analyses were performed by Acme Analytical Laboratories, Vancouver, British Columbia, Canada. Replicate samples and U.S. Geological Survey standard reference materials were included in the analytical suite as unknowns to ensure accuracy and precision.