Probing Upper Mantle Heterogeneity: Os and Pb Isotopic Compositions of Individual Sulfide Grains in Abyssal Peridotites

Abyssal peridotites from mid-ocean ridges are unique samples of the depleted upper mantle that can be used to understand a variety of processes from melting and melt extraction to the compositional evolution of the interior of the earth. Traditional work on abyssal peridotites has focused on either bulk rock compositions or on the compositions of clinopyroxenes, which are the main repository of trace elements in these rocks. However, recent (e.g., Alard et al., 2000; Luguet et al., 2003; Harvey et al., 2006) and previous (Meijer, 1980; Morgan, 1985) works on peridotites have indicated that Fe-Ni-Cu sulfides, present at trace levels in abyssal peridotites, may contain a significant proportion of both the Pb and Os budget of the upper mantle. As the isotopic compositions of Pb and Os provide important information about the long term evolution of the mantle, analysis of single sulfide grains can provide unique information not available from basalt or whole-rock peridotite studies: the spatial scale is small compared to basaltic melt sampling, adjacent silicate mineralogy can be evaluated, and secondary sulfide and alteration minerals can be avoided. Thus, while sulfides represent <0.01% of peridotites modally, they are a promising way to examine abyssal peridotites that may be the key to understanding the long term evolution of Os and Pb in the Earth. Peridotites were obtained from the Oblique Segment of the Southwest Indian Ridge and from the Gakkel Ridge. These locations are unique in having comprehensive on-axis sample suites that can be directly related to the spatial scale of mantle heterogeneity, whereas most abyssal peridotites come from fracture zones. In polished thin sections, sulfide contents range from relatively abundant (modally ~0.01%) to absent. Sulfide presence is negatively correlated with alteration in the peridotites. This observation, combined with sulfide morphology and earlier work on sulfide alteration (e.g., Luguet et al., 2003), shows that the sulfides are of mantle, not hydrothermal, origin. The majority of sulfides are pentlandites, with some vein sulfides consisting of chalcopyrite. We have adapted single grain sulfide techniques from studies of sulfide inclusions in diamonds, to allow us to extract Os, Pb and transition metals from sulfides down to 5 μg. Os concentrations in pentlandites are ~0.4-5 ppm, 2-3 orders of magnitude higher than bulk rock abyssal peridotite concentrations. In contrast, chalcopyrites, associated with veins in the peridotites, do not contain measurable quantities of Os. Preliminary Pb data indicates that pentlanditic sulfides have Pb concentrations close to ~9 ppm. 187Os/188Os isotopic compositions range from 0.1209 to 0.1278 in sulfides away from hotspots, typical of depleted mantle and in agreement with the depleted Sr and Nd isotopic composition of clinopyroxenes from the same samples. To date, we have not found any anomalously unradiogenic Os compositions in the sulfides, as has been observed at the Kane Fracture Zone (Harvey et al., 2006). In one sample from the Oblique Segment, proximal to Bouvet Hotspot, elevated Os (187Os/188Os=0.1499) and Pb isotopic compositions indicate interaction between the depleted upper mantle and the hotspot.