Re-Os SYSTEMATICS AND HSE DISTRIBUTION IN METAL FROM Ochansk (H4) CHONDRITE

Introduction: Previous studies of the Re-Os systematics of chondrites have documented considerable variation in the Re/Os ratios of whole rock samples [1,2]. For some whole rock chondrites, Re-Os systematics display large deviations from the primitive isochron that are considerably larger than deviations in other isotope systems [1]. Possible interpretation of these facts is that the Re-Os system in chondrites is particularly sensitive to post-formation alteration processes, thus providing a useful tool to examine such processes. Significant variations that have been detected in highly siderophile element (HSE) patterns for ordinary chondrites [3] support this conclusion. We report Re-Os isotope data for metal separates from the Ochansk H4 chondrite coupled with abundance data for Ru, Pd, Ir, and Pt, determined in the same samples by isotope dilution. We chose this meteorite mainly because it is an observed fall with minimal signs of weathering, and its low metamorphic grade (H4) and shock stage (S3). Experimental methods and samples: A single fragment of Ochansk (~ 2 g, free of fusion crust) was gently crushed in agate mortar to preserve chondrules. Chondrules and chondrule fragments were handpicked and removed at all stages of sample processing, so that the samples reported here represent mostly matrix metal. The sample was divided into three fractions by size and magnetic separation (labeled as FM, MM, and CM, fine, medium and coarse metal, respectively). In addition, a handpicked aliquant of coarse metal (mmsize, labeled as "CM hpk") was processed again via mortar, to remove a non-metal "coating". We also subdivided an aliquant of fine metal with a hand magnet. The resulting fractions are labeled FM1, FM3, and FM5, where FM5 represents the finest and purest metal (~ 95%, judging from the amount of residue after acid digestion). Samples were digested with aqua regia using quartz Carius tubes [3,4]. For isotope dilution we used a mixed Re-Os spike and a mixed HSE spike (Ru, Pd, Ir, and Pt). Osmium was separated by solvent extraction, followed by microdistillation using a H2SO4 dichromate mixture as oxidant [1]. Anion exchange resin was used to separate Re, Ir, Pt, Ru and Pd. Rhenium and Os were measured in negative mode on the Sector 54 mass spectrometer at the University of Maryland. Platinum, Ir, Ru and Pd were measured on the Plasma54 ICP-MS at DTM using faraday cups and static multicollection. Total analytical blanks were 5 pg Re, 5 pg Os, 14 pg Ir, 25 pg Pt, 14 pg Ru, and 150 pg Pd. Results and Discussion Re-Os systematics in Ochansk metal: Figure 1 shows Re-Os results for Ochansk metal. Our data display a good agreement with the group IIA iron meteorite isochron [5], as well as with whole rock data for Ochansk [1]. Our isochron is also in good agreement with Re-Os isochron for St. Séverin [2]. The isochron precision for Ochansk (±150 Ma) is limited by the small range in Re/Os ratios of this set, only from 0.43 to 0.47. However, if we assume a Solar System initial Os ratio for these samples, then the uncertainties in the calculated model ages range from 10 to 30 Ma, illustrating the resolution potential of the Re-Os system to constrain the timing of alteration in chondrites.