HR: 0800h
AN: V51B-0566    [Abstracts]
TI: Natural Rhenium Isotope Variations
AU: * Miller, C A
EM: camiller@whoi.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States
AU: * Miller, C A
EM: camiller@whoi.edu
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, United States
AU: Peucker-Ehrenbrink, B
EM: behrenbrink@whoi.edu
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, United States
AU: Ball, L
EM: lball@whoi.edu
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, United States
AB: Analyses of industrial and natural materials show ‰-scale Re isotope variation. Data are expressed using δ notation where δ¹⁸⁷Re = \Large(\normalsize \frac{¹⁸⁷Re/¹⁸⁵Re_smpl}{¹⁸⁷Re/¹⁸⁵Re_std} \Large)\normalsize-1 × 10,000 relative to NIST SRM-989 (¹⁸⁷Re/¹⁸⁵Re = 1.7025; Gramlich et al., 1973). Re is extracted from solid samples with an aggressive chemical leach and is purified by anion chromatography using AG1x8 100-200 mesh resin. Column-induced fractionation is observed with early eluant δ¹⁸⁷Re = showing a positive anomaly of up to 1‰. The pooled isotopic composition of the eluted Re converged with that of the reservoir after elution of ~85% of total Re. Total Re recovered was 100% within uncertainty. Analyses are done by MC-ICP-MS (WHOI Plasma Mass Spectrometry Facility, ThermoFinnigan Neptune) after doping with W (SRM 3163) to allow internal exponential correction of mass bias (¹⁸⁶W/¹⁸³W = 1.98594; Völkening et al., 1991). Analyte concentrations are 20 ng Re g^-1 and 75 ng W g^-1. Masses 180(W), 182(W), 183(W), 185(Re), 186(W), 187(Re), 190(Os), and 192(Os) are monitored in static mode. Tailing effects of Re into W and vice-versa have been evaluated and are negligible. Some Re-rich samples with unknown but presumably very large ¹⁸⁷Os/¹⁸⁸Os, such as ReS₂ and MoS₂, are stripped of Os by sparging (Hassler et al., 2000 ). Comparison of sparged and unsparged solutions splits can show significant differences up to 1.5 per mil, for an Archaean MoS₂. Uncertainties for raw ¹⁸⁷Re/¹⁸⁵Re and ¹⁸⁶W/¹⁸³W ratios typically range from 8 to 30 ppm. Long-term reproducibility of NIST SRM-989 is 0.04 ‰ (2s). Industrial Re, represented by SRM-989, a Re filament from H. R. Cross, and commercially available perrhenic acid (HReO₄) exhibit a total isotopic range of ~0.3‰ with δ¹⁸⁷Re values of 0, 0.17, and 0.27‰ respectively. To date, variations in δ¹⁸⁷Re have been observed in all classes of natural materials analyzed. Samples of Devonian Ohio shale from a Kentucky weathering profile with highly variable Re concentrations (Jaffe et al., 2002) display a range of about 0.6‰ and are themselves 0.5‰ lighter than an unweathered drill core sample taken from the site. Meteorites show variation of a similar range as represented by Allende (-0.69‰) and Canyon Diablo Troillite (-0.21‰). Sulfides analyzed include two MoS₂ samples, one Archaean (0.55‰) one Proterozoic (-0.10‰), and a modern ReS₂ (- 0.27‰). Finally, two water samples taken from the Berkeley Pit show δ¹⁸⁷Re of - 0.13‰ (surface) and -0.27‰ (76 m depth). As sample data were generated and reduced throughout the developmental period and therefore according to some variable parameters (e.g. analyte levels, frequency of standard bracketing, different bracketing standards . . .) it is difficult to properly determine sample reproducibility at this stage. When evaluated under all conditions, 2s sample reproducibility can be as good as ±0.03‰ (3 separately processed sample aliquots, 6 total analyses), and as poor as 0.35‰ (2 separately processed sample aliquots, 5 total analyses). The effect of observed Re isotope variations on Re-Os dating is negligible.
DE: 1039 Alteration and weathering processes (3617)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting