Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.v51b0566m&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #V51B-0566
Physics
1039 Alteration And Weathering Processes (3617), 1041 Stable Isotope Geochemistry (0454, 4870)
Scientific paper
Analyses of industrial and natural materials show ‰-scale Re isotope variation. Data are expressed using δ notation where δ187Re = \Large(\normalsize \frac{187Re/185Resmpl}{187Re/185Restd} \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.
Ball Lewis
Miller Charles A.
Peucker-Ehrenbrink Bernhard
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