TABLE A2.Procedure for Re-Os dating at the University of Alberta Radiogenic Isotope Facility

Molybdenite separates were prepared by pulverizing the vein samples in a porcelain disk mill, and molybdenite was separated from other sulfide and gangue phases using heavy liquid techniques, magnetic separation, and by flotation using high-purity water. Finally, molybdenite grains were handpicked under a binocular microscope.

The 187Re and 187Os concentrations in molybdenite were determined by isotope dilution mass spectrometry at the University of Alberta Radiogenic Isotope Facility. Dissolution of molybdenite separates and equilibration of sample and tracer Re and Os were done using the Carius tube method (Shirey and Walker, 1995).

Samples were dissolved and equilibrated with a known amount of 185Re and isotopically normal Os in 3 ml of reverse aqua regia (2:1, 16 N HNO3: 12 N HCl) at 240°C for 24 hours. Os and Re were separated by solvent extraction, microdistillation, and anion chromatography techniques (Selby and Creaser, 2004). The purified Os and Re fractions were loaded onto Ba-coated Pt filaments and measured with Faraday collectors using negative thermal ionization mass spectrometry (Creaser et al., 1991; Völkening et al., 1991) on a Micromass Sector 54 mass spectrometer. Total procedure blanks are on the order of <5 pg for Re, and <2 pg for Os.

Model Re-Os ages were calculated (after Stein et al. 2001) based on the equation: t = ln(187Os/187Re + 1)/, where t is the model age, and  is the 187Re decay constant (1.666 x 10-11 a-1; Smoliar et al. 1996). Errors (2σ) include uncertainties in Re and Os isotopic measurements, spike and standard Re and Os isotope compositions, calibration and gravimetric uncertainties of 185Re and 187Os, and uncertainties in the 187Re decay constant. Uncertainties in weights of sample and tracer solution do not affect the calculated age and are not considered.

The molybdenite powder HLP-5 (Markey et al., 1998) is analyzed to assess the accuracy in Re-Os molybdenite age determinations. The average Re-Os age for this standard over a four-year period is 221.59 ± 0.45 Ma (1σ uncertainty, n = 11), which is identical to the value of 221.0 ± 1.0 Ma reported by Markey et al. (1998). The Henderson molybdenite standard is also analyzed to assess accuracy, and yielded an age of 27.71 ± 0.13 Ma in accord with the recommended age value (Markey et al., 2007).

References

Creaser RA, Papanastassiou DA, Wasserburg GJ (1991) Negative thermal ion mass spectrometry of osmium, rhenium and iridium. Geochim Cosmochim Acta 55:397–401.

Markey RJ, Stein HJ, Morgan JW (1998) Highly precise Re-Os dating for molybdenite using alkaline fusion and NTIMS. Talanta 45:935–946.

Markey RJ, Stein HJ, Hannah JL, Selby D, Creaser RA (2007) Standardizing Re-Os geochronology: A new molybdenite Reference Material (Henderson, USA) and the stoichiometry of Os salts. Chem Geol 244:74–87.

Selby D, Creaser RA (2004) Macroscale NTIMS and microscale LA-MC-ICP-MS Re-Os isotopic analysis of molybdenite: Testing spatial restrictions for reliable Re-Os age determinations, and implications for the decoupling of Re and Os within molybdenite. Geochim Cosmochim Acta 68:3897–3908.

Shirey SB, Walker RJ (1995) Carius tube digestion for low-blank rhenium-osmium analysis. Anal Chem 67:2136–2141.

Smoliar MI, Walker RJ, Morgan JW (1996) Re-Os ages of Group IIA, IIIA, IVA, and IVB iron meteorites. Science 271:1099–1102.

Stein HJ, Markey RJ, Morgan JW, Hannah JL, Scherstén A (2001) The remarkable Re–Os chronometer in molybdenite: how and why it works. Terra Nova 13:479–486.

Völkening J, Walczyk T, Heumann K (1991) Osmium isotope ratio determinations by negative ion mass spectrometry. Int J Mass Spectrom Ion Proc 105:147–159.