Analytical Methods

Re-Os molybdenite geochronology

Analytical methods are described in detail by Selby and Creaser (2004) and Markey et al. (2007). Aliquots of molybdenite of typically 0.01-0.03g are dissolved, together with a 185Re+188Os+190Os tracer in inverse (Lefort) aqua regia comprising 6ml 15N HNO3 + 2ml 10N HCl at 200°C for 24 hours. Chemical separation of Os uses solvent extraction of OsO4 into chloroform followed by back-extraction into HBr, and microdistillation for final purification. Re is recovered from the nitric acid solution by solvent extraction into acetone, anion exchange chromatography in column form (1 ml resin bed). Rhenium and Os are loaded onto Ni and Pt filaments, respectively, and analyzed by N-TIMS using a ThermoScientific Triton instrument. Total procedural blank abundances of Re and Os are typically <3 pg Re and < 1 pg Os, with a blank 187Os/188Os of 0.3. The Henderson molybdenite age standard RM8599 is regularly analyzed to monitor accuracy. From 2015-2022, 39 analyses of this standard yield an average age of 27.78 ± 0.07 Ma, in accord with the certified value (27.66 ± 0.1 Ma, Wise and Watters, 2011).

Re-Os shale and graphite geochronology

Analytical methods for shales are described in detail by Selby and Creaser (2003), Kendall et al. (2009) and Toma et al. (2022). Between 0.3 and 0.8 g of black shale powder, or < 0.1 g graphite, are dissolved together with a mixed 185Re+190Os tracer and 8 mL of a CrVI+H2SO4 solution (containing 0.25 g of CrO3 per 1 mL of 4N H2SO4) in Carius tubes at 240°C for 72 hours. The CrO3 used is Fluka Chemika purchased from 2004-2006. Chemical separation of Os uses solvent extraction of OsO4 into chloroform followed by back-extraction into HBr, and microdistillation for final purification. Re is recovered from the CrVI+H2SO4 solution by solvent extraction into acetone, anion exchange chromatography in both column form (1 ml resin bed) and as single-beads for final purification. Rhenium and Os are loaded onto Ni and Pt filaments, respectively, and analyzed by N-TIMS using a ThermoScientific Triton instrument. Total procedural blank abundances of Re and Os are typically 10-15 pg Re and <0.2 pg Os, with a blank 187Os/188Os of 0.3. Most of the Re blank is attributed to the CrVI+H2SO4 solution.

Re-Os sulfide geochronology

Analytical methods are described in detail by Hnatyshin et al. (2016, 2020). Aliquots of sulfide minerals up to 0.4g are dissolved, together with either a 185Re+190Os tracer or a 185Re+188Os+190Os tracer in inverse (Lefort) aqua regia comprising 6ml 15N HNO3 + 2ml 10N HCl at 220°C for 48 hours. Chemical separation of Os uses solvent extraction of OsO4 into chloroform followed by back-extraction into HBr, and microdistillation for final purification. Re is recovered from the nitric acid solution by solvent extraction into acetone, anion exchange chromatography in both column form (1 ml resin bed) and as single-beads for final purification. Rhenium and Os are loaded onto Ni and Pt filaments, respectively, and analyzed by N-TIMS using a ThermoScientific Triton instrument. Total procedural blank abundances of Re and Os are typically <3 pg Re and < 0.2 pg Os, with a blank 187Os/188Os of 0.3.

Mass spectrometry

Isotopic analysis uses a ThermoScientific Triton instrument installed in 2008. This instrument is dedicated to Re-Os geochronology, and has never used filaments made of Re since manufacture. The instrument has logged nearly 20,000 analyses at August 2022. Ni filaments for analysis of Re are constructed from 0.006” width Ni wire and Pt filaments for analysis of Os from 0.005” Pt wire, crimped in the center to facilitate loading. Both filament types are glowed in air prior to loading Re or Os. For Re loaded on Ni wire, an activator of aqueous BaNO3 is used. For Os loaded on Pt wire, an activator of 0.1N NaOH saturated with Ba(OH)2 is used. Isotope ratios of Re and Os are measured as ReO4- and OsO3- by N-TIMS (Creaser et al., 1991) at ~ 810°C and 715°C, respectively. Re from most sample types is analyzed by static Faraday collector analysis. For sulfides, shales and graphite, Os is typically analyzed by a pulse counting, single SEM in peak-hopping mode. Molybdenite Os is typically analyzed by static Faraday collector analysis. Raw Re ratios are corrected for isobaric oxide interferences, and standardized to 185Re/187Re = 0.5974 empirically using long-term standard analysis values. For Os, raw ratios are corrected offline for isobaric oxide interferences, mass fractionation and spike contributions. The value for oxygen 17O/16O and 18O/16O used in oxide corrections is determined by N-TIMS analysis of a 192Os spike by measurement at masses 240, 241 and 242. For monitoring of instrument performance, we use in-house Re and Os standards. The Re standard is made from 99.999% Re, and the Os from Johnson-Matthey (NH4)2OsCl6. For the Re standard using loads of 5ng and analysis by static Faraday collector, an average value for 185Re/187Re of 0.5977 ± 0.0011 (2s) is determined (n=208, 2016-2022). For Os, average 187Os/188Os ratios of 0.10688 ± 0.00042 (2s, n=390, 2008-22) and 0.10687 ± 0.00056 (2s, n=1252, 2008-22) using Faraday and SEM collectors, respectively. This 187Os/188Os value is in accord with other published data for Johnson-Matthey Os (Li et al. 2010, Geochemical J. 44, 73-80). The Durham Os standard DROsS is also analyzed, which gives an average 187Os/188Os ratio of 0.16091 ± 0.00016 (n=6, August 2022) in accord with published measurements (Saintilan et al. 2020, Sci. Reports 8, 1496).