HIGH SPATIAL RESOLUTION RE-OS DATING OF MOLYBDENITE AND BLACK SHALES USING LA-ICP-“TRIPLE QUAD” MS

The Re-Os geochronometer is widely used to date Re-rich phases such as molybdenite, pyrite, organic compounds in petroleum and organic-rich black shales. Conventional Re-Os isotopic analysis via N-TIMS produces accurate results but requires highly specialized and labor-intensive chemistry to fully extract all Os from the sample. Moreover, the amount of material required for these analyses necessitates bulk sampling and a loss of spatial resolution. Recent advances in ICP-MS/MS technology now allow for the chemical separation of parent ¹⁸⁷Re from daughter ¹⁸⁷Os in the ICP-MS, allowing for in-situ analysis with laser ablation ICP-MS/MS at spatial resolutions at the ≈100 µm scale.

Presented here are new in-situ measurements of Re-Os isotopic analyses from a suite of molybdenites of known age (Kingsgate ≈220 Ma, Bingham Canyon ≈38 Ma, and NIST8599 ≈28 Ma) as well as a suite of Paleozoic organic-rich black shales of known stratigraphic age (Stark shale ≈305 Ma, Mecca shale ≈309 Ma, lower Marcellus shale ≈390 Ma, and the Niutitang Formation ≈540 Ma). All Re-Os geochronology data was calibrated using the Moly Hill molybdenite for Re/Os ratios and MASS-3 for Os isotopic compositions. Black shales analyzed have Re contents between 100 and >1,000 ng/g and molybdenite have Re contents between 3 and >100 µg/g. Using optimized laser ablation conditions of 6 J/cm², 15 Hz, and a spot size between 100 and 120 µm in diameter there are minimal matrix effects between black shales and molybdenite when obtaining Re-Os dates. Results show precisions on the order of 1% to 10% for Re-Os model dates while isochron date precision is 2% to 20%. Re-Os dates and initial Os compositions are within the uncertainties of published ages and compositions. In-situ sampling of black shale whole rock samples allow sub-sampling of discrete phases leading to larger spreads in the ¹⁸⁷Re/¹⁸⁹Os ratio than homogenized powdered whole rock, thus improving the precision on the measured Re-Os date. In addition to Re-Os isotopic data, the ICP-MS/MS is also able to collect a wide range of other major and trace elements that can improve geological interpretation, chronostratigraphic relations, understanding of processes associated with ore formation, metal accumulation in black shales, and the mobility and timing of metal migration during diagenesis and oil generation.