Assessment of the 187Re decay constant by cross calibration of Re–Os molybdenite and U–Pb zircon chronometers in magmatic ore systems

Abstract

The past decade has seen renewed interest in 187Re–187Os geochronology using a variety of matrices including sulfide minerals, shales and meteorites. The most widely used value of the 187Re decay constant (λ 187Re) is 1.666 ± 0.005 × 10−11 a−1 (±0.31%), which is based on cross calibration of Re–Os and Pb–Pb chronometers for certain meteorites [Smoliar M. I., Walker R. J., and Morgan J. W. (1996) Re–Os isotope constraints on the age of Group IIA, IIIA, IVA, and IVB iron meteorites. Science 271, 1099–1102]. However, other recent studies have yielded alternate values of λ 187Re, based upon either direct counting experiments or analysis of meteorites. Here, we provide an independent assessment of λ 187Re, using methodology, sample materials, and preparation of Os standard solutions different from those of Smoliar et al. (1996). Combining Re–Os age data for molybdenite formed in magmatic ore deposits, with the U–Pb zircon age of the magmatic rocks, a refined λ 187Re value is determined by averaging 11 individual cross-calibration experiments spanning ca. 2700 Ma of Earth history. Using the U decay constants of Jaffey [Jaffey A. H., Flynn K. F., Glendenin L. E., Bentley W. C., and Essling A. M. (1971) Precision measurement of half-lives and specific activities of 235U and 238U. Phys. Rev. 4, 1889–1906], a value for λ 187Re of 1.6668 ± 0.0034 × 10−11 a−1 is determined. Using the λ 238U value of Jaffey et al. (1971) and λ 235U value of Schoene [Schoene B., Crowley J. L., Condon D. J., Schmitz M. D., and Bowring S. A. (2006) Reassessing the uranium decay constants for geochronology using ID-TIMS U–Pb data. Geochim. Cosmochim. Acta 70, 426–445], a value for λ 187Re of 1.6689 ± 0.0031 × 10−11 a−1 is determined. These values are nominally higher (ca. 0.1 and ca. 0.2%) than the value determined by Smoliar et al. [Smoliar M. I., Walker R. J., and Morgan J. W. (1996) Re–Os isotope constraints on the age of Group IIA, IIIA, IVA, and IVB iron meteorites. Science 271, 1099–1102], but within calculated uncertainty. Further refinement of λ187Re by cross calibrating the molybdenite and U–Pb zircon chronometers should be possible by utilizing high precision, single-grain, chemical abrasion zircon U–Pb analyses.