Geochronology

Uranium-lead dates

Lead occurs in four stable isotopes ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb. All but ²⁰⁴Pb are formed by the radiogenic decay of U and Th. The isotopic ratios of Pb in minerals that contain U and Th can be used  to identify the date of mineral formation provided the mineral has remained closed (no loss or gain of Pb, U or Th after formation) (Faure, 1977).

Methods available

U-Pb dating is done by LA-ICP-MS on zircon and monazite minerals. The method for igneous rocks is U-ISTP02 on 20 mineral grains. The method for sediments is U-ISTP03 on 60 mineral grains. The separation of zircon and monazite is included in the cost of the analysis.

Re dates

Rhenium has two naturally occurring isotopes, ¹⁸⁵Re which is stable and ¹⁸⁷Re which decays to stable ¹⁸⁷Os (Faure, 1977). As rhenium effectively substitutes for Mo in molybdenite but Os is not incorporated into the mineral, the ¹⁸⁷Os present in molybdenite is from the decay of ¹⁸⁷Re (Stein et al., 2001). Using the concentration of ¹⁸⁷Os, 187 Re and the decay constant for ¹⁸⁷Re the age of the molybdenite formation can be determined.

Method available

Method Re-ISTP01 is offered for molybdenite. Rock or drill core must be received whole as steel jaw crushing will contaminate the sample with Re. Age can only be determined for rocks of >0.5 Ma, and the molybdenite separate must contain >100ppm Re.

Ar dates

The isotope ⁴⁰Ar is the stable daughter product of ⁴⁰K decay. Potassium bearing samples are irradiated to change ³⁹K to ³⁹Ar and measured for their ⁴⁰Ar/ ³⁹Ar composition. As the radiogenic ⁴⁰Ar is a result of ⁴⁰K decay, the mineral formation time can be determined based on the ⁴⁰Ar/ ³⁹Ar and the half-life of ⁴⁰K (Faure, 1977).

Method available

Method Ar-ISTP01 is performed on targeted minerals. Rock and drill core should be submitted intact or crushed only as sample prep is included. Measurement is by irradiation and step heating in a mass spectrometer.

References & further reading

Faure, G., 1977. Principles of Isotope Geology. Second Edition. John Wiley and Sons.

Stein, H.J., Markey, R.J., Morgan, J.W., Hannah, J.L., and Schersten, A., 2001. The remarkable Re-Os chronometer in molybdenite: how and why it works. Terra Nova, Vol13, No. 6, pp 479-486.