The actinide elements U and Th are among the heaviest naturally-occurring elements. They are radioactive and decay by alpha-decay down a complex decay series to Pb, and also by fission.
U has two main radioactive isotopes, 235U and 238U, that decay to isotopes of Pb, 207Pb and 206Pb respectively. Th also has one common (long-lived) radioactive isotope, 232Th, that decays to 208Pb. Because the ionic radius and chemical behaviour of U and Th are so similar (although U exists in nature in two oxidation states), U-Th/Pb in common minerals offers three coupled decay schemes that together, provide a powerful means of dating the time of mineral formation in geological samples.
Pb is a metal in the Carbon Group.
The half-life of 238U is comparable to the age of Earth and roughly 1/2 has decayed since solar system formation. The half-life of 235U is much shorter that the age of Earth, so most has since decayed away. Because the decay rates of 238U and 235U are very different and both systems share the same parent and daughter elements, these radioactive decay schemes may be applied in combination to model open-system behaviour since closure of the parent-daughter system.
Some minerals that crystallise within magmas or during metamorphic or diagenetic events will concentrate U and/or Th into their crystal structures, yet because its ionic radius is not compatible with their crystal structures, Pb will be largely excluded from these minerals.
In particular, the common minerals zircon (Zr2SiO4) and baddeleyite (ZrO2) may contain high concentrations of U and (to a lesser extent) Th, whereas monazite (Th, REE, PO4) will concentrate Th and (to a lesser extent) U.
Whilst these minerals usually form with low concentrations of Pb, determination of precise dates still requires the determination of and correction for the isotopic composition of Pb that may have been incorporated into mineral at the time that it formed. Pb incorporated into mineral at the time that it formed is referred to as “common Pb”.
U-Th/Pb dating: mineral characteristics
- high U and/or Th contents
- low common-Pb contents
- resilient structure (U-Pb system closed)
- zircon (ZrSiO4)
- monazite (Th, REE, PO4)
- baddeleyite (ZrO2)
- TIMS Pb-Pb evaporation
- conventional (TIMS) single-zircon
- ion microprobe
Common problems encountered during mineral dating:
- between-sample mineral cross-contamination: usually occurs during sampling, crushing, milling and/or heavy mineral separation
- within-mineral radiogenic-Pb loss or redistribution: discordant or scattered analyses may result from high-U minerals, multiple metamorphic disturbance events or weathering
- complex age spectrum or too many age populations; usually arises from poor sample selection and results in ambiguous interpretation of results and/or imprecision on dates for populations
Age structure within individual zircons
This zircon was detected in a 2.6 Ga old granite from the NW Yilgarn of Western Australia (near Murchison Shire, W of Meekatharra). The back-scattered electron (BSE) and cathodoluminescence (CL) images show surface microstructure and ion-probe analysis sites. An interpretive schematic structural diagram shows zones within the grain which are distinguishable on the basis of the BSE and CL image and which ion-microprobe analysis indicates have different dates (uncertainties given on U–Pb dates are ±1sigma). This zircon was a xenocryst with an old core around which a younger rim formed when the grain was incorporated into the host granite melt.
Such age structure within individual grains presents a problem for TIMS and evaporation methods. It was this problem that led to the development of U-Pb zircon dating by ion-microprobe.