Introduction to Radioisotope Geochronology/Part 3 - Commonly Used Decay Schemes

U-Pb

U-Pb geochronology is often regarded as the gold standard of geochronology because unlike all other chronometers it exploits two independent decay schemes, ²³⁵U to ²⁰⁷Pb and ²³⁸U to ²⁰⁶Pb and both the ²³⁸U and ²³⁵U decay constants are relatively precise and accurate (Jaffey et al., 1971). The advantage of two independent chronometers in the same mineral is that it is possible to detect small amounts of open system behavior such as Pb loss or the inheritance of an older mineral. This is a major factor in our ability to make reliable, high-precision age determinations as we can evaluate whether a number of analyses represents a single time of mineral growth. The different half lives of ²³⁸U and ²³⁵U, ca. 4.5 and ca. 0.7 Ga respectively, means that by the Neoproterozoic much smaller amounts of ²³⁵U (relative to ²³⁸U) remain due to the higher decay rate, therefore smaller amounts ²⁰⁷Pb are produced per increment relative to ²⁰⁶Pb. Although all three dates can be calculated from most published analyses, the relative precisions are related to the analytical technique employed (see section 5 for further discussion of age uncertainties). The U-Pb method is most-often applied to U-bearing accessory minerals such as zircon found in igneous rocks and the most common way to graphically represent the data is the Concordia diagram.

²³⁸U to ²⁰⁶Pb decay series

When the radiometric ages of the two U-Pb isotopic systems are in agreement it is referred to as being ‘concordant’. Tracing this concordance through geologic time produces a ‘concordia’ in ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U (referred to as a Wetherill concordia). The concordia can also be visualized using ²⁰⁶Pb/²⁰⁷Pb and ²³⁸U/²⁰⁶Pb (A Tera-Wasserburg concordia). U-Pb dating is performed on a mineral that accepts U in the crystal lattice during mineral formation. In zircon, Pb is not readily incorporated into the crystal lattice during mineral formation. Therefore any measurable lead in the mineral is that which was generated by radiometric decay. When the mineral is a closed-system for U and Pb the U/Pb isotopic ratios of an analysis will lie on the concordia. For other mineral systems that incorporate “common-Pb” into the crystal lattice will also contain some proportion of radiogenic Pb. When several analyses with different proportions of common- and radiogenic-Pb is plotted on a Tera-Wasserburg concordia diagram a linear regression can be plotted through these analyses anchored at the common-Pb composition. Where this regression (or discordia) crosses the concordia at its lower-age intercept is interpreted as the age at which the mineral began to accumulate radiogenic-Pb (or became a closed system).

Analyical Methods

There are a number of different analytical approaches that can be used to determine the U/Pb ratio and isotopic composition of U-bearing accessory minerals. The different techniques types discussed in more detail below.

ID-TIMS

Isotope dilution where minerals are dissolved in the presense of tracer isotopes

• Isotope Dilution
• Thermal Ionisation Mass Spectromtry

Visualising U-Pb data

U-Pb concordia plot

Detail of U-Pb concordia plot

Two separate dates for a zircon based on each individual decay scheme may be calculated and plotted on a concordia diagram (Fig. 1). On a conventional (Wetherill) concordia diagram the X and Y axis are the ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²³⁸U ratios respectively, and the concordia curve represents the simultaneous solution of the decay equations for a given age. A third 207Pb/206Pb date can be determined from only Pb isotopic measurements by knowing both the ²³⁵U and ²³⁸U decay constants and the present day ²³⁵U/²³⁸U ratio which is assumed to be 137.88 (Steiger and Jager, 1977) or 137.818 (Hiess et al., 2012). Calculation of the U/Pb dates requires determination of the Pb*/U ratio (Pb* denotes radiogenic Pb).

Accuracy of the U-Pb system

Gravimetric Calibration of U-Pb System

• U and Pb reference materials
• How to calibrate a mixed U-Pb tracer solution
• Mineral Standards for normalisation

U-decay constants

• 238U
• 235U
• 234U

Closed-System behaviour?

Pb-loss

It has been known for several decades that zircons often show evidence of post-crystallization Pb loss. This has the effect of lowering the U/Pb ratios and the derived dates (see Fig. 2). Silver and Duetsch (Silver and Duetsch, 1963) demonstrated that Pb-loss was correlated with radiation damage. Since that time it has become widely appreciated that the Pb-loss is not by thermally activated volume diffusion but rather by fast-pathway diffusion from damaged parts of the crystal lattice. In order to minimise/eliminate the effects of post-crystallisation Pb loss it is possible to pre-treat zircons using techniques to remove domains that have lost Pb leaving zircon domains that have remained closed systems.

• Air Abrasion

The first approach developed involved physically abrading away the exterior portions of the zircons (Krogh, 1982a), based on the observation that the outer portions were richest in U and thus susceptible to radiation damage and Pb loss. At the same time Krogh (Krogh, 1982b) also demonstrated that the selection of the least magnetic zircons often corresponded to the lowest U contents and had least amount of Pb loss. These approaches were widely applied until the development of a new technique described as ‘chemical abrasion’ (Mattinson, 2005).

• Chemical Abrasion

This technique involves annealing zircon grains at 800-900°C followed by partial dissolution. This method effectively “mines out” or preferentially dissolves the higher U parts of the zircon that have been damaged by radiation and are thus susceptible to fast-pathway diffusion of Pb from the zircon crystal. This method seems to offer the promise of the effective elimination of open system behaviour in most zircon. Microbeam techniques (see section 4.1.2) have not typically employed pre-treatment techniques as they assume that Pb loss is restricted to the exterior portions of grains which they attempt to avoid during the in-situ analyses.

References

1 ^ Jaffey, Flynn, Glendenin, Bentley, and Essling. Precision Measurement of Half-Lives and Specific Activities of ²³⁵U and ²³⁸U, Physical Review C, Vol. 4, No. 5, Nov. 1971.

2 ^ Subcommission on Geochronology: Convention on the Use of Decay Constants in Geo- and Cosmochronology, compiled by R.H. Steiger and E. Jager. Earth and Planetary Science Letters, 36 (1977) 359-362 [4]

3 ^ ²³⁸U/²³⁵U Systematics in Terrestrial Uranium-Bearing Minerals. Science, Vol. 335 no. 6076 pp. 1610-1614. [5]