Zircon Geochronology

Zircon (ZrSiO4) is a common accessory mineral in igneous rocks. It contains trace levels (1-1000 parts per million) U and Th and their decay to the Pb isotopes is the basis for geochronology. Zircon is also very resilient to geological processes and zircon can be added to the structure like rings on a tree. This is the attribute that requires a measurement technique capable of resolving internal structures within a single grain of zircon.

The in situ measurement of Pb isotopes by SIMS requires the elimination of isobaric interferences of Hf-Si molecules (e.g. 180Hf-28Si at mass 208) which equates to mass resolution 5,500. SHRIMP I was designed for simultaneous high sensitivity and high mass resolution allowing the resolution to eliminate the molecular isobars, while maintaining the sensitivity to analyse the trace levels of Pb in zircon.

While the Pb isotope composition is taken as measured, the measured U/Pb ratio is affected by the different ionisation efficiencies of the two elements. Furthermore, subtle variations in conditions at the sputter site can cause large changes in measured Pb/U. Following the suggestion of Andersen and Hinthorne (1973), the Pb/U was found to vary systematically with the measured UO/U ratio. When the analyses are normalized back to a common UO/U ratio, the measured Pb/U for a standard material (i.e. known and uniform age) can be calibrated to better than 2%.

This allows geologically useful information to be obtained from only a few nanograms of material.

Monazite Geochronology

Monazite is commonly a metamorphic mineral and so can yield information not available from zircon. While most of the isobaric interferences are resolved, a family of interferences based on doubly charged REEThO2 causes problems, especially for 204Pb which has an isobar at 144NdThO2++. These isobars require mass resolution beyond the capabilities of these mass spectrometers. They can be eliminated through energy filtering or by peak stripping the contribution as monitored at another peak. For example, the correction to 204Pb can be monitored by measuring the abundance of 143NdThO2++ at mass 203.5.

Other Minerals

The list of minerals that has been measured by SHRIMP dating continues to grow. Besides zircon and monazite, it includes titanite (sphene), apatite, rutile, xenotime, columbite, allanite, perovskite, baddeleyite. The main limitation to SHRIMP geochronology in these minerals is the concentration of U and Th and hence the concentration of radiogenic Pb leading to a limitation in precision due to counting statistics. In addition, the relative abundance of common Pb can be a limiting factor.

In general, minerals with a narrow range of compositions are better analysed than minerals that have variable and wide ranging substitutions. This is most problematic in the calibration of U/Pb.