Quantities required to obtain a Pb-U date from a high-U mineral using SIMS and to assess its concordance include:

• Pb isotopic ratios (counts of 204Pb, 206Pb, 207Pb and 208Pb)

• 206Pb/238U ratio (from which 207Pb/235U ratio can be calculated)

• 208Pb/232Th ratio

However, the SIMS secondary ion spectrum includes a complex range of charged and multiply-charged molecular species and different species are generated with different efficiencies, so the inter-element ion ratios measured during SIMS analysis are not equivalent to those present in the target mineral.

Inter-element (e.g. Pb/U and Pb/Th) ratios measured by SIMS are influenced by:

• concentration of elements in target (sputtered volume)

• element secondary ion sputter yield/incident ion

• probability that the element forms the ion species measured

• extraction efficiency of secondary ion optics for the ion species

• other instrumental operating parameters

Therefore, measured Pb/U and Pb/Th ratios must be corrected for these parameters using correction factors based on measurement of a standard of the same mineral that has known Pb, U and Th abundance and that is analysed at the same time as the unknowns.

Processing of U-Pb geochronology data acquired via SIMS is lengthy and complex. An overview (pdf) of SIMS data processing is downloadable from here. This document also provides access details for a software package for the processing of SIMS data.