Pushing the Limits of Geological SIMS: Optimization of Techniques and Strategies for New Capabilities

The basis of secondary ion mass spectrometry is a focused primary ion beam that sputters a sample in a very controlled manner such that compositions of atomic monolayers may be resolved. In geological SIMS, molecular interferences must be resolved in the mass spectrometer requiring high mass resolution. SHRIMP was the first geological SIMS instrument and immediately opened a completely new scale of analysis but also demonstrated the fundamental limits of this technique. Isotope ratios of trace elements in a few nanograms of material are limited primarily by counting statistics. Advances in SIMS instrumentation are no longer responsive to all potential capabilities particularly with regards to primary spot size, isotope ratio precision and accuracy, mass resolution of the mass spectrometer and so forth. In other words, no single SIMS instrument to date possesses all the capabilities necessary for ion imaging and ratio measurement. Advances in SHRIMP instrumentation are now focused on specific analytical qualities - SHRIMP RG with an ultra-high mass resolution capability, and the SHRIMP SI with a simple and robust measurement capability for stable isotope measurements. Each instrument therefore is targeted to answer specific geologic problems. For example, applications amenable to the SHRIMP RG have included Hf-W isotopes in meteoritic zircon, rapid trace element analysis routines, and applications requiring high abundance sensitivity. For SHRIMP II, multiple collection development is ongoing for a variety of applications including rapid detrital zircon age determinations, common Pb measurements (including 204Pb), zircon Ti thermometry, O and S isotopic measurements. Advances in SIMS instrumental capabilities alone are not the sole issue in analytical improvements. Optical (reflected light, transmitted light) and electron imaging techniques (cathodoluminescence, BSEI, EBSD) are crucial for an understanding of the petrographic context of analytical sites. Better documentation within a single crystal allows data to be collected from targeted zones and limit the possible measurement of mixed domains. All too frequently, the right sample, whether by selection or serendipity, will often transcend the specific analytical technique used and open new scientific doors.