Reliable geochronology of rare-metal pegmatites has been a significant challenge due to the typical dearth of suitable mineral geochronometers (e.g., zircon, baddeleyite) for in-situ techniques. Recent advances in mass-spectrometry now allow for in-situ age determinations of more abundant minerals (e.g., micas) using the Rb–Sr (and potentially K–Ca) decay system. This approach allows for rapid and cost-effective acquisition of geochronometric data with simultaneous collection of in-situ geochemical data that may be useful in advancing our understanding of rare-metal pegmatites.
Analysis of mica specimens from rare-metal LCT-class pegmatites around the world acquired from the South Australian Museum and the Tate Museum, University of Adelaide, demonstrates that many are hyper-radiogenic with negligible initial strontium. These characteristics suggest that single spot 87Rb/87Sr age calculations assuming purely radiogenic growth of 87Sr (akin to U–Pb systems) are often feasible. Preliminary analysis of lithian-micas from Archean pegmatites in WA (Tabba Tabba, Londonderry, Wodgina), Mesoproterozoic pegmatites from the NT, Neoproterozoic pegmatites from Brazil, and Cretaceous pegmatites from California all return precise age data well within the expected age uncertainty for these deposits. We also found that unlike most minerals, crystal orientation of micas has a significant effect on isotopic ratio quantification via LA-ICP-MS, and thus the calculated ages. Caesium-beryl also shows potential as a reliable Rb–Sr geochronometer, though further development is needed. By contrast, our data indicate that neither spodumene nor amblygonite take in appreciable Rb, or Lu, for feasible in-situ Rb–Sr or Lu–Hf dating with current analytical technology.