MICRO-SAMPLING OF LOW PB ("YOUNG") MONAZITE – OPTIONS, RESTRICTIONS AND POTENTIAL

The presence of monazite in a wide variety of rock-types, its ability to incorporate both actinide and lanthanide elements, and the observation that it reacts across a range of pressure, temperature, and deformation conditions combine to make monazite a powerful petrochronometer. It is common for multiple episodes of growth, dissolution and re-precipitation to be preserved within distinct intra-crystal compositional domains that are typically at, or below, the 10’s of micron scale. Whilst the presence of multiple compositional and/or age domains at such scales provides an excellent opportunity to decipher detailed geologic histories, it also presents a number of analytical challenges that must be surmounted in order to unlock the true potential of monazite. ID-TIMS is capable of producing ages with uncertainties <0.1%, and although significant advances have been made in micro-sampling methods, the typical scale and complexity of intra-crystalline zoning places limits on the utility of this technique. For Paleozoic and older monazite the Electron Probe MicroAnalyzer yields ages at high spatial resolution. However, for Tertiary monazite, the combination of Low Pb concentrations and disequilibrium in the ²³⁸U decay chain restricts its use. For ‘young’ monazite, it is therefore necessary to employ in-situ techniques - either SIMS or LA-ICPMS. Drawing on case studies from the Himalaya and other Tertiary orogenic belts, this presentation will highlight the start-of-the-art in determining in-situ ages for ‘young’ monazite, including recent developments in laser systems (e.g. decreased wavelength and improved ablation cell designs) and improvements in instrumentation (e.g. higher sensitivity, concurrent measurement of isotopes and trace elements). Together, these advances enable rapid and simultaneous production of moderately precise (1 - 2%) isotope and elemental data at high (< 7μm) spatial resolution in petrographic context. Current limitations, including mass spectrometry and availability of reference materials as well as potential solutions to these problems will be discussed in the context of ongoing research projects.