Laser Microprobe (U-Th)/He Thermochronology of Detrital Minerals

A persistent concern in detrital mineral geochronology is the need to obtain a representative sampling of crystallization or cooling ages in the source region. Methods with high throughput --- e.g., laser microprobe 40Ar/39Ar thermochronology of muscovite and U-Pb thermochronology of zircon --- have a distinct advantage in this regard. Both techniques have advanced to the point that the dozens of analyses necessary to obtain a representative sampling can be done quickly and with sufficiently precision for high-quality research. Datasets obtained using methods that are far more labor intensive --- e.g., single-grain (U-Th)/He and fission track dating of minerals such as zircon --- typically include many fewer analyses. Consequently, we have less confidence that the cooling age distribution in the dataset represents the cooling age distribution in the source region. Of greater concern are analytical protocols that increase the probability of non-representative sampling. One example is the practice of picking zircon grains that are inclusion-free and euhedral (or nearly so) for conventional (U-Th)/He dating. While this practice is essential for successful conventional (U-Th)/He dating, it unavoidably leads to the systematic exclusion of grains that actually may represent significant portions of the source terrain. We describe a new approach to detrital mineral (U-Th)/He thermochronology that, in principle, provides a higher- fidelity record of the source region cooling history than the conventional technique. It involves the use of an excimer laser microprobe to ablate portions of the grain interiors from detrital zircons in a polished grain mount. (Prior to analyses, the grains can be mapped using backscattered electron and cathodoluminesence imagery.) The amounts of evolved 4He are typically so small that they are best measured using a magnetic-sector mass spectrometer rather than a quadrupole mass spectrometer of the type typically used for conventional (U- Th)/He dating. U and Th concentrations can be measured in or around the same ablation pit using laser ablation, inductively coupled plasma source mass spectrometry or secondary ionization mass spectrometry. The greatest difficulty in widespread application of the technique, at present, is the lack of reliable U and Th concentration standards. In addition to making ill-formed grains amenable to analysis, the method allows a user to target specific areas within grains and thus avoid regions with inclusions or complex chemical zoning, both of which complicate conventional dating. This presentation will be a review of the current state of development of this promising technique, as well as a critical evaluation of its strengths and weaknesses compared to conventional (U-Th)/He thermochronology.