Next Steps in (U-Th)/He Laser Microprobe Geochronology and Thermochronology

Laser microprobes permit the direct determination of radiogenic He concentrations and concentration gradients in minerals. At the Massachusetts Institute of Technology, we developed a method to acquire multiple (U-Th)/He dates for laser ablation pits in single monazite grains (Boyce et al., 2006, Geochim. Cosmochim. Acta 70, 3031-3039). The method involves: 1) ultrahigh-vacuum ablation of polished grain mounts with an ArF excimer laser; 2) conventional gas purification and He isotope dilution analysis using a quadrupole mass spectrometer; 3) measurement of the ablation pit volume with an interferometric microscope (in order to calculate radiogenic He concentration from abundance data); and 4) determination of parent isotope abundances from elemental concentration data obtained by electron microprobe analysis. This technique has been used successfully to date monazites as young as 730ka with an analytical precision of better than 3 percent (2SE). Expanding the utility of this technique is a major research focus at the new Noble Gas Geochronology and Geochemistry Laboratory at Arizona State University. Efforts are underway to increase the effective spatial resolution of the technique and permit applications to minerals with lower U+Th abundances (e.g., apatite, titanite, and zircon) by using a split flight tube magnetic sector mass spectrometer for the He analyses and by testing a variety of microanalytical techniques (e.g., SIMS, LA-ICPMS) for parent element measurements. Special emphasis is being placed on the measurement of intracrystalline gradients in U, Th, and He that result from both crystallization zoning and diffusive loss. Such patterns are typically complex in the accessory minerals used for (U-Th)/He thermochronology, and the hope is that laser microprobe methods can be employed to better understand the nature of such complexities and their impact on conventional (U- Th)/He and 3He/4He thermochronology. Ultimately, our goal is to resolve natural 4He diffusion profiles is sufficient detail to permit the reconstruction of thermal histories in orogenic systems.