HR: 17:00h
AN: V22G-05    [PDF]
TI: Laser-Ablation (U-Th)/He Geochronology
AU: * Hodges, K
EM: kvhodges@mit.edu
AF: Massachusetts Institute of Technology, 54-1120 MIT, Cambridge, MA 02139 United States
AU: Boyce, J
EM: jwboyce@mit.edu
AF: Massachusetts Institute of Technology, 54-1120 MIT, Cambridge, MA 02139 United States
AB: Over the past decade, ultraviolet laser microprobes have revolutionized the field of $^{40}$Ar/$^{39}$Ar geochronology. They provide unprecedented information about Ar isotopic zoning in natural crystals, permit high-resolution characterization of Ar diffusion profiles produced during laboratory experiments, and enable targeted dating of multiple generations of minerals in thin section. We have modified the analytical protocols used for $^{40}$Ar/$^{39}$Ar laser microanalysis for use in (U-Th)/He geochronologic studies. Part of the success of the $^{40}$Ar/$^{39}$Ar laser microprobe stems from fact that measurements of Ar isotopic ratios alone are sufficient for the calculation of a date. In contrast, the (U-Th)/He method requires separate analysis of U+Th and $^{4}$He. Our method employs two separate laser microprobes for this process. A target mineral grain is placed in an ultrahigh vacuum chamber fitted with a window of appropriate composition to transmit ultraviolet radiation. A focused ArF (193 nm) excimer laser is used to ablate tapered cylindrical pits on the surface of the target. The liberated material is scrubbed with a series of getters in a fashion similar to that used for $^{40}$Ar/$^{39}$Ar geochronology, and the $^{4}$He abundance is determined using a quadrupole mass spectrometer with well-calibrated sensitivity. A key requirement for calculation of the $^{4}$He abundance in the target is a precise knowledge of the volume of the ablation pit. This is the principal reason why we employ the ArF excimer for $^{4}$He analysis rather than a less-expensive frequency-multiplied Nd-YAG laser; the excimer creates tapered cylindrical pits with extremely reproducible and easily characterized geometry. After $^{4}$He analysis, U and Th are measured on the same sample surface using the more familiar technique of laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Our early experiments have been done using a frequency-quintupled Nd-YAG microprobe (213nm), While the need to analyze U+Th and He in separate ablation experiments results in considerably worse spatial resolution than that typically possible for $^{40}$Ar/$^{39}$Ar laser microprobe dating, it is possible to site the LA-ICPMS ablation pit within a few microns of the pit used for He extraction, or to simply re-occupy and enlarge the original ablation pit. The potential effective spatial resolution of the technique is thus on the order of a few tens to roughly 100 microns. As a proof-of-concept exercise, we have applied this technique to fluorapatite from Cerro de Mercado, Durango, Mexico, which has a generally accepted (U-Th)/He age of 32.1 $\pm$ 3.4 Ma (2 sigma) based on single-crystal fusion analyses reported by House et al. (2000, EPSL). Using the approach described above, we made 48 separate age measurements on a 12 mm polished section cut through a single crystal of Durango fluorapatite perpendicular to its c axis. The measured dates yield a mean of 34.9 $\pm$ 5.1 Ma (2 sigma), with a total dispersion of dates comparable to that reported by House et al. Much of the apparent age variation observed in both studies is due to documented U+Th heterogeneities in single crystals of the Durango fluorapatite. Nevertheless, the consistency of the laser ablation and conventional results for this material is striking. Compared to conventional laser and furnace methods of (U-Th)/He geochronology, the laser microprobe approach offers substantially improved spatial resolution, and the ability to avoid (or at least minimize) alpha-ejection corrections. In addition, the method affords improved sample throughput, such that age estimates for homogeneous materials can be made with considerably higher precision based on a larger number of analyses.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting