HR: 16:45h
AN: V22G-04    [PDF]
TI: Zonation-Dependent $\alpha$-ejection Correction by Laser Ablation ICP-MS Depth Profiling: Toward Improved Precision and Accuracy of (U-Th)/He Ages
AU: * Hourigan, J K
EM: jeremy.hourigan@yale.edu
AF: Dept. of Geology and Geophysics, Yale University P.O. Box 208109, New Haven, CT 06511 United States
AU: Reiners, P W
EM: peter.reiners@yale.edu
AF: Dept. of Geology and Geophysics, Yale University P.O. Box 208109, New Haven, CT 06511 United States
AU: Nicolescu, S
EM: stefan.nicolescu@yale.edu
AF: Dept. of Geology and Geophysics, Yale University P.O. Box 208109, New Haven, CT 06511 United States
AU: Plank, T
EM: tplank@bu.edu
AF: Department of Earth Sciences, 685 Commonwealth Avenue, Boston University, Boston, MA 02215 United States
AU: Kelley, K
EM: kelleyk@bu.edu
AF: Department of Earth Sciences, 685 Commonwealth Avenue, Boston University, Boston, MA 02215 United States
AB: (U-Th)/He dating of zircon and apatite constrains the timing and rates of shallow crustal exhumation by yielding ages corresponding to closure temperatures of about $180\deg$C and $70\deg$C, respectively. Relatively long $\alpha$ stopping distances in both minerals, however, typically require significant corrections for He ejected from dated crystals ($\alpha$-ejection correction). Good reproducibility of $\alpha$-ejection corrected ages from volcanic standards as well as predictable behavior of ages in intercalibration studies indicate that these corrections generally produce accurate and sufficiently precise corrected ages. However, the default $\alpha$-ejection correction assumes homogeneous distribution of U and Th parent concentrations in dated crystals, an assumption that is often violated to varying degrees. Recent work has shown that heterogeneous U-Th distribution in single zircon crystals can produce both systematic age biases and poor precision in some crystal populations when a standard homogeneous $\alpha$-ejection correction (HAC) is applied. We have developed a method for characterizing U-Th zonation in single crystals, prior to dating, by rim-to-core depth profiling by a 213 nm laser ablation ICP-MS. Compositionally homogeneous Sri Lankan zircon standards are used for standardization and to determine depth-dependent U/Zr and Th/Zr fractionation, which is insignificant relative to intragrain U-Th concentration variations. Time-resolved traces are converted to depth profiles using an empirically calibrated drilling rate of 0.6 $\mu$m/s. We calculate zonation-dependant $\alpha$-ejection correction (ZAC) factors by inputting fractionation-corrected U-Th profiles into an $\alpha$-ejection model of a spherical crystal with a surface-to-volume ratio equivalent to the analyzed grain, and assuming that compositional zonation has radial symmetry. Our data on zircons from a range of samples, including Fish Canyon tuff and Tardree rhyolite zircons, show that ZAC and HAC corrected ages can differ by up to 20%. Single-grain ZAC-corrected ages are always more accurate than HAC-corrected ages, typically by 5-10% relative to accepted ages. In some cases, even ZAC-corrected ages remain up to 5-15% different from accepted zircon ages. This suggests that crystal morphology and incomplete characterization of 3D zoning constitute significant sources of error. Ongoing developments include the use of multiple depth profiles to better characterize 3D zoning, and a new ZAC code with a more realistic bipyramidal prism morphology.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1094 Instruments and techniques
DE: 3620 Crystal chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting