HR: 09:15h
AN: V31G-06    [Abstracts]
TI: Inside CA-TIMS Zircon Analysis: the Interplay Among Natural Radiation Damage, Annealing, Solubility, and U-Pb Isotopic Systematics
AU: * Mattinson, J M
EM: mattinson@geol.ucsb.edu
AF: Department of Earth Science, University of California, Santa Barbara, CA 93106-9630, United States
AU: Nasdala, L
EM: luta.nasdala@univie.ac.at
AF: Institut für Mineralogie und Kristallographie, Universität Wien Althanstr. 14, A-1090, Wien, A-1090, Austria
AU: Lengauer, C
EM: christian.lengauer@univie.ac.at
AF: Institut für Mineralogie und Kristallographie, Universität Wien Althanstr. 14, A-1090, Wien, A-1090, Austria
AU: Wirth, R
EM: wirth@gfz-potsdam.de
AF: GeoForschungsZentrum, Telegrafenberg, D-14473, Potsdam, D-14473, Germany
AB: The CA-TIMS method of U-Pb zircon analysis (Mattinson, 2005) has demonstrated remarkable effectiveness at isolation of closed-system, concordant zircon by selective removal of zircon domains that have lost Pb. However, our understanding of exactly how CA-TIMS works has been far from complete. Here we report analysis of a series of Sri Lankan zircon crystals with a range of U+Th concentrations. Natural radiation damage (D-alpha, units = 10E18 alpha-decays/g) in these samples ranges from ca. 0.9 (lightly damaged) to ca. 10.6 (totally metamict). The zircon grains are unzoned, greatly simplifying interpretation of the relationships among radiation damage, annealing, solubility, and U-Pb systematics. We made Raman and X-ray powder diffraction measurements before and after CA-TIMS annealing treatment (1,000 °C, 48 hrs). A subset of the annealed zircon samples was then dissolved in a series of up to 24 partial dissolution steps at progressively increasing temperatures from 80-215 °C; all using 50% HF for 12 hrs, and with complete U-Pb analysis of each step. Annealing at 1,000 °C has resulted in significant but still incomplete recovery of the radiation damage. Raman spectra indicate ca. 70% reconstitution of the short-range order, based on FWHM measurements. XRD indicates ca. 85% recovery of the long-range order based on unit cell dimensions. This is consistent with transmission electron microscope results; the remaining amorphous volume fraction appeared insignificant whereas there is still notable disorder of the (dominating) crystalline zircon fraction. For completely metamict zircon, there was no recovery of zircon structure, only a mixture of amorphous material and nano-scale ZrO2. Dissolution behavior correlates closely to original (and thus, residual) radiation damage. The originally completely metamict zircon completely dissolved at 80 °C. A D-alpha = 6 zircon was completely dissolved after the 140 °C step. In contrast, after the 160 °C step, a D-alpha = 4.7 zircon was only 17% dissolved, and D-alpha = 3 – 1.66 zircon samples were only 1.5 – 3% dissolved. Thus, the highly selective removal of badly damaged zircon (most likely to have been affected by Pb loss) by early CA-TIMS step(s) evidently is primarily due to the proportional amount of post-annealing residual radiation damage. In terms of isotopic systematics, specifically CA-TIMS plateau behavior, we observe a transition between original D-alpha = ca. 6 and above (badly damaged to totally metamict) and D-alpha = ca. 4.7 and below. The former do not yield plateau results for 206Pb/238U ages, but the latter do. The transition is close to a post-annealing damage level equal to the "first percolation point" where amorphous domains become interconnected.
DE: 1040 Radiogenic isotope geochemistry
DE: 1094 Instruments and techniques
DE: 1115 Radioisotope geochronology
DE: 1194 Instruments and techniques
DE: 3620 Mineral and crystal chemistry (1042)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting