Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology
Charles W. Magee Jr.1,2, Martin Danišík3, and Terry Mernagh2,41Australian Scientific Instruments, 111/113 Gladstone St. Fyshwick ACT 2609 2Research School of Earth Sciences, Australian Nati onal University, Canberra ACT 2600 3GeoHistory Facility, John de Laeter Centre, TIGeR, Applied Geology, Curtin University, Perth, WA 6845, Australia 4Geoscience Australia, Symonston ACT 2609
Received: 02 Mar 2016 – Accepted for review: 05 Apr 2016 – Discussion started: 11 Apr 2016
Abstract. Isotopologue deficits of up to 200 ‰ below ideal mixing are observed in UO2+ species during SIMS gechronological analyses using the SHRIMP IIe instrument. These are identified by bombarding natural U-bearing minerals with an 18O2-primary beam. The large anomalies are associated with repeat analyses down a single SIMS sputtering crater (Compston et al., 1984), analysis of high-uranium, radiation damaged zircon, and analysis of baddeleyite. Analysis of zircon under routine conditions yield UO2+ isotopologue anomalies generally within a few percent of equilibrium. The conditions under which the isotopologue anomalies are observed are also conditions in which the UOx-based corrections, or calibration, for relative U vs. Pb ionization efficiencies fail. The existence of these isotopologue anomalies suggest that failure of the various UOx species to equilibrate with each other is the reason that none of them will successfully correct the U / Pb ratio. No simple isotopologue-based correction is apparent. However, isotopologue disequilibrium appears to be a more sensitive tool for detecting hi U calibration breakdowns than Raman spectroscopy, which showed sharper peaks for ~ 37 Ma high uranium 20 zircons than for reference zircons OG1 and Temora. U-Th-Sm/He ages were determined for aliquots of reference zircons OG1 (755 ± 71 Ma) and Temora (323 ± 43 Ma), suggesting that the broader Raman lines for the Temora reference zircons may be due to something other than accumulated radiation damage. Isotopologue abundances for UO+ and ThO+ and their energy spectra are consistent with most or all molecular species being 25 the product of atomic recombination when the primary beam impact energy is greater than 5.7 kV. This, in addition with the large UO2+ instrumentally generated isotopologue disequilibria, suggest any attempts to use SIMS to detect naturally occurring isotopologue deviations could be tricky.
Magee Jr., C. W., Danišík, M., and Mernagh, T.: Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology, Geosci. Instrum. Method. Data Syst. Discuss., doi:10.5194/gi-2016-7, in review, 2016.