Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology

Abstract. The current limitation in the accuracy and precision of inter-element analysis in secondary ion mass spectrometry (SIMS) is the ability to find measurable quantities that allow relative differences in ionization and transmission efficiency of secondary ions to be normalized. In uranium–thorium–lead geochronology, the ability to make these corrections, or "calibrate" the data, results in an accuracy limit of approximately 1 %. This study looks at the ionization of uranium and thorium oxide species, which are traditionally used in U–Pb calibration, to explore the conditions under which isotopologues, or molecular species whose composition differs only in the isotopic composition of one or more atoms in the molecule, remain in or deviate from equilibrium.

Isotopologue deficits of up to 0.2 (200 ‰) below ideal mixing are observed in UO₂⁺ species during SIMS gechronological analyses using the SHRIMP IIe SIMS 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 UO_x-based corrections, or calibration, for relative U vs. Pb ionization efficiencies fail. The existence of these isotopologue anomalies suggest that failure of the various UO_x 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 high-U calibration breakdowns than Raman spectroscopy, which showed sharper peaks for  ∼  37 Ma high-uranium 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 the product of atomic recombination when the primary beam impact energy is greater than 5.7 keV. This, in addition to the large UO₂⁺ instrumentally generated isotopologue disequilibria, suggests that any attempts to use SIMS to detect naturally occurring isotopologue deviations could be tricky.