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The current limitation in the accuracy and precision of inter-element\udanalysis in secondary ion mass spectrometry (SIMS) is the ability to find measurable quantities that allow\udrelative differences in ionization and transmission efficiency of secondary\udions to be normalized. In uranium–thorium–lead geochronology, the\udability to make these corrections, or "calibrate" the data, results in an\udaccuracy limit of approximately 1 %. This study looks at the ionization\udof uranium and thorium oxide species, which are traditionally used in U–Pb\udcalibration, to explore the conditions under which isotopologues, or\udmolecular species whose composition differs only in the isotopic composition\udof one or more atoms in the molecule, remain in or deviate from equilibrium. \udIsotopologue deficits of up to 0.2 (200 ‰) below ideal mixing are\udobserved in UO2+ species during SIMS gechronological analyses using\udthe SHRIMP IIe SIMS instrument. These are identified by bombarding natural\udU-bearing minerals with an 18O2− primary beam. The large\udanomalies are associated with repeat analyses down a single SIMS sputtering\udcrater (Compston et al., 1984), analysis of high-uranium, radiation-damaged\udzircon, and analysis of baddeleyite. Analysis of zircon under routine\udconditions yield UO2+ isotopologue anomalies generally within a few\udpercent of equilibrium. The conditions under which the isotopologue anomalies\udare observed are also conditions in which the UOx-based corrections, or\udcalibration, for relative U vs. Pb ionization efficiencies fail. The\udexistence of these isotopologue anomalies suggest that failure of the various\udUOx species to equilibrate with each other is the reason that none of\udthem will successfully correct the U  / Pb ratio. No simple\udisotopologue-based correction is apparent. However, isotopologue\uddisequilibrium appears to be a more sensitive tool for detecting high-U\udcalibration breakdowns than Raman spectroscopy, which showed sharper peaks\udfor  ∼  37 Ma high-uranium zircons than for reference zircons OG1 and\udTemora. U–Th–Sm / He ages were determined for aliquots of reference\udzircons OG1 (755±71 Ma) and Temora (323±43 Ma), suggesting that\udthe broader Raman lines for the Temora reference zircons may be due to\udsomething other than accumulated radiation damage.\ud \udIsotopologue abundances for UO+ and ThO+ and their energy spectra\udare consistent with most or all molecular species being the product of atomic\udrecombination when the primary beam impact energy is greater than 5.7 keV.\udThis, in addition to the large UO2+ instrumentally generated\udisotopologue disequilibria, suggests that any attempts to use SIMS to detect\udnaturally occurring isotopologue deviations could be tricky

Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology