Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology

dc.contributor.authorMagee, Charles
dc.contributor.authorDanišík, Martin
dc.contributor.authorMernagh, Terrence
dc.date.accessioned2021-06-18T02:27:08Z
dc.date.available2021-06-18T02:27:08Z
dc.date.issued2017
dc.date.updated2020-11-23T10:32:20Z
dc.description.abstractThe 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 uraniumthorium- 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 O-18(2)- 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 high-U calibration breakdowns than Raman spectroscopy, which showed sharper peaks for similar to 37 Ma high-uranium zircons than for reference zircons OG1 and Temora. U-ThSm 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 UO2+ and ThO2+ 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.en_AU
dc.description.sponsorshipWe thank Trevor Ireland, Michael Wingate, and Yuri Kostitsyn for the loan of samples; Simon Bodorkos for help with the SQUID data reduction software, Patrick Burke for SHRIMP technical assistance; Chris May (TSW Analytical) for help with the solution ICP-MS work; and the management of Australian Scientific Instruments for allowing the publication of this in-house research. Martin Danišík was supported by the AuScope NCRIS2 programme, Australian Scientific Instruments Pty Ltd., Australian Research Council (ARC) Discovery funding scheme (DP160102427), and Curtin Research Fellowship. We thank Kenji Horie and Trevor Ireland for constructive reviewsen_AU
dc.format.mimetypeapplication/pdfen_AU
dc.identifier.issn2193-0856en_AU
dc.identifier.urihttp://hdl.handle.net/1885/237827
dc.language.isoen_AUen_AU
dc.provenanceThis work is distributed under the Creative Commons Attribution 3.0 License.en_AU
dc.publisherCopernicus GmbH on behalf of the European Geosciences Unionen_AU
dc.relationhttp://purl.org/au-research/grants/arc/DP160102427en_AU
dc.rights© 2017 The Authorsen_AU
dc.rights.licenseCreative Commons Attribution licenceen_AU
dc.rights.urihttp://creativecommons.org/licenses/by-nc-nd/3.0/en_AU
dc.sourceGeoscientific Instrumentation, Methods and Data Systemsen_AU
dc.source.urihttps://gi.copernicus.org/articles/6/523/2017/en_AU
dc.titleExtreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronologyen_AU
dc.typeJournal articleen_AU
dcterms.accessRightsOpen Accessen_AU
local.bibliographicCitation.issue2en_AU
local.bibliographicCitation.lastpage536en_AU
local.bibliographicCitation.startpage523en_AU
local.contributor.affiliationMagee, Charles, College of Science, ANUen_AU
local.contributor.affiliationDanišík, Martin, Curtin Universityen_AU
local.contributor.affiliationMernagh, Terry, College of Science, ANUen_AU
local.contributor.authoremailu9704596@anu.edu.auen_AU
local.contributor.authoruidMagee, Charles, u9704596en_AU
local.contributor.authoruidMernagh, Terry, u5645128en_AU
local.description.notesImported from ARIESen_AU
local.identifier.absfor040203 - Isotope Geochemistryen_AU
local.identifier.absseo970104 - Expanding Knowledge in the Earth Sciencesen_AU
local.identifier.ariespublicationu4485658xPUB1088en_AU
local.identifier.citationvolume6en_AU
local.identifier.doi10.5194/gi-6-523-2017en_AU
local.identifier.scopusID2-s2.0-85037721301
local.identifier.thomsonID000417338100001
local.identifier.uidSubmittedByu4485658en_AU
local.publisher.urlhttps://gi.copernicus.orgen_AU
local.type.statusPublished Versionen_AU

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