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^(238)U/^(235)U measurement in single-zircon crystals: implications for the Hadean environment, magmatic differentiation and geochronology

Tissot, François L. H. and Ibanez-Mejia, Mauricio and Boehnke, Patrick and Dauphas, Nicolas and McGee, David and Grove, Timothy L. and Harrison, T. Mark (2019) ^(238)U/^(235)U measurement in single-zircon crystals: implications for the Hadean environment, magmatic differentiation and geochronology. Journal of Analytical Atomic Spectrometry, 34 (10). pp. 2035-2052. ISSN 0267-9477. doi:10.1039/c9ja00205g.

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Owing to the challengingly small amounts of uranium (U) they contain, the isotopic composition (^(238)U/^(235)U) of single zircon grains has never been measured. Leveraging methods we designed for analysis of small sample amounts and modern MC-ICPMS instruments, we show that precise (±0.04 to ±0.25‰) single-zircon ^(238)U/^(235)U measurements are now possible. We report data for 31 single grains from the Jack Hills conglomerate, and 3 reference zircon localities (FC-1, R33 and Temora). Consistent with the reducing conditions implied by the small Ce anomalies of many Hadean zircon, Jack Hills grains display only small δ238U variations (from −0.60 to −0.12‰). The distribution is centered on the average chondritic and bulk continental crust value, arguing against the widespread existence of Oklo-type reactors in the early Earth. The subtle δ^(238)U variations in Jack Hills zircons are more plausibly explained by a small (∼0.10%) mass-dependent equilibrium isotope fractionation between at least one U-bearing accessory mineral and silicate melts, during magmatic differentiation under reducing conditions. In contrast, the large δ^(238)U difference between pooled titanite and pooled zircon fractions from the Fish Canyon Tuff sample suggests larger isotope effects during igneous fractional crystallization under oxidizing conditions (∼QFM+2), with preferential removal of ^(235)U from the melt and into zircon, and/or other accessory phases. We estimate that ∼50% of zircon dated by the CA-ID-TIMS method would be amenable to single-grain U isotope measurements, making this method widely applicable to future studies. This would enable (i) improvements in precision and accuracy of U–Pb and Pb–Pb dates, (ii) accurate investigation of U-series disequilibrium contribution to U–Pb discordance, and (iii) accurate re-evaluation of U decay constants.

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URLURL TypeDescription Information
Tissot, François L. H.0000-0001-6622-2907
Ibanez-Mejia, Mauricio0000-0002-7839-2425
Dauphas, Nicolas0000-0002-1330-2038
Alternate Title:238U/235U measurement in single-zircon crystals: implications for the Hadean environment, magmatic differentiation and geochronology
Additional Information:© 2019 The Royal Society of Chemistry. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. The article was received on 15 Jun 2019, accepted on 16 Jul 2019 and first published on 26 Jul 2019. This work was supported by an NSF grant (EAR1824002) and Crosby Postdoctoral Fellowships (MIT) to FT and MI, a Chamberlin Postdoctoral Fellowship (University of Chicago) to PB, grants from NSF (PG EAR1444951 and CSEDI EAR1502591) and NASA (LARS NNX17AE86G, EW NNX17AE87G, and SSW NNX15AJ25G) to ND, an MIT Ferry Fund award and NSF award (EAR1439559) to DM, and a NASA (EW 80NSSC17K0773) grant to TG. Author contributions: FT, PB, MI and ND initiated the study; FT, PB and MI designed the research; TMH provided the Jack Hills zircons; FT and MI performed the research. FT performed the measurements on ND's and DM's instruments and analyzed the data (including modeling). FT, MI and PB interpreted the data. FT wrote the manuscript with contributions from all co-authors. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the ESI.† Additional data related to this paper may be requested from the authors.
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Massachusetts Institute of Technology (MIT)UNSPECIFIED
University of ChicagoUNSPECIFIED
Issue or Number:10
Record Number:CaltechAUTHORS:20190821-095956243
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:98071
Deposited By: Tony Diaz
Deposited On:21 Aug 2019 18:40
Last Modified:16 Nov 2021 17:36

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