Jump to main content
Jump to site search

Issue 10, 2019
Previous Article Next Article

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

Author affiliations

Abstract

Owing to the challengingly small amounts of uranium (U) they contain, the isotopic composition (238U/235U) 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 238U/235U 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 δ238U 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 δ238U 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 235U 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.

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

Back to tab navigation

Supplementary files

Article information


Submitted
15 Jun 2019
Accepted
16 Jul 2019
First published
26 Jul 2019

This article is Open Access

J. Anal. At. Spectrom., 2019,34, 2035-2052
Article type
Paper

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

F. L. H. Tissot, M. Ibanez-Mejia, P. Boehnke, N. Dauphas, D. McGee, T. L. Grove and T. M. Harrison, J. Anal. At. Spectrom., 2019, 34, 2035
DOI: 10.1039/C9JA00205G

This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Material from this article can be used in other publications provided that the correct acknowledgement is given with the reproduced material and it is not used for commercial purposes.

Reproduced material should be attributed as follows:

  • For reproduction of material from NJC:
    [Original citation] - Published by The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS) and the RSC.
  • For reproduction of material from PCCP:
    [Original citation] - Published by the PCCP Owner Societies.
  • For reproduction of material from PPS:
    [Original citation] - Published by The Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology, the European Photochemistry Association, and RSC.
  • For reproduction of material from all other RSC journals:
    [Original citation] - Published by The Royal Society of Chemistry.

Information about reproducing material from RSC articles with different licences is available on our Permission Requests page.


Social activity

Search articles by author

Spotlight

Advertisements