Single-grain 238U/235U measurements in Early Earth zircons: Implications for the Hadean environment, magmatic differentiation and geochronology

Hadean zircons are the only lithic witnesses of the first ~500 Myr of Earth's crust evolution. As these zircons are detrital, each grain has a potentially unique story to tell about its formation conditions. Here, we report the first high-precision U isotope data of single-zircon grains, using 31 Jack Hills grains and 3 reference zircon localities (FC-1, R33, Temora) [1]. This work is motivated by the possibility that Oklo-type natural reactors were more prevalent in the Hadean [2], that zircon ²³⁸U/²³⁵U could reflect the conditions, extent, and/or nature of crystallization [3], and how single-zircon ²³⁸U/²³⁵U variations may influence the precision and accuracy of high- precision U-Pb and Pb-Pb dating [4].

Single Hadean zircon show resolvable δ²³⁸U variations between -0.60 and -0.12 permil (‰). This range is centered on the chondritic and bulk continental crust value, arguing against (i) a widespread existence of Oklo-type reactors in the Hadean and (ii) any contribution of U from sources with excess ²³⁸U (e.g., mineralized ore sediments), consistent with the reducing conditions implied by the small Ce anomalies of many Hadean zircon. These variations are more plausibly explained by vibrational isotope effects than by nuclear field shift during magmatic differentiation, and suggest preferential removal of ²³⁵U from the melt into zircons, and/or other accessory phases, during fractional crystallization.

Contrary to previous claims [5-7], we show that single-zircon ²³⁸U/²³⁵U measurements can be performed with sufficient precisions (±0.04 to ±0.25 ‰) on the small sample loads relevant to high-precision U-Pb and Pb-Pb geochronological studies. This will make it possible to (i) improve both the precision and accuracy of U-Pb and Pb-Pb chronology, (ii) properly investigate the contributions of intermediate daughter product disequilibrium to U-Pb dates discordance, as well as (iii) re-evaluate the accuracy of U decay constants.

[1] Tissot et al. (2019) JAAS, accepted.

[2] Draganic et al. (1983) Prec. Res. 20, 283-298.

[3] Tissot et al. (2017) GCA 213, 593-617.

[4] Tissot & Dauphas (2015) GCA 167, 113-143

[5] J. Hiess et al. (2012) Science 335, 1610-1614.

[6] McLean et al. (2015) GCA 164, 481-501.

[7] Livermore et al. (2018) GCA 237, 171-183.