Japan Geoscience Union Meeting 2018

Presentation information

[EE] Poster

S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Tectonophysics

[S-IT22] Interaction and Coevolution of the Core and Mantle in the Earth and Planets

Tue. May 22, 2018 10:45 AM - 12:15 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Tsuyoshi Iizuka(University of Tokyo), Hidetoshi Shibuya(Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University), Taku Tsuchiya(愛媛大学地球深部ダイナミクス研究センター, 共同), Kenji Ohta(Department of Earth and Planetary Sciences, Tokyo Institute of Technology)

[SIT22-P28] Zr isotope constraints on early Earth differentiation

*Tsuyoshi Iizuka1, Keita Itano1, Shinji Yamamoto2 (1.University of Tokyo, 2.Yokohama National University)

Keywords:Hadean, Nb-Zr isotopes, early Earth differentiation

The radionuclide 92Nb decays to 92Zr with a half-life of 37 Ma [1]. Because Nb and Zr can fractionate from each other during partial melting of the mantle and mineral crystallization as well as metal-silicate differentiation, Nb-Zr isotope systematics can potentially place chronological constraints on early planetary differentiation. Considering the initial abundance of 92Nb in the solar system (92Nb/93Nb = 1.7 × 10-5), yet, the possible Zr isotopic variation on the Earth would be highly restricted [2]. For instance, even a 4.53 Ga reservoir having a Nb/Zr ratio four times higher than that of CHUR is expected to display a 92Zr anomaly of only +10 ppm. Such reservoir with an elevated Nb/Zr ratio might be formed if Fe-Ti oxide and sulfide minerals presented as a liquidus phase during differentiation on the infant Earth.
Here we report our search for a Zr isotopic vestige of early Earth differentiation using ancient terrestrial zircons. Zirconium isotopic ratios in single zircon grains were measured using MC-ICP-MS with the desolvating nebulization technique, which allows us to achieve analytical precision of ±<10 ppm. So far, the high-precision Zr isotopic analysis has been applied to detrital zircons with ages up to 4.3 Ga from the Jack Hills in the Yilgarn Craton, Western Australia and igneous zircons from the 4.0–3.6 Ga orthogneisses in the Acasta Gneiss Complex, northwestern Canada. These Acasta rock samples were previously studied for zircon Lu–Hf isotope systematics and the results indicated that their magmatic sources contained Hadean crustal components [3]. Our Zr isotopic data for the Jack Hills and Acasta zircons display no resovable 92Zr anomalies at the level of analytical precision. By combining the Hf isotopic data for these zircon grain, we indicate the development of a 4.5 Ga enriched reservoir on the Earth that had a Lu/Hf of 0.07 and a Nb/Zr less than 0.4. Such reservoir could form via fractional crystallization processes within the shallow or upper mantle.
[1] Holden (1990) Pure Appl. Chem. 62, 941–958. [2] Iizuka et al. (2016) Earth Planet. Sci. Lett. 439, 172–181. [3] Iizuka et al. (2009) Chem. Geol. 259, 230–239.