¹Jun Sakuma, ^2,3Hisashi Asanuma, ²Naoto Takahata, ^4,5Akira Yamaguchi, ¹Tsuyoshi Iizuka
American Mineralogist, 111, 1683–1692
Link to Article [https://msaweb.org/MSA/AmMin/TOC/2026/Abstracts/AM111P1683.pdf]
¹Department of Earth and Planetary Science, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
²Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-0882, Japan
³Graduate School of Human and Environmental Studies, Kyoto University, Sakyo, Kyoto 606-8501, Japan
⁴National Institute of Polar Research, Tokyo 190-8518, Japan
⁵Department of Polar Science, School of Multidisciplinary Science, SOKENDAI, Tokyo 190-8518, Japan

Copyright: The Mineralogical Society of America

Precise and accurate ages of asteroidal crusts are fundamental for reconstructing the timeline of magmatic, metamorphic, and impact events in the early Solar System. Zirconolite (CaZrTi2O7) is an accessory mineral found in a wide range of crustal rocks on both the Earth and Moon and has proven to be a potentially useful U-Pb chronometer. However, this mineral is rare in asteroidal meteorites, and its use for early Solar System chronology has been limited.

We present the in situ occurrence, U-Pb chronology, and chemistry of zirconolite in the andesitic meteorite Erg Chech 002, which represents a sample of the oldest known asteroidal crust. Zirconolite occurs as needle- and fiber-shaped and stubby crystals with widths of ∼3 μm and lengths up to ∼30 μm. Electron and ion microprobe analyses yielded concordant U-Pb data with a weighted mean 207Pb/206Pb age of 4557.9 ± 4.3 Ma (2σ), rendering it the Solar System’s oldest known zirconolite. Yet, this age is distinctly younger than reported high-precision 207Pb/206Pb ages varying from 4565.6 to 4566.2 Ma, which were obtained by acid leaching of pyroxene and whole-rock samples of the meteorite.

From its mineralogical and REE–(U Th)–(Nb Ta) characteristics, we argue that the zirconolite age represents the timing of a shock metamorphism of the parent asteroid’s crust. Our results suggest that 207Pb/206Pb dating for acid-leached samples can be affected by including even a tiny amount of metamorphic zirconolite, calling for caution in interpreting the high-precision 207Pb/206Pb age data. On the basis of thermodynamic and geochemical considerations, we infer further occurrences of zirconolite in alkali-silica-rich asteroidal rocks that rapidly cooled from high temperatures.