[S-GC36] 固体地球化学・惑星化学

[SGC36-01] 太陽系最古のジルコノライト

佐久間 盾¹、浅沼 尚²、高畑 直人²、*飯塚 毅¹ (1.東京大学理学系研究科、2.東京大学大気海洋研究所)

キーワード：ウランー鉛年代測定、安山岩質隕石、初期太陽系、エルグチェック002、副成分鉱物

Precise and accurate ages for asteroidal crusts are fundamental to reconstructing the timeline of magmatic and impact events in the early Solar System. Zirconolite (CaZrTi₂O₇) is an accessory mineral found in a wide range of crustal rocks on the Earth and Moon, and has proven to be an ideal U–Pb chronometer. Until now, however, it has not been found from asteroidal crustal rocks. The lack of asteroidal zirconolite may reflect the fact that the meteorite record of asteroidal crust is dominated by basaltic rocks depleted in alkali elements, in which zirconolite is hard to form. Here we report the occurrence, chemistry, and U–Pb chronology of zirconolite in the unique meteorite Erg Chech 002 that represents a sample of andesitic asteroidal crust. The zirconolite occurs as needle- and fiber-like crystals with lengths of ~30 µm and widths of ~3 µm. Electron probe microanalyses reveal that it has high contents of U, Th, REEs, and Nb, like terrestrial and lunar zirconolite. High-spatial resolution ion microprobe analyses give a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 4560 ± 8 Ma, making it the oldest zirconolite dated so far. Our results provide firm evidence for the contemporaneous emergence of andesitic and basaltic asteroidal crusts, and demonstrate that, given its occurrence in other alkali-undepleted samples, zirconolite can serve as a powerful chronometer for refining early Solar System chronology.