In situ U Pb chronology and chemistry of zirconolite in the andesitic meteorite Erg Chech 002

Abstract

Precise and accurate ages for 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. The zirconolite occurs as needle- and fiber-shaped, stubby crystals, with widths of 3 micrometers and lengths of up to 30 micrometers. Electron and ion microprobe analyses yielded concordant U Pb data with a weighted mean 207Pb 206Pb age of 4557.9 4.3 Ma 2sigma, 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.