The ⁴⁰Ar/³⁹Ar dating technique applied to planetary sciences and terrestrial impacts

Abstract

The ⁴⁰Ar/³⁹Ar technique is a powerful geochronological method derived from the K/Ar technique that can help to unravel the evolution of the solar system. The ⁴⁰Ar/³⁹Ar system can not only record the timing of volcanic and metamorphic processes on asteroids and planets, it finds domain of predilection in dating impact events throughout the solar system. The ⁴⁰Ar/³⁹Ar method is a robust analytical technique when the events to be dated are well understood and data are not over interpreted. The power of the ⁴⁰Ar/³⁹Ar technique resides in the ability to check the validity of age data internally by statistical means and multiple lines of evidence, and hence to evaluate when Ar age data are unreliable. Yet, too many ‘ages’ reported in the literature are still based on over-interpretation of perturbed age spectra. This review is by no means exhaustive and is centred on the most recent applications of the ⁴⁰Ar/³⁹Ar technique applied to planetary material, not the history of the planetary bodies themselves, or a historical review of the development of the argon dating technique. This paper presents selected case-study examples on terrestrial impact structures, the Moon and meteorites with ages recalculated using the latest decay constants. Currently, only 21 terrestrial impact structures are precisely and accurately dated, and the only proven age concordance is between the Chixculub impact and the Cretaceous/Paleogene mass extinction. ⁴⁰Ar/³⁹Ar dating of volcanic events on the Moon suggests that volcanism was concentrated between 3.8 and 3.1 Ga. The study of lunar volcanism would also benefit from dating of volcanic spherules for which only few data are available. Rigorous filtering of the ⁴⁰Ar/³⁹Ar age database of lunar melt breccias yielded concordant ages with high precision for two major basins of the Moon, but more precise age data would be needed to further test and validate the Late Heavy Bombardment (LHB) hypothesis. ⁴⁰Ar/³⁹Ar dating of lunar impact spherules shows an increase of ages <400 Ma suggesting a recent increase in the impact flux. The impact history of the LL parent body (bodies?) has yet to be well constrained but may mimic the LHB observed on the Moon, which would indicate that the LL parent body was quite large. Basaltic meteorites (HEDs) show an ⁴⁰Ar/³⁹Ar age range between 4.1 and 3.4 Ga, suggesting a diffuse LHB event; however, the spread of apparent ages may be a data-interpretation artefact, and the LHB parent body (bodies?) might have experienced a bombardment closer to the duration of the LHB age range than expected. Martian meteorites contain clues on Mars atmospheric and mantle argon compositions.

KEY WORDS:

• ⁴⁰Ar/³⁹Ar
• geochronology
• planetary sciences
• impact craters
• meteorites
• extraterrestrial geology

Acknowledgements

I thank M. Norman for inviting me to participate to this special volume and for his handling of the manuscript. A. Frew is thanked for analytical assistance with the significant amount of planetary materials measured at the argon–argon laboratory at Curtin. E. Thern is thanked for a critical reading of the manuscript. I thank constructive and critical reviews by A. Deutsch and V. Fernandes, although the latter reviewer's opinions on lunar geochronology certainly diverged from those expressed in this article.