Mercer, C. M., Young, K. E., Weirich, J. R., Hodges, K. V., Jolliff, B. L., Wartho, Jo-Anne and van Soest, M. C. (2015) Refining lunar impact chronology through high spatial resolution 40Ar/39Ar dating of impact melts. Science Advances, 1 (1). e1400050. DOI 10.1126/sciadv.1400050.

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Abstract

Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope
geochronology of returned samples. However, interpreting the results of such studies can often be difficult
because the provenance region of any sample returned from the lunar surface may have experienced
multiple impact events over the course of billions of years of bombardment. We illustrate this problem with
new laser microprobe 40Ar/39Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a
straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document
multiple impact melt–forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably,
published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation
of multiple impact events through 40Ar/39Ar geochronology is likely not to have been possible using
standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe
technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations
that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin
impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations
from multiple samples to improve our current understanding of the lunar impact record, and to establish
the absolute ages of important impact structures encountered during future exploration missions in the
inner Solar System.

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