title: Refining lunar impact chronology through high spatial resolution (40)Ar/(39)Ar dating of impact melts.
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 (40)Ar/(39)Ar 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 (40)Ar/(39)Ar 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.
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Cameron M
authors.initials: CM
authors.lastname: Mercer
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Kelsey E
authors.initials: KE
authors.lastname: Young
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: John R
authors.initials: JR
authors.lastname: Weirich
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Kip V
authors.initials: KV
authors.lastname: Hodges
authors.affiliation: Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA.
authors.forename: Bradley L
authors.initials: BL
authors.lastname: Jolliff
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Jo-Anne
authors.initials: JA
authors.lastname: Wartho
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Matthijs C
authors.initials: MC
authors.lastname: van Soest