Exploring the variability of argon loss in Apollo 17 impact melt rock 77135 using high-spatial resolution 40Ar/39Ar geochronology

Cameron M. Mercer, Kip Hodges, Bradley L. Jolliff, Matthijs Van Soest, Jo Anne Wartho, John R. Weirich

Research output: Contribution to journalArticle

Abstract

40Ar/39Ar incremental heating experiments on whole-rock lunar samples commonly provide evidence of varying degrees of radiogenic 40Ar (40Ar*) loss. However, these experiments provide limited information about whether or not 40Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP) 40Ar/39Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on 40Ar/39Ar dates. Potassium-rich glass and K-feldspar in the mesostasis are the dominant sources for Ar released during low-temperature steps of published 40Ar/39Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate- to high-temperature steps. Additionally, UVLAMP analysis of a mm-scale plagioclase clast demonstrates the potential to use stranded 40Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or 40Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon's evolution.

Original languageEnglish (US)
JournalMeteoritics and Planetary Science
DOIs
StateAccepted/In press - Jan 1 2019

Fingerprint

impact melts
geochronology
plagioclase
argon
spatial resolution
melt
rocks
ultraviolet lasers
clast
laser ablation
high resolution
lunar rocks
minerals
rock
impactors
heating
glass
electron probes
moon
dating

ASJC Scopus subject areas

  • Geophysics
  • Space and Planetary Science

Cite this

Exploring the variability of argon loss in Apollo 17 impact melt rock 77135 using high-spatial resolution 40Ar/39Ar geochronology. / Mercer, Cameron M.; Hodges, Kip; Jolliff, Bradley L.; Van Soest, Matthijs; Wartho, Jo Anne; Weirich, John R.

In: Meteoritics and Planetary Science, 01.01.2019.

Research output: Contribution to journalArticle

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