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Phosphate ages in Apollo 14 breccias: Resolving multiple impact events with high precision U–Pb SIMS analyses

Snape, J.F. ; Nemchin, A.A. ; Grange, M.L. ; Bellucci, J.J. ; Thiessen, F. ; Whitehouse, M.J.

Geochimica et Cosmochimica Acta, 02/2016, Vol.174, C, pp.13-29 [Periódico revisado por pares]

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  • Título:
    Phosphate ages in Apollo 14 breccias: Resolving multiple impact events with high precision U–Pb SIMS analyses
  • Autor: Snape, J.F. ; Nemchin, A.A. ; Grange, M.L. ; Bellucci, J.J. ; Thiessen, F. ; Whitehouse, M.J.
  • Assuntos: Mass Spectrometry – Analysis ; Apatites – Analysis ; Phosphate Minerals – Analysis ; Zirconium – Analysis ; Breccia – Analysis
  • É parte de: Geochimica et Cosmochimica Acta, 02/2016, Vol.174, C, pp.13-29
  • Descrição: To access, purchase, authenticate, or subscribe to the full-text of this article, please visit this link: http://dx.doi.org/10.1016/j.gca.2015.11.005 Byline: J.F. Snape [Joshua.snape@nrm.se] (a,*), A.A. Nemchin (a,b), M.L. Grange (b), J.J. Bellucci (a), F. Thiessen (a), M.J. Whitehouse (a) Abstract The U--Pb systems of apatite and merrillite grains within four separate Apollo 14 impact melt breccia samples were analysed by secondary ion mass spectrometry. No systematic difference was identified between the .sup.207Pb/.sup.206Pb ages of the apatites and merrillites. A combined .sup.207Pb/.sup.206Pb age of 3927 [plus or minus] 2 Ma (95% conf.) is determined for three of these samples (14305,103: 3926 [plus or minus] 4 Ma; 14306,150: 3926 [plus or minus] 6 Ma; 14314,13: 3929 [plus or minus] 4 Ma). By combining these data with the ages previously obtained for zircons in Apollo 12 impact melt breccia fragments and the lunar meteorite SaU 169, a weighted average age of 3926 [plus or minus] 2 Ma (95% conf.) is obtained, which is attributed to the formation of the Imbrium basin. An age of 3943 [plus or minus] 5 Ma is determined for the fourth breccia (14321,134), which is similar to ages of 3946 [plus or minus] 15 Ma and 3958 [plus or minus] 19 Ma, obtained from several older phosphates in 14305,103 and 14314,13. The weighted average of these three older ages is 3944 [plus or minus] 4 Ma (95% conf.). This is indistinguishable to the age (3938 [plus or minus] 4 Ma; 2[sigma]) obtained for a different Apollo 14 impact melt breccia in a previous study. After investigating likely sources for this older ~3940 Ma age, we conclude that the Humorum or Serenitatis basin forming events are likely candidates. The potential identification of two large impact events within ~15 Myrs has important implications for the rate of lunar bombardment around 3.95--3.92 Ga. This study demonstrates the importance of high-precision age determinations for interpreting the impact record of the Moon, as documented in lunar samples. Author Affiliation: (a) Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden (b) Department of Applied Geology, Curtin University, Perth, WA 6845, Australia * Corresponding author. Article History: Received 4 February 2015; Accepted 4 November 2015 (miscellaneous) Associate editor: Maud Boyet
  • Idioma: Inglês

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