TY - JOUR
T1 - Phosphate ages in Apollo 14 breccias
T2 - Resolving multiple impact events with high precision U-Pb SIMS analyses
AU - Snape, J. F.
AU - Nemchin, A. A.
AU - Grange, M. L.
AU - Bellucci, J. J.
AU - Thiessen, F.
AU - Whitehouse, M. J.
N1 - Funding Information:
The authors thank the astronauts of Apollo 14 for risking their lives to collect the samples. Comments from Bradley Jolliff, Tsuyoshi Iizuka and one anonymous reviewer significantly improved the manuscript. This work was funded by grants from the Knut and Alice Wallenberg Foundation (2012.0097) and the Swedish Research Council (VR 621-2012-4370) to MJW and AAN. The NordSIMS facility is operated under a joint Nordic agreement; this is NordSIMS publication #428. The research has made use of NASA’s Astrophysics Data System.
Publisher Copyright:
© 2015 Elsevier Ltd.
PY - 2016/2/1
Y1 - 2016/2/1
N2 - 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 207Pb/206Pb ages of the apatites and merrillites. A combined 207Pb/206Pb age of 3927±2Ma (95% conf.) is determined for three of these samples (14305,103: 3926±4Ma; 14306,150: 3926±6Ma; 14314,13: 3929±4Ma). 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±2Ma (95% conf.) is obtained, which is attributed to the formation of the Imbrium basin. An age of 3943±5Ma is determined for the fourth breccia (14321,134), which is similar to ages of 3946±15Ma and 3958±19Ma, obtained from several older phosphates in 14305,103 and 14314,13. The weighted average of these three older ages is 3944±4Ma (95% conf.). This is indistinguishable to the age (3938±4Ma; 2σ) obtained for a different Apollo 14 impact melt breccia in a previous study. After investigating likely sources for this older ~3940Ma age, we conclude that the Humorum or Serenitatis basin forming events are likely candidates. The potential identification of two large impact events within ~15Myrs has important implications for the rate of lunar bombardment around 3.95-3.92Ga. This study demonstrates the importance of high-precision age determinations for interpreting the impact record of the Moon, as documented in lunar samples.
AB - 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 207Pb/206Pb ages of the apatites and merrillites. A combined 207Pb/206Pb age of 3927±2Ma (95% conf.) is determined for three of these samples (14305,103: 3926±4Ma; 14306,150: 3926±6Ma; 14314,13: 3929±4Ma). 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±2Ma (95% conf.) is obtained, which is attributed to the formation of the Imbrium basin. An age of 3943±5Ma is determined for the fourth breccia (14321,134), which is similar to ages of 3946±15Ma and 3958±19Ma, obtained from several older phosphates in 14305,103 and 14314,13. The weighted average of these three older ages is 3944±4Ma (95% conf.). This is indistinguishable to the age (3938±4Ma; 2σ) obtained for a different Apollo 14 impact melt breccia in a previous study. After investigating likely sources for this older ~3940Ma age, we conclude that the Humorum or Serenitatis basin forming events are likely candidates. The potential identification of two large impact events within ~15Myrs has important implications for the rate of lunar bombardment around 3.95-3.92Ga. This study demonstrates the importance of high-precision age determinations for interpreting the impact record of the Moon, as documented in lunar samples.
UR - http://www.scopus.com/inward/record.url?scp=84947785852&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2015.11.005
DO - 10.1016/j.gca.2015.11.005
M3 - Article
AN - SCOPUS:84947785852
SN - 0016-7037
VL - 174
SP - 13
EP - 29
JO - Geochimica Et Cosmochimica Acta
JF - Geochimica Et Cosmochimica Acta
ER -