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Impact history of the Apollo 17 landing site revealed by U-Pb SIMS ages
Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
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Number of Authors: 52017 (English)In: Meteoritics and Planetary Science, ISSN 1086-9379, E-ISSN 1945-5100, Vol. 52, no 4, p. 584-611Article in journal (Refereed) Published
Abstract [en]

Secondary ion mass spectrometry (SIMS) U-Pb ages of Ca-phosphates from four texturally distinct breccia samples (72255, 76055, 76015, 76215) collected at the Apollo 17 landing site were obtained in an attempt to identify whether they represent a single or several impact event(s). The determined ages, combined with inferences from petrologic relationships, may indicate two or possibly three different impact events at 3920 +/- 3 Ma, 3922 +/- 5 Ma, and 3930 +/- 5 Ma (all errors 2 sigma). Searching for possible sources of the breccias by calculating the continuous ejecta radii of impact basins and large craters as well as their expected ejecta thicknesses, we conclude that Nectaris, Crisium, Serenitatis, and Imbrium are likely candidates. If the previous interpretation that the micropoikilitic breccias collected at the North Massif represent Serenitatis ejecta is correct, then the average Pb-207/Pb-206 age of 3930 +/- 5 Ma (2 sigma) dates the formation of the Serenitatis basin. The occurrence of zircon in the breccias sampled at the South Massif, which contain Ca-phosphates yielding an age of 3922 +/- 5 Ma (2 sigma), may indicate that the breccia originated from within the Procellarum KREEP terrane (PKT) and the Imbrium basin appears to be the only basin that could have sourced them. However, this interpretation implies that all basins suggested to fall stratigraphically between Serenitatis and Imbrium formed within a short (<11 Ma) time interval, highlighting serious contradictions between global stratigraphic constraints, sample interpretation, and chronological data. Alternatively, the slightly older age of the two micropoikilitic breccias may be a result of incomplete resetting of the U-Pb system preserved in some phosphate grains. Based on the currently available data set this possibility cannot be excluded.

Place, publisher, year, edition, pages
2017. Vol. 52, no 4, p. 584-611
National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-143609DOI: 10.1111/maps.12814ISI: 000400591600002OAI: oai:DiVA.org:su-143609DiVA, id: diva2:1103984
Available from: 2017-05-31 Created: 2017-05-31 Last updated: 2018-09-18Bibliographically approved
In thesis
1. The evolution of lunar breccias: U-Pb geochronology of Ca-phosphates and zircon using Secondary Ion Mass Spectrometry
Open this publication in new window or tab >>The evolution of lunar breccias: U-Pb geochronology of Ca-phosphates and zircon using Secondary Ion Mass Spectrometry
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Planetary bodies in our Solar System, including the Moon, were exposed to an intense asteroid bombardment between ~4.5-3.8 Ga, shaping their surfaces and leaving visible “footprints” in the form of large impact basins. The end of this period (~4.0-3.85 Ga), might have been marked by a cataclysmic increase in impacts, the so-called Late Heavy Bombardment (LHB), although this remains highly contentious. Since destructive processes, such as tectonics or erosion, have destroyed ancient (< 3.0 Ga) impact structures on Earth, studies of the early Solar System are mainly restricted to lunar samples, since impact structures are much better preserved on the Moon.

In this thesis, we have therefore analysed impact breccias from three Apollo landing sites (Apollo 12, 14, and 17) with the overall aim to gain a better understanding of the lunar impact history. This endeavour included comprehensive textural and petrological analyses of the breccias and grains of interest (i.e. Ca-phosphates and zircon), as well as obtaining precise U-Pb Secondary Ion Mass Spectrometry (SIMS) ages. The U-Pb ages of Ca-phosphates obtained are consistent with the age of the Imbrium impact at ~3925 Ma, whereas an older age of ~3930 Ma yielded by Ca-phosphates in an Apollo 17 breccia might be linked to the formation of the Serenitatis basin. Furthermore, an impact event at ~3940 Ma was identified in zircon grains in Apollo 14 breccias, which is in agreement with older Ca-phosphate ages yielded in a previous study. The identification of three possible impact events within ~15 myr has important implications for the lunar bombardment history.

However, there is a possibility that partial Pb loss from older grains during a relatively late event (e.g. Imbrium) might result in apparently older ages in Ca-phosphates. Incomplete resetting of the U-Pb system was recorded in zircon grains in an Apollo 12 breccia, leading to meaningless U-Pb ages which cannot be interpreted unambiguously as either magmatic or as impact events. Nevertheless, the U-Pb ages of several zircon grains occurring in lithic clasts in Apollo 14 breccias can plausibly be linked to magmatic activity, exhibiting several magmatic events between ~4286 Ma and ~4146 Ma. The data obtained in this thesis, together with previously published zircon and Ca-phosphate data, indicate several spikes in the magmatic and impact history during the first ~600 myr of lunar history. This study highlights the importance of combining high-precision age determination with thorough petrological and textural analyses in order to exclude meaningless ages and to interpret the impact and magmatic history of the Moon. 

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2018
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 374
Keywords
Moon, impact breccias, Apollo missions, impact event, Late Heavy Bombardment, zircon, Ca-phosphates, U-Pb, SIMS, geochronology
National Category
Geosciences, Multidisciplinary Geology Geochemistry
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-160018 (URN)978-91-7797-396-6 (ISBN)978-91-7797-397-3 (ISBN)
Public defence
2018-11-09, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2018-09-18 Created: 2018-09-18 Last updated: 2018-09-18Bibliographically approved

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