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Apollo 12 breccia 12013: Impact-induced partial Pb loss in zircon and its implications for lunar geochronology
Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
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Number of Authors: 52018 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 230, p. 94-111Article in journal (Refereed) Published
Abstract [en]

Apollo 12 breccia 12013 is composed of two portions, one grey in colour, the other black. The grey portion of the breccia consists mainly of felsite thought to have formed during a single crystallisation event, while the black part is characterized by presence of lithic fragments of noritic rocks and individual plagioclase crystals. In this study, U-Pb analyses of Ca-phosphate and zircon grains were conducted in both portions of the breccia. The zircon grains within the grey portion yielded a large range of ages (4154 +/- 7 to 4308 +/- 6 Ma, 2 sigma) and show decreasing U and Th concentrations within the younger grains. Moreover, some grains exhibit recrystallisation features and potentially formation of neoblasts. The latter process requires high temperatures above 1600-1700 degrees C leading to the decomposition of the primary zircon grain and subsequent formation of new zircon occurring as neoblasts. As a result of the high temperatures, the U-Pb system of the remaining original zircon grains was most likely open for Pb diffusion causing partial resetting and the observed range of (20)(7)pb/Pb-206 ages. The event that led to the Pb loss in zircon could potentially be dated by the U-Pb system in Ca-phosphates, which have a weighted average (207)pb/ Pb-206 age across both lithologies of 3924 +/- 3 Ma (95% conf.). This age is identical within error to the combined average (20)(7)pb/Pb-206 age of 3926 +/- 2 Ma that was previously obtained from Ca-phosphates within Apollo 14 breccias, zircon grains in Apollo 12 impact melt breccias, and the lunar meteorite SaU 169. This age was interpreted to date the Imbrium impact. The zircon grains located within the black portion of the breccia yielded a similar range of ages (4123 +/- 13 to 4328 +/- 14 Ma, 2 sigma) to those in the grey portion. Given the brecciated nature of this part of the sample, the interpretation of these ages as representing igneous crystallisation or resetting by impact events remains ambiguous since there is no direct link to their source rocks via textural relationships or crystal chemistry. Similarly, the currently available zircon data set for all lunar samples may be distorted by partial Pb loss, resulting in meaningless and misleading age distribution patterns. Therefore, it is crucial to fully understand and recognize the processes and conditions that may lead to partial resetting of the U-Pb system in zircon in order to better constrain the magmatic and impact history of the Moon.

Place, publisher, year, edition, pages
2018. Vol. 230, p. 94-111
Keywords [en]
Apollo 12, Impacts, Zircon, U-Pb dating, SIMS, Partial resetting
National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-156784DOI: 10.1016/j.gca.2018.03.023ISI: 000430999100006OAI: oai:DiVA.org:su-156784DiVA, id: diva2:1213038
Available from: 2018-06-04 Created: 2018-06-04 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, because 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. p. 46
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)
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Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

Available from: 2018-10-17 Created: 2018-09-18 Last updated: 2018-10-16Bibliographically approved

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