December 8 – Licentiate Thesis Presentation by Fiona Thiessen

U-Pb SIMS ages of Ca-phosphates from lunar breccias and their implications for the impact history of the Apollo 17 landing site by Fiona Thiessen

Date and time: Tuesday, December 8 at 10.00
Place: De Geersalen, Geohuset (link to the house plan)

The background image is from the Lunar Reconnaissance Orbiter Camera (NASA/GSFC/Arizona State University)

The intensity and length of the impact bombardment throughout the early Solar System is still an unresolved key element in planetary sciences. The idea of the proposed Late Heavy Bombardment (LHB) is mainly based on radiometric ages of lunar samples, which revealed a cluster of ages at ~3.8-4.0 Ga. However, these ages might reflect a sample bias since the samples from the Apollo and Luna missions were collected at restricted areas on the near side of the Moon and are potentially dominated by ejecta excavated in only a few impact events. Moreover, the isotopic systems of the analyzed rocks are susceptible to resetting, and information about earlier impacts might have been erased by the last ~3.8 Ga impact event(s). In this study, different impact breccias from separate Apollo landing sites were analyzed in order to test the theory of the LHB. This endeavor will help to constrain the impact history of the Moon and provide a better understanding of the impact flux throughout the early Solar System. The methods used in this study include zircon and phosphate U-Pb geochronology, cathodoluminescence (CL) imaging, as well as geochemical and petrological investigation of the analyzed samples.
To date, the Ca-phosphate minerals apatite and merrillite within four different Apollo 17 breccias have been analyzed. The U-Pb Secondary Ion Mass Spectrometry (SIMS) ages, together with chemical and petrological observations, indicate that these breccias might have
formed during two or three impact events at 3920±3 Ma (2σ), 3922±5 Ma (2σ) and 3930±5 Ma (2σ). These findings are in contrast to previous studies which interpreted the breccias as originating from a single impact event; either Serenitatis or Imbrium. Based on ejecta distribution calculations for major impact events, the likely sources for these breccias are Nectaris, Crisium, Serenitatis and Imbrium. The link between the breccias and impact basins remains ambiguous, but the precise U-Pb dating constants of this study led to the identification at least two basin forming events within the narrow time interval of ~8 million years.


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