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Large-Scale Redistribution of Heterogeneous PKT Material by the Imbrium Impact - Evidence from Petrology and U-Pb Zircon Dating of a Complex Apollo 16 Breccia
| Content Provider | Semantic Scholar |
|---|---|
| Author | Vanderliek, D. M. Becker, Harry Rocholl, Alexander B. E. |
| Copyright Year | 2019 |
| Abstract | Introduction: The prevalence of isotopic ages at 3.9 Ga in lunar impactites from the Apollo landing sites led to the hypothesis of a spike in the impactor flux in the inner solar system at this time (lunar cataclysm, e.g. [1-5]). Ages >4.0 Ga were much less common in previous isotopic dating studies of Apollo and Luna samples. In contrast, Neukum and Ivanov [6] assumed an exponential decay of the accretion flux after formation of the terrestrial planets and predicted that many of the large lunar basins were formed prior to 4 Ga. Recent work has concluded that the interpretation of the isotopic ages of Apollo rocks sampled in the proximity of the “young” Imbrium basin may be biased because of resetting of chronometers during subsequent impact and an overall sampling bias of Apollo samples caused by the proximity of the young Imbrium basin (e.g. [7-9] and references therein). To address these issues, we present new UPb age data for zircons and petrologic information on the complex breccia 67915 and compare these data with previous U-Pb work on zircons that occur in lunar impactites from Apollo 15 and 16 landing sites. Materials and Methods: Sample 67915 (Apollo 16) has been collected from Outhouse Rock and interpreted to sample the Descartes Formation [10]. The block may have been excavated by a young impact that formed North Ray crater. The sample contains heterogenous clasts, including anorthositic norites, troctolite, ferroan anorthosites, granulites, SiO2-alkali feldspar enriched clasts and mineral fragments cemented by glass. Ar-Ar age data for clasts and matrix vary from 3.7 to 4.1 Ga [11-13]. To identify Zrphases and to map the mineralogy of thin sections, Quantitative Evaluation of Minerals by Scanning Electron Micros-copy (QEMSCAN) has been applied at the Institute of Mineralogy and Economic Geology at the Rheinisch-Westfälische Technische Hochschule Aachen. Back Scattered Electron (BSE) and Cathodoluminescence (CL) imaging of Zr-bearing phases have been per-formed on selected grains at the GFZ Potsdam to investigate their texture. U-Th-Pb concentrations and isotope ratios have been measured using the secondary ionization mass spectrometer (SIMS) CAMECA IMS 1280 HR at the GFZ Potsdam. Zircons and spot positions were selected considering zircon size, textures and petrogenetic context. The UPb concentration data was calibrated against the zircon reference material 91500 [14] using the empirical relationship between Pb/U vs. UO2/UO. Results: Zircons in sample 67915 are either located within rock clasts or occur as mineral clasts in the fragmented matrix. Zircons are commonly small with diameters of 5 – 10 μm, except for one large grain of about 50 μm in diameter. Zircon textures are blocky, granular and skeletal. No distinct features have been observed using CL. Few zircons contain baddeleyite inclusions. The SIMS data has been acquired in two sessions. In the first session, zircon grains in section 67915,84 have been measured (,84 n=7). In session two zircon grains of both sections (,84 n=1, ,76 n=8) were analyzed. Measurements of session one are all slightly reverse discordant yielding a concordia intercept age of 4230 ± 33 Ma. Despite two slightly discordant analyses, the second session yields a concordia age of 4209 ± 16 Ma, in accordance with the first session. 207Pb/206Pb ages (n=16) range from 4200 to 4241 Ma (Fig. 1) with typical in run uncertainties of 9 35 Ma (2 s. d.). |
| Starting Page | 2379 |
| Ending Page | 2379 |
| Page Count | 1 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.hou.usra.edu/meetings/lpsc2019/pdf/2379.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
