An (U-Th)/He age for the small Monturaqui impact structure,Chile |
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| Institution: | 1. School of Environment, The University of Auckland, Private Bag 92019, Auckland, 1142, New Zealand;2. Department of Earth & Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, USA;3. School of Earth and Space Sciences, PO Box 871404, Arizona State University, Tempe, AZ, 85287, USA;4. GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, 24148, Kiel, Germany;5. NASA Ames Research Center, Moffett Field, CA, 94035, USA;6. SETI Institute, Mountain View, CA, 94043, USA;7. Departmento de Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chile |
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| Abstract: | Single-crystal (U-Th)/He dating of 32 apatite and zircon crystals from an impact breccia yielded a weighted mean age of 663 ± 28 ka (n = 3; 4.2 % 2σ uncertainties) for the Monturaqui impact structure, Chile. This ~350 m diameter simple crater preserves a small volume of impactite consisting of polymict breccias that are dominated by reworked target rock clasts. The small size, young age and limited availability of melt material for traditional geochronological techniques made Monturaqui a good test to define the lower limits of the (U-Th)/He system to successfully date impact events. Numerical modeling of 4He loss in apatite and zircon crystals shows that, for even small craters such as Monturaqui, the short-lived compressional stage and shock metamorphic stage can account for the observed partial to full resetting of (U-Th)/He ages in accessory minerals. Despite the distinctly different 4He diffusion parameters of apatite and zircon, the 2σ-overlapping youngest ages are recorded in both populations of minerals, which supports the inference that the weighted mean of the youngest (U-Th)/He population is the age of formation of this impact structure. |
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