Multichronometer studies of terrestrial impact sites inform best practices for extraterrestrial studies
Abstract
The impact of asteroids, comets, and meteorites is among the most ubiquitous surface processes in the inner Solar System, and the tempo of impact through time will be a high-value research target for future missions of exploration. However, such research will require the analysis of carefully collected samples and the application of high-precision isotope geochronology. Unfortunately, robust dating of impact-produced rocks (impactites) is challenging. Most contain a mixture of pre-existing and newly formed material, and some isotopic systematics can be disturbed easily during subsequent thermal events. It is critical to develop strategies for optimal sample selection and precise analysis of small sample masses through terrestrial analog studies.
Applying a battery of chronometers - U/Pb, 40Ar/39Ar, and (U-Th)/He - and multiple analytical protocols to a diverse suite of impactites, we are conducting such a study of the West Clearwater impact structure at Lake Wiyâshâkimî, Quebec, Canada. Our results thus far suggest that efforts to select the "best" samples based on traditional field criteria alone (e.g., the avoidance of samples that appear relatively weathered) are only partially successful. The isotopic systematics of pre-impact clasts of individual minerals (xenocrysts) and rocks (xenoliths) are variably reset in West Clearwater impactites, implying that systematic study of such relicts can help increase confidence in dates obtained for melt materials. U/Pb analysis of impact-related zircons by thermal ionization mass spectrometry (TIMS) yielded the most precise age currently available for this impact structure - 286.55 ± 0.10 Ma - but such zircons are not found in most samples. We found that screening large numbers of zircons through comparably low-precision laser ablation U-Pb analyses helped identify zircons that were most amenable to high-precision TIMS work. All other methods have yielded highly dispersed results so far, but the combination of low-precision and high-precision chronometric data obtained through a variety of techniques can improve overall confidence in age assignments for impact craters. Unfortunately, not all impactites are amenable to the application of multiple isotopic chronometers, and thus an important criterion for sample selection is mineralogical diversity.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.P31H3803B
- Keywords:
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- 5464 Remote sensing;
- PLANETARY SCIENCES: SOLID SURFACE PLANETSDE: 5470 Surface materials and properties;
- PLANETARY SCIENCES: SOLID SURFACE PLANETSDE: 5494 Instruments and techniques;
- PLANETARY SCIENCES: SOLID SURFACE PLANETSDE: 5499 General or miscellaneous;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS