Discordant K-Ar and young exposure dates for the Windjana sandstone, Kimberley, Gale Crater, Mars

• Published in: Journal of Geophysical Research: Planets
• Main Authors: Vasconcelos, P. M., Farley, K. A., Malespin, C. A., Mahaffy, P., Ming, D., McLennan, S. M., Hurowitz, J. A., Rice, M. S.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley-Blackwell Publishing 2016
• Subjects: 21Ne; 36Ar; 3He; K-Ar; Mars; 1104 Aquatic Science; 1107 Forestry; 1111 Soil Science; 1606 Physical and Theoretical Chemistry; 1901 Earth and Planetary Sciences (miscellaneous); 1902 Atmospheric Science; 1904 Earth-Surface Processes; 1906 Geochemistry and Petrology; 1908 Geophysics; 1910 Oceanography; 1911 Palaeontology; 1912 Space and Planetary Science; 2303 Ecology; 2312 Water Science and Technology; 2505 Materials Chemistry; 2507 Polymers and Plastics; 3104 Condensed Matter Physics; Yellowknife; Yellowknife Bay
• Online Access: https://espace.library.uq.edu.au/view/UQ:414027/UQ414027_OA.pdf; https://espace.library.uq.edu.au/view/UQ:414027

K-Ar and noble gas surface exposure age measurements were carried out on the Windjana sandstone, Kimberley region, Gale Crater, Mars, by using the Sample Analysis at Mars instrument on the Curiosity rover. The sandstone is unusually rich in sanidine, as determined by CheMin X-ray diffraction, contributing to the high KO concentration of 3.09 ± 0.20 wt % measured by Alpha-Particle X-ray Spectrometer analysis. A sandstone aliquot heated to ~915°C yielded a K-Ar age of 627 ± 50 Ma. Reheating this aliquot yielded no additional Ar. A second aliquot heated in the same way yielded a much higher K-Ar age of 1710 ± 110 Ma. These data suggest incomplete Ar extraction from a rock with a K-Ar age older than 1710 Ma. Incomplete extraction at ~900°C is not surprising for a rock with a large fraction of K carried by Ar-retentive K-feldspar. Likely, variability in the exact temperature achieved by the sample from run to run, uncertainties in sample mass estimation, and possible mineral fractionation during transport and storage prior to analysis may contribute to these discrepant data. Cosmic ray exposure ages from He and Ne in the two aliquots are minimum values given the possibility of incomplete extraction. However, the general similarity between the He (57 ± 49 and 18 ± 32 Ma, mean 30 Ma) and Ne (2 ± 32 and 83 ± 24 Ma, mean 54 Ma) exposure ages provides no evidence for underextraction. The implied erosion rate at the Kimberley location is similar to that reported at the nearby Yellowknife Bay outcrop.