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

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 K2O concentration of 3.090.20wt % measured by Alpha-Particle X-ray Spectrometer analysis. A sandstone aliquot heated to similar to 915 degrees C yielded a K-Ar age of 62750Ma. Reheating this aliquot yielded no additional Ar. A second aliquot heated in the same way yielded a much higher K-Ar age of 1710110Ma. These data suggest incomplete Ar extraction from a rock with a K-Ar age older than 1710Ma. Incomplete extraction at similar to 900 degrees 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-3 and Ne-21 in the two aliquots are minimum values given the possibility of incomplete extraction. However, the general similarity between the He-3 (5749 and 18 +/- 32Ma, mean 30Ma) and Ne-21 (2 +/- 32 and 83 +/- 24Ma, mean 54Ma) 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.