¹Hikaru Hyuga,¹Yuichiro Cho,¹Yayoi N. Miura,¹Takashi Mikouchi,^1,2Seiji Sugita
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70022]
¹Department of Earth and Planetary Science, University of Tokyo, Bunkyo, Japan
²Planetary Exploration Research Center, Chiba Institute of Technology, Narashino, Japan
Published by arrangement with John Wiley & Sons

Dating rocks with a 2σ precision of 200 Ma is required to understand the history of Martian habitability and volcanic activity since ~4000 Ma. In situ K-Ar dating using a spot-by-spot laser ablation technique has been developed for isochron dating on Mars. The precision of isochron ages is determined mainly by the relationship between the laser spot diameter and the grain size of the sample. However, the achievable precision of age estimates using a realistic mineralogy of Martian rocks has yet to be investigated. We simulated isochrons under various conditions, including different laser spot sizes, K and Ar measurement errors, and numbers of analyses based on the mineral abundances of representative Martian meteorites (NWA 817, Zagami, and NWA 1068) analyzed using an electron probe microanalyzer. We found that attaining a precision of 200 Ma necessitates an isochron data range, defined as the ratio of the maximum to minimum K concentrations, of >6, a laser spot diameter of 250 μm, and measurement errors of <10% for both K and Ar. Reducing the laser spot size and selecting a sample with a large grain size are effective in obtaining a large K range. Furthermore, minimizing the variance in measurement errors between K and Ar is essential to increase the accuracy of the age estimates. We demonstrate that the precision required for in situ dating on Mars is achievable with realistic instrument settings, thus demonstrating the feasibility of establishing an in situ K-Ar geochronology for Mars.

¹Mohammad Tauseef,¹Ingo Leya,²Beda Hofmann
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70029]
¹Space Research and Planetary Sciences, Physics Institute, University of Bern, Bern, Switzerland
²Natural History Museum Bern, Bern, Switzerland
Published by arrangement with John Wiley & Sons

We present isotope concentrations of the light noble gases He and Ne for samples from five well-documented strewnfields and two individual meteorites from the Omani desert. Cosmogenic (22Ne/21Ne)cos for the strewnfield samples are low, as expected considering the total known masses. A (22Ne/21Ne)cos of 1.210 for the LL6 chondrite RaS 267 from Oman indicates a small pre-atmospheric size of less than 10 cm. The CRE ages for the Omani meteorites calculated using 21Necos range from 1 to 20 Ma. Using the (22Ne/21Ne)cos and previously established correlations, new shielding-corrected 14C and 14C-10Be terrestrial ages are calculated. For the strewnfield samples, the new ages are similar to the earlier ages but are more consistent. The new terrestrial age for RaS 267 is more than 20% lower than the previous age. Motivated by this success, we reinvestigated meteorites from other hot deserts (Acfer, Adrar, and Nullarbor regions) and Antarctica using literature data for 14C and (22Ne/21Ne)cos, along with the newly established correlations between 14C production rates and (22Ne/21Ne)cos. For these meteorites, the new terrestrial ages are systematically younger than the ages calculated earlier using a shielding-independent approach. Using shielding-corrected 14C terrestrial ages, the long-term puzzling problem that there is a lack of meteorites with short terrestrial ages disappears. The new histogram, though with only a limited number of data, shows the expected decrease in the number of meteorites with increasing terrestrial age. Therefore, the unexpected shape in the terrestrial age histogram was most likely due to a bias in the 14C dating system, that is, ages of small meteorites are overestimated.