Analytical capability of K-Ar isochron dating on Mars: assessment from mineral compositions of Martian meteorites
Many in situ potassium-argon (K-Ar) dating instruments under development use laser ablation to perform local analyses of several hundred um on rocks. Laser-induced breakdown spectroscopy (LIBS) and noble gas mass spectrometry (MS) are combined to achieve multiple spot analyses of the same rock to obtain K-Ar isochrons. The range and error of the data points on an isochron determine the accuracy and precision of dating. The range of the data on the isochron is governed by the relationship between the laser spot size and mineral size in the target rocks. A smaller laser spot size increases the range of the measured K concentration but decreases the amount of Ar extracted, which deteriorates the measurement accuracy. Because of this trade-off, the optimal laser spot size, which would give the best dating accuracy, would be somewhere in the middle. The mineral composition and spatial distribution in Martian rocks determine this optimum spot size. However, no extensive studies have been conducted to consider optimum laser spot size taking the mineral compositions of Martian rocks into account. Thus, it has been unknown how accurate the LIBS-MS method can be for in situ dating on Mars. In this study, we quantify the precision of dating Martian rocks by the LIBS-MS method and determine the instrumental conditions necessary for achieving the required precision. The dating precision was quantitatively evaluated by simulating isochrons that reflect the mineral composition of Martian rocks, which were obtained with electron probe microanalysis of three Martian meteorites. Our results indicate that a dating precision of 200 Myr could be achieved by reducing the laser spot size to 250 um and improving the measurement accuracy of K and Ar concentrations to 10%. We determined the instrumental conditions necessary to achieve the required dating precision for the LIBS-MS instrument currently developing.