K-Ar dating of rocks on Mars: Requirements from Martian meteorite analyses and isochron modeling

Abstract

Radiometric age dating of Martian rocks and surfaces at known locations for which crater densities can be determined is highly desirable in order to fully understand Martian history. Performing K-Ar age dating of igneous rocks on Mars by robots, however, presents technical challenges. Some of these challenges can be defined by examining Ar-Ar data acquired on Martian meteorites, and others can be evaluated through numerical modeling of simulated K-Ar isochrons like those that would be acquired robotically on Martian rocks. Excess 40Ar is present in all shergottites. Thus for Martian rocks, the slopes of K-Ar isochrons must be determined to reasonable precision in order to calculate reliable ages. Model simulations of possible isochrons give an indication of some requirements in order to define a precise rock age: Issues addressed here are: how many K-Ar analyses should be made of rocks thought to have the same age; what range of K concentrations should these analyzed samples have; and what analytical uncertainty in K-Ar measurements is desirable. Meteorite data also are used to determine the D/a^2 diffusion parameters for Ar in plagioclase and pyroxene separates of several shergottites and nakhlites. These data indicate the required temperatures and times for heating similar Martian rocks in order to extract Ar. Quantitatively extracting radiogenic 40Ar could be difficult, and degassing cosmogenic Ar from mafic phases even more so. Considering all these factors, robotic K-Ar dating of Martian rocks may be achievable, but will be challenging.