OPTICALLY STIMULATED LUMINESCENCE DATING PROPERTIES OF GYPSUM AND ANHYDRITE: POTENTIALLY IMPORTANT GEOCHRONOMETRIC MINERALS IN MARTIAN SEDIMENTS

Optically stimulated luminescence (OSL) dating, also known simply as optical dating, is a dosimetric dating technique commonly applied to terrestrial quartz and feldspars for determining the depositional ages for Quaternary sediments and the landforms they compose. Sedimentary depositional ages are important for understanding the paleoclimatic and geomorphologic history of features and processes present on the surface of Earth or Mars. Gypsum (calcium sulfate dihydrate) and anhydrite (anhydrous calcium sulfate) are two of the most stable and abundant sulfate species found on Earth, and have been discovered in Martian sediments by the Mars Exploration Rovers (MER) as wells as remote sensing methods (OMEGA). Quartz has not been found in abundance on the surface of Mars. For these reasons, it is important to examine the luminescence characteristics of calcium sulfates to determine whether sulfate minerals can be used as geochronometers and/or how their presence in Martian sediments could influence in-situ optical dating on Mars. In this study, adapted single aliquot regenerative dose (SAR) experimental procedures have been used to characterize the radiation dose response and signal stability of two natural gypsum samples formed in different environments as well as a synthetic gypsum and synthetic anhydrite sample. The adapted SAR method successfully corrected for sensitivity change in the samples. The radiation dose response for the calcium sulfates was monotonic in all cases with saturation doses ranging from ~700 Gy to ~6000 Gy. Short-term signal fading was observed ranging from ~5% to ~23% per decade. These results suggest that gypsum itself could be a candidate geochronometric material on Mars' surface and its OSL properties will need to be considered in developing protocols for in-situ optical dating experiments.