Quantifying secular variations in the chemical composition of the Martian crust provides unique insights into the processes that have guided the evolution of the Martian crust-mantle system. Using global abundances for a suite of elements determined by the Gamma Ray Spectrometer (GRS) on board the Mars Odyssey spacecraft and global mapping of apparent surface age adapted from existing geologic maps in the USGS Martian Geologic Investigation series, we report the average abundance of K, Th, Fe, Cl, H, and Si for the major Martian geologic epochs (Noachian, Hesperian, and Amazonian). Average GRS-determined K and Th abundances generally decrease by 9% and 7%, respectively, between the Hesperian and the Amazonian, possibly implying evolving magma chemistry throughout major resurfacing events (although the effects of surficial alteration processes cannot be entirely discounted). GRS-determined Fe and Cl averages increase by 12% and 19%, respectively, with younger apparent relative surface age, suggesting the possible mobilization and transport of these elements through aqueous processes (although an igneous origin for the variation in Fe also cannot be excluded). While H abundance does vary with surface age, the relationship is likely not governed by geologic processes. No statistically reliable apparent surface age relation was found for Si.