Compositional and morphological analyses of basin rim units in Argyre Planitia are consistent with local olivine-rich materials that become relatively olivine-poor in the process of being transformed from rocky materials to finer particulate regolith. Where high-thermal-inertia rocky surfaces are observed, the composition is more mafic than the surrounding fines. This relationship between rocks and soils appears to be common on Mars as a similar pattern is observed in other locations, such as Gusev Crater, Isidis Planitia, and Nili Fossae. Despite the lack of local detectable alteration products, these trends may be an indication that most Martian dark regions are not similar in composition to the primary igneous composition from which they are derived. The Martian crust may be significantly more mafic, and alteration of these surfaces may be more pervasive than has been inferred from the bulk surface mineralogy derived from orbital observations.