Quantitative geochemical mapping of martian elemental provinces

We present an exploratory approach to the interpretation of the elemental maps produced by the Odyssey Gamma-Ray Spectrometer (GRS). These maps benefit from a direct detection of elemental mass fractions and are used to delineate and characterize elementally homogeneous provinces in the mid-latitudinal martian surface on the basis of chemistry alone. This approach is different from assessing the elemental composition of regions previously defined by their geologic context. Multivariate statistical approaches are discussed and a combination of principal component and clustering analyses is applied on the GRS-based hydrogen, chlorine, potassium, silicon, iron, and calcium maps. At least three principal components must be considered to properly describe the compositional variability seen in the maps. The main component is likely driven by the degree of mantling through a GRS perspective, i.e. by materials enriched in mobile elements (Cl, H) and finer than 10-20 cm, at horizontal and depth scales of hundreds of kilometers and tens of centimeters, respectively. Elemental diversity is found in both mantled and less-mantled provinces, suggesting both local and regional sources for the surficial materials. The less-mantled regions appear to have compositions which include basaltic igneous rocks. Although there is an absence of obvious natural clusters in the data, a solution between five and eight elemental provinces seems optimal and is discussed (Amazonis-Tharsis and Sabaea-Arabia, Tempe and the southern highlands, Chryse and Utopia, Elysium-Tartarus, Acidalia-Arabia). Future investigation of the defined elemental provinces will involve integrating other types of data and geological information.