The evolution of a heterogeneous Martian mantle: Clues from K, P, Ti, Cr, and Ni variations in Gusev basalts and shergottite meteorites

Martian basalts represent samples of the interior of the planet, and their composition reflects their source at the time of extraction as well as later igneous processes that affected them. To better understand the composition and evolution of Mars, we compare whole rock compositions of basaltic shergottitic meteorites and basaltic lavas examined by the Spirit Mars Exploration Rover in Gusev Crater. Concentrations range from K-poor (as low as 0.02 wt.% K2O) in the shergottites to K-rich (up to 1.2 wt.% K2O) in basalts from the Columbia Hills (CH) of Gusev Crater; the Adirondack basalts from the Gusev Plains have more intermediate concentrations of K2O (0.16 wt.% to below detection limit). The compositional dataset for the Gusev basalts is more limited than for the shergottites, but it includes the minor elements K, P, Ti, Cr, and Ni, whose behavior during mantle melting varies from very incompatible (prefers melt) to very compatible (remains in the residuum). The range in partitioning behavior of these elements provides leverage on interpreting igneous processes. Models are presented that demonstrate how concentrations of these elements in Gusev basaltic magmas would change by simple igneous processes (fractional crystallization, crustal contamination, and mantle partial melting). The Gusev basalts may be related by two-stage batch melting of a primitive (K-rich) mantle source to first, generate the K-rich, Columbia Hills basalts and second, the lower K Adirondack basalts, leaving behind a K-poor residuum. The mantle source for the Gusev basalts is separate from the more depleted (K-poorer) source region of the shergottites. This indicates that primitive, K-rich mantle persisted until the Early Hesperian during formation of the Columbia Hills basalts. We suggest that separate mantle reservoirs developed by inhomogeneous partial melting (at melt fractions greater than 0.05) to form the Martian crust in the first 1 Ga of the planet's history.