APXS-Derived Compositional Characteristics of Vera Rubin Ridge and Murray Formation, Gale Crater, Mars: Geochemical Implications for the Origin of the Ridge

The resistant similar to 50 m thick Vera Rubin ridge (VRR) situated near the base of Mount Sharp, Gale crater, Mars, has been deemed a high priority science target for the Mars Science Laboratory mission. This is because of (1) its position at the base of the 5 km layered strata of Mount Sharp and (2) the detection of hematite from orbit, indicating that it could be the site of enhanced oxidation. The compositional data acquired by the Alpha Particle X-ray Spectrometer (APXS) during Curiosity's exploration of VRR help to elucidate questions pertaining to the formation of the ridge. APXS analyses indicate that VRR falls within the compositional range of underlying lacustrine mudstones, consistent with a continuation of that depositional environment and derivation from a similar provenance. Lower Fe concentrations for VRR compared to the underlying strata discounts the addition of large amounts of hematite to the strata, either as cement or as detrital input. Compositional trends are associated with VRR cross-cut stratigraphy, indicating postdepositional processes. Higher Si and Al and lower Ti, Fe, and Mn than the underlying mudstone, particularly within distinct patches of gray/blue bedrock, are consistent with the addition of Si and Al. Lateral and vertical compositional variations suggest enhanced element mobility and fluid flow (possibly via multiple events) through VRR, increasing toward the top of the ridge, consistent with the action of warm (similar to 50-100 degrees C), locally acidic saline fluids as inferred from the mineralogy of drilled samples. Plain Language Summary Curiosity has explored the resistant Vera Rubin ridge (VRR) at the base of Mount Sharp, Gale crater, Mars, owing to (1) its position within the 5 km layered rocks of Mount Sharp, which record changes in Mars environment through time, and (2) the detection of hematite from orbit. The Alpha Particle X-ray Spectrometer (APXS) measures the elemental composition of rocks. APXS analyses indicate that VRR has a similar composition to underlying mudstones, consistent with continued deposition in a lake. Lower iron discounts the addition of large amounts of hematite, holding together mineral grains either as cement or as detrital grains. Other elemental trends cut across layering, indicating postdepositional processes. Lateral and vertical compositional variations suggest enhanced element mobility and fluid flow (possibly via multiple events) through VRR, particularly at the top of the ridge and within gray/blue patches of bedrock, consistent with the action of warm (similar to 50-100 degrees C), acidic saline fluids inferred from the mineralogy of drilled samples.