ELEMENT MOBILITY IN HAWAIIAN WEATHERING PROFILES: CONSTRAINTS FOR LOWER MT. SHARP IN GALE CRATER, MARS

Investigation of sedimentary rocks in Gale Crater, Mars, by the Alpha Particle X-ray Spectrometer (APXS) onboard the rover Curiosity has revealed evidence of weathering and alteration under diverse conditions. Within the lowermost units of Mt. Sharp (Sol 720+), silica-rich rocks (55-75 wt% SiO₂) occur as strata-bound deposits and as fracture-associated halos. With increasing silica, most oxides decrease (e.g., Al₂O₃, FeO, Na₂O). This is not due entirely to silica addition; TiO₂ increases and K₂O and P₂O₅ vary, but all 3 are apparently retained in altered rocks. Two preferred geologic models are: (1) silica retention during acidic alteration with open-system element mobilization, occurring either in situ or in the sediment source region, and (2) sedimentary mixing of a relatively unaltered, basaltic protolith with a weathered, more silicic protolith.

To evaluate these models, we conducted a field study at Maunakea and Kohala in Hawai’i, sampling fresh lava, weathering profiles, and sediment deposits (outwash gravels and glacial drift). Post-shield hawaiites and mugearites are similar in composition to the rocks of Gale Crater. Geochemical weathering trends in Hawai’i are dominated by circumneutral weathering of basalt, often with open system leaching of silica and mobile elements (Na, K, Ca, Mg) and formation of Fe-, Al-, Ti-oxide/hydroxides and minor clays. Retention of P and Zn is common, indicating a pH > 6. Acid sulfate alteration, which is localized near cinder cones, leads to immobile element enrichment (Si, Ti) and the further leaching of most cations, including less mobile Al, Fe. Notably, K and P are retained, which is a feature of alteration by acidic, high ionic strength fluids.

Our results, compiled with previous work on Hawaiian weathering [e.g., Morris et al., LPSC 2000], indicate that both models above are likely applicable to Lower Mt. Sharp units. The geochemical trends seen in Hawaiian acid sulfate alteration are similar to Mt. Sharp; circumneutral open system weathering is clearly not evident. K₂O is apparently an important tracer because it is retained during alteration under acidic conditions. We propose that elevated K₂O (~1 wt%) and K₂O/TiO₂ in the Murray fm., relative to other Lower Mt. Sharp rocks, point toward contributions of an alkali basaltic protolith.