Silica coatings in the Ka'u Desert, Hawaii, a Mars analog terrain: A micromorphological, spectral, chemical, and isotopic study

High-silica materials have been observed on Mars, both from orbit by the CRISM spectrometer and in situ by the Spirit rover at Gusev Crater. These observations potentially imply a wet, geologically active Martian surface. To understand silica formation on Mars, it is useful to study analogous terrestrial silica deposits. We studied silica coatings that occur on the 1974 Kilauea flow in the Ka'u Desert, Hawaii. These coatings are typically composed of two layers: a similar to 10 mu m layer of amorphous silica, capped by a similar to 1 mu m layer of Fe-Ti oxide. The oxide coating is composed of similar to 100 nm spherules, suggesting formation by chemical deposition. Raman spectroscopy indicates altered silica glass as the dominant phase in the silica coating and anatase and rutile as dominant phases in the Fe-Ti coating; jarosite also occurs within the coatings. Oxygen isotopic contents of the coatings were determined by secondary ion mass spectrometry (Cameca 7f and NanoSIMS). The measured values, delta O-18(Fe-Ti) = 14.6 +/- 2.1%, and delta O-18(silica) = 12.1 +/- 2.2% (relative to SMOW), are enriched in O-18 relative to the basalt substrate. The observations presented are consistent with a residual formation mechanism for the silica coating. Acid-sulfate solutions leached away divalent and trivalent cations, leaving a silica-enriched layer behind. Micrometer-scale dissolution and reprecipitation may have also occurred within the coatings. Chemical similarities between the Hawaiian samples and the high-silica deposits at Gusev suggest that the Martian deposits are the product of extended periods of similar acid-sulfate leaching.