SULFATE MINERALS, HEMATITE, AND SILICA IN BASALTIC CAVES AT CRATERS OF THE MOON NATIONAL MONUMENT, IDAHO: A POTENTIAL MARS ANALOG

The Holocene basaltic lavas of Craters of the Moon National Monument (CoM) are potentially the most Mars-like of all terrestrial basalts, at least for Fe₂O₃T content (17-19%). These lavas have caves that preserve abundant precipitated and alteration materials including sulfates, silica, and hematite that have also been observed on Mars at Gusev Crater and Meridiani Planum. A prior study (Peck, 1974) even identified jarosite in a CoM cave. In this study, samples of ceiling coatings, floor mounds, and fresh basalt were collected from four caves to examine the mineralogy and geochemistry of the deposits and their relationship to the Fe-rich basaltic substrate.

    Minerals were identified using powder XRD, and bulk geochemistry was determined by WD-XRF on fused samples. SEM was used to determine the textural relationship between the phases. Calcite dominated the white ceiling coatings while hematite and silica (along with remnant basaltic minerals) dominated the brown ones. Sulfate minerals were found only in the floor mounds, which were dominated by Na-sulfate, carbonate, and bicarbonate minerals including thenardite, burkeite, mirabilite, and trona. Fe sulfates were absent or very minor. The origin of these minerals relates to groundwater interaction with the basalts: the calcite is a precipitate, and the hematite/silica coatings likely formed by in-situ alteration of the basalts as groundwater seeped through. The mounds are likely biological in origin, either through microbial processes or the alteration of guano.

    The hematite and silica- bearing alteration materials can be compared to Mars, and could serve as a comparison for non-thermal groundwater related alteration of high-Fe basalt. Hematite appears to form at the expense of olivine, which is abundant in the fresh basalt but absent from the altered sample. Fe, Ca, and Al decrease with alteration, while Si, Na, and K increase. The Na (rather than Mg or Fe) sulfates make the CoM precipitates less like Mars. Differences in starting composition and environment likely cause this distinction: CoM basalts are more Na-rich, and Na sulfates are stable at higher pH (often associated with basaltic weathering on Earth). Thenardite is as soluble as some Mg sulfate minerals from Gusev, suggesting that water abundance alone cannot explain the difference.