New Mexico Mineral Symposium — Abstracts

Palagonite tuff samples from two maar volcanoes in central New Mexico were examined in order to characterize the process of alteration of basaltic glass. This work is part of a larger study of altered glass as a possible analog to the soil of Mars.

Isleta volcano is a 2.8 million year old feature located just west of Isleta Pueblo. The Canjilon Hill complex, dated at 2.6 million years, is north of Bernalillo. In both areas phreatomagmatic explosions produced deposits of glassy basaltic ash mixed with olivine and palagonite crystals.

The glass is orange-brown sideromelane, the characteristic product of rapid quenching of basaltic liquid. Alteration to palagonite occurs exclusively in zones up to 30 microns thick at the surfaces of glass grains and on the walls of vesicles. Palagonitization generally involveS 10-20% hydration of the glass combined with selective leaching of elements and oxidation of iron from Fe²⁺ to Fe³⁺. Sodium, calcium, managanese and phosphorus are strongly depleted in the palagonite relative to the glass, while silicon, aluminum, iron and potassium remain relatively constant or display apparent enrichment. Titanium and magnesium are concentrated in sections of the palagonite rinds and depleted in other sections.

Material leached from basaltic glass during palagonitization may be lost to the system or may be locally reprecipitated. Zeolites, apparently chabazite and analcite, as well as calcite and amorphous silica, occur along with palagonite in some samples. These minerals are indicative of low temperature (below 80°C) alteration, possibly in a post-eruptive hydrothermal environment.

This research is supported by NASA grant NSG-7579 to Klaus Keil.