EXCEPTIONAL PRESERVATION OF SECONDARY MINERALS IN HAWAIIAN LAVA TUBES (Invited Presentation)

Basaltic lava tubes in dry climates provide an environment in which secondary mineral deposits can form as a result of biological and non-biological processes. In this study we report the presence, context, and identifications of secondary sulfate, oxide, carbonate, and amorphous silica phases in lava tubes from the southern (Kula Kai, in <2000 yr flows) and northern (1843 flow) slopes of Mauna Loa on Hawaii. Both cave systems lie within rain shadow areas with limited vegetation.

The environments within these lava tubes can be humid despite low average annual rainfall, though they lack regularly flowing water. The ceilings and walls preserve some mineral features associated with high-temperature conditions at and shortly after the time of formation of the caves, including thin crusts rich in specular hematite. Other secondary mineral deposits would have formed later at lower temperatures, and include in some cases cm-thick brittle coatings of gypsum or soluble Na-sulfate salts, dendritic features composed of amorphous silica, and soft “puffballs” of calcite. In the Kula Kai caves these coatings are fine grained, but gypsum crystals coating lava features in the 1843 lava flow caves are larger, in some cases forming blades several cm in length.

Cave minerals were analyzed by XRD. Thin, hard coats on sampled chunks of lava were removed and powdered using a dental drill; all other samples were powdered using an agate mortar and pestle with no liquids to avoid destroying soluble salts.

These mineral deposits likely formed by precipitation, as water vapor (potentially including some sea spray) interacted with the reactive substrate basalt. The sulfate minerals differ significantly from those formed by currently active fumaroles at nearby Kilauea, which is dominated by more acidic phases.

Soluble minerals can be preserved under these conditions, protected from direct interaction with rain and sunlight within these caves, which also regulate humidity conditions. Lava tube environments provide a unique context for the preservation of soluble and ephemeral mineral features in volcanic terrains, and could likely preserve such features on Mars. Where such delicate mineral features can be preserved, associated biosignatures also have a high preservation potential.