THE ORIGIN AND ZONATION OF SUBLIMATES AND PRECIPITATES IN ACTIVE HAWAIIAN LAVA TUBES

The distribution of diverse sulfate, chloride, and fluoride minerals is spatially and temporally zoned around active lava tube systems in Hawaii. A zone of anhydrous Ca-Na-K sublimates is surrounded by an outer acid steam zone of hydrous Na-Mg-Fe-Al sulfates, Fe-chloride, and Ca-Na-Mg-Al fluorides. Gas released from lava traveling within tube systems reacts with the surrounding atmosphere, wallrock, and precipitation to produce both sublimates and precipitates. The highest temperature assemblages (measured by thermocouple) consist of subhedral anhydrite, glauberite, and magnesioferrite that form at temperatures in excess of 1000°C within incandescent tubes. This is the highest temperature occurrence of magmatic vapor phase anhydrite documented to date. Copper-bearing Na₂SO₄ (I) and KNaSO₄ coat rocks at 700-800°C, immediately outside the incandescent zone. The lower temperature polymorphs of these minerals, thenardite and aphthitalite, form in cooler (<300°C) near surface fumaroles. Anhydrite, along with vesicular molten sulfur, also forms between 100 and 400°C. Surface and near surface fumaroles produce a suite of Mg-Fe-Al-Na hydrous acid sulfate minerals including copiapite, halotrichite, and tamarugite with hydrous Mg-sulfates. Although not previously reported from tholeiitic shield volcanoes, locally dominant fluorine minerals, such as meniaylovite and heklaite, are also present on surface exposures. Cooling lava flows surrounding the active tubes are suffused by acidic steam that produces an assemblage of gypsum, ralstonite, opal, and Fe-chloride and hydroxides. Draining of the lava tube is accompanied by inward collapse of the alteration zones and produces precipitates dominated by hydrous Na-Mg-K sulfates including thenardite, mirabilite, blödite, aphthitalite, picromerite, and Mg-sulfates between 30 and 60°C. As the tubes cool to ambient temperature, these highly soluble sulfates are removed by meteoric water encroachment and gypsum becomes the dominant mineral.