SILICA COATINGS on YOUNG BASALTS AT KILAUEA, HAWAI'I: RAPID ONSET OF ACID-SULFATE WEATHERING

Young basalts from Kilauea, on Hawai’i, frequently feature visually striking white, yellow and orange coatings. These coatings provide an opportunity to study the early onset of acid-sulfate weathering, a process common to many active volcanic environments. Previous work on basalts collected in the Ka’u Desert from the 1974 flow along Kilauea’s SW rift zone revealed coatings that consist of a ~10 µm layer of amorphous silica, capped by a ~1 µm layer of Fe-Ti oxides. The Fe-Ti oxide layer is composed primarily of anatase and rutile and displays depositional morphology. The silica coating displays a texture most consistent with formation by leaching and produces a Raman spectrum similar to that of altered basaltic glass. Coating δ¹⁸O was determined by ion probe methods; both layers are enriched in ¹⁸O relative to the basalt substrate, indicating formation in a disequilibrium environment with extensive evaporative fractionation.

Recent field work has involved collection of basalts of various ages (1969-2010) to observe the onset of coating formation and their evolution through time. Bright coatings similar to those from the Ka’u Desert were observed on lavas emplaced as recently as 2007. Spatter ramparts from 1997 at Napau Crater feature silica and Fe-Ti-rich coatings with a similar morphology to the 1974 flows. Spatter vents from 2007 near Pu’u O’o display pale silica coatings with identical Raman spectra to the 1974 silica, but no apparent Fe-Ti coating. Coatings were not commonly observed on the July 2010 flows from the TEB vent, but mineralization and acid bleaching occurs immediately adjacent to fumarolic vents along the active lava tubes. At all field sites, coatings tend to occur most prominently on smooth, low-vesicularity lavas (spatter, volcanic bombs, dense pahoehoe breakouts). Old but recently exposed surfaces appear to regrow bright glazes with similar qualities. These observations suggest that silica coatings form on a ~1 year timescale. Acid-sulfate solutions leach away basaltic cations, leaving behind a residual silica layer. Fe³⁺ and Ti⁴⁺ liberated by leaching become insoluble and precipitate on the surface as oxides. Alteration continues after local volcanism has ceased, suggesting the influence of regional vog or acid rain, but proximity to fumaroles may accelerate the alteration process.