Insights on formation mechanism of colorful silica coatings on Kilauean basalts from field observations and silicon isotopes

Many young basalts from Kilauea, on the big island of Hawai'i, feature visually striking white, yellow, orange and blue coatings. These coatings represent an opportunity to study the early onset of acid-sulfate weathering in volcanic environments. The coatings, first investigated on 1974 flows in the Ka'u Desert, consist of a 10-50 μm thick layer of amorphous silica, capped, in some cases, by a ~1 μm layer of Fe-Ti oxide. Both layers contain %-level enrichments of S, and jarosite is present as a minor phase. The coatings often display residual morphologies consistent with formation by leaching, but occasionally demonstrate depositional or accumulative morphology. Coated basalts of various ages (1969-2010) were collected from several sites along the SW and E rift zones of Kilauea to characterize variability in coating properties and the timing of coating formation. As early as one month after emplacement, some lava surfaces feature translucent, discontinuous 2-3 μm silica glazes, and lavas emplaced as recently as 2007 feature mature coating morphologies. Coatings tend to occur most prominently on smooth, low-vesicularity lavas, such as spatter and volcanic bombs. Rougher surfaces tend not to preserve coatings due to enhanced erosion. Older but recently exposed or broken surfaces away from active eruptions appear to regrow bright glazes with similar qualities, suggesting the importance of regional vog or acid rain in coating formation. Field observations illustrate the timing and complexity of silica coating growth, but do not address the coating formation mechanism and degree of Si mobility. Recent work, including in situ silicon isotope analysis via SIMS and detailed structural analysis via ²⁹Si NMR and Raman spectroscopy, strongly suggests that the coatings are depositional and that Si was mobile during coating formation. ²⁹Si NMR spectra indicate that the coating is structurally identical to amorphous silica gel and contains unusually high structural water content for a natural silica (5.4% H₂O). The spectra show no evidence for residual basaltic structure in the silica coatings, implying that Si must have undergone dissolution and re-precipitation. Si isotopes fractionate with the precipitation of clays and opaline silica, and have been employed as an indicator of weathering intensity. δ³⁰Si of the silica coatings was measured via ion probe (O^- primary beam) and normalized against the RSQ quartz standard. The silica coating was determined to have δ³⁰Si_NBS-28 = -1.8 ± 1.0% (2σ). Hawaiian basalts have δ³⁰Si = -0.5%, so the silica coating is ~1% lighter than its substrate. This fractionation implies that Si was mobile during coating formation. Basalt was dissolved in volcanically-derived acidic solution, then a fraction (tens of %) of aqueous Si precipitated as amorphous silica; the remaining dissolved Si was lost from the system. Most divalent and trivalent cations liberated by basalt dissolution remained soluble, but Fe³⁺ and Ti⁴⁺ became insoluble and precipitated on the surface as oxides.