| Young basaltic lavas on the Big Island of Hawaii frequently feature brightly colored surface coatings. These coatings, the product of interaction of volcanically-derived acidic fluids with basaltic substrates, provide an opportunity to study the rates and mechanisms of early onset chemical weathering in a natural setting. Lava flows of various ages, from hours to ∼40 years, at sites along Kilauea's southwest and east rift zones and at Mauna Loa were visited and sampled to determine the nature and extent of this alteration phenomenon. The coatings are composed of a layer of amorphous silica, 1–80 μm thick, capped in some cases by a 1 μm layer of Fe-Ti oxide. Raman, infrared, and 29Si nuclear magnetic resonance (NMR) spectra indicate that the coating is structurally identical to hydrous silica gel and contains unusually high structural Si-OH contents for a natural silica sample (5.4 wt% as H2O). Opaque coatings were observed on surfaces as young as one year old, and incipient siliceous alteration was observed to begin within days. Coating thickness varies with flow age, proximity to acid sources such as volcanic plumes emanating from Kilauea Caldera and Pu'u O'o, and climate. Coatings form preferentially on lavas with glassy surface layers, such as dense pahoehoe and spatter ramparts.;The silicon isotope compositions of silica coatings and basalts were measured to determine the degree of Si mobility during coating formation. Coatings are enriched in 30Si/28Si by 1–1.5 per mil relative to their basalt substrates. This fractionation is opposite in direction to that previously reported for opal precipitation, suggesting that aqueous Si speciation may strongly affect the sign and magnitude of the fractionation factor. Experiments in which fresh basalt glass was partially dissolved in dilute hydrochloric or hydrofluoric acid reproduced Hawaiian silica coating morphologies. Fluids collected from all experiments displayed δ 30Si fractionations, but the direction of fractionation depended on fluid chemistry.;The morphologic, spectroscopic and geochemical observations presented here are most consistent with a dissolution-reprecipitation mechanism for silica coating formation. Acidic solutions dissolved near-surface basalt, then precipitated in situ a portion of the dissolved Si as amorphous silica. |
