Silica coatings on young Hawaiian basalts: Constraints on formation mechanism from silicon isotopes

Young basalts from Kilauea, on the big island of Hawai’i, frequently feature visually striking, white, orange and blue coatings, consisting of a 10-50 μm layer of amorphous silica, capped, in some cases, by a ~1 μm layer of Fe-Ti oxide [1]. The coatings provide an opportunity to study the early onset of acid-sulfate weathering, a process common to many volcanic environments. Silicon isotopes fractionate with the precipitation of clays and opaline silica, and have been demonstrated to be an indicator of weathering intensity [2,3]. Here we report in situ measurements of !30Si of the silica coatings and their implications for coating formation. The analyzed coated basalt was collected from 1997 overflow lavas at the rim of the Pu’u O’o cone. The sample was mounted in cross section. Analyses were conducted on a Cameca 7f-Geo ion microprobe with a Oprimary beam (~30 μm spot) and two Faraday cups. The silica coating was measured against the Rose Quartz standard (RSQ). We tested whether the coating’s amorphous structure introduced a matrix effect by analyzing quartz and fused glass of the same composition (Ge214). Measurements of those materials were the same within error, so although we have not ruled out other relevant matrix effects (e.g. water content), we accept RSQ as a viable standard for analyzing amorphous silica. The silica coating was determined to have !30Si = -1.8 ± 1.0‰ (2+). Hawaiian basalts have !30Si % -0.5‰ [2], so the silica coatings are ~1‰ lighter than the substrate. The sign of this fractionation is consistent with previously reported values for secondary silica [2-4], and implies that, although the silica coatings have a residual/leaching morphology [1], Si was mobile during coating formation. Basalt dissolved in acidic solution, then a fraction of aqueous Si precipitated as amorphous silica; the remaining dissolved Si (tens of percent) was lost from the system. Ongoing work includes confirmation of the SIMS analysis by ICP-MS and replication of observed coating morphologies and isotopic properties in flow-through alteration experiments.