Steven M Chemtob1,2, George R. Rossman1, Edward D Young3, Karen Ziegler4, Fréderic Moynier2,5, John M. Eiler1, Joel Hurowitz6
Silicon isotopes are fractionated by a host of low-temperature aqueous processes, making them potentially useful as a weathering proxy. Here we characterize the silicon isotope signature of chemical weathering of young glassy basalts at Kilauea Volcano, Hawaii. Fresh basalts (<40 years old) frequently feature opaque amorphous silica surface coatings up to 80 µm thick. MC-ICP-MS measurements of these amorphous silica coatings and cements indicate they are 30Si-enriched (δ30SiNBS-28 = + 0.92 to +1.36‰) relative to their basaltic substrate (δ30Si = -0.18‰). This sense of fractionation is unusual, as clays and opals are typically 28Si-enriched relative to the dissolved reservoirs from which they precipitated. Mechanisms capable of producing 30Si-enriched secondary minerals were explored by conducting batch alteration experiments on fresh basaltic glass. Acidic alteration of basalt glass produced silica-rich surface layers resembling the Hawaiian surface coatings over a wide variety of experimental conditions and fluid chemistries (HCl, H2SO4, HF). Differences in fluid chemical composition affected the direction and magnitude of Si isotope fractionation. Basalt leaching in HCl or H2SO4 produced 30Si-enriched fluids (1000 ln aprecip-fluid @ -0.8‰). In contrast, HF-bearing experiments produced highly 28Si-enriched fluid compositions (1000 ln aprecip-fluid up to +8‰). Fractionations were larger with higher acid strength and at lower fluid-rock ratio. The experiments suggest that fluid chemistry and environmental conditions impact the Si isotope signature of chemical weathering. The d30Si values of the natural silica coatings support a formation mechanism in which silica was released from basalt during acidic leaching, then subsequently transported and evaporatively deposited on flow surfaces. The coating 30Si-enriched isotope compositions were achieved by Rayleigh fractionation.