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.