A two-stage model for the formation of smectite from detrital volcanic glass under shallow-marine conditions

Abstract

A downcore increase in smectite content at the expense of decreasing amounts of detrital volcanic glass in a 5 m-long, mud-dominated, late Holocene core from the Firth of Thames, New Zealand, is consistent with the smectite being derived from glass by a two-stage reaction. The first stage appears to be a parabolic kinetics, diffusion-controlled dissolution reaction; the second, a first-order precipitation process. The rate constants, calculated from both the proportion of smectite and the proportion of glass, indicate a rate of transformation in this low-energy marine embayment that is about 2 orders of magnitude greater than the rate of formation of clay minerals from the subaerial weathering of glass in the adjacent catchment. The compositional requirements for the formation of the smectite occur in interstitial waters having slightly reduced marine salinity, due to mixing of river and seawater, and lower concentrations of Ca 2+ and Na + and higher concentrations of dissolved silica relative to those in ocean water.