Experimental pathways of clay formation and kinetics of basalt alteration in poor drainage systems: Implications for weathering

Abstract

Basalt being the most dominant rock on the earth’s crust, contributes significantly to the global elemental cycle through weathering. In recent years, the potential of basalt weathering has been continuously scrutinized as a carbon dioxide removal (CDR) strategy. An accurate estimation of such large-scale processes requires a deeper insight into the mechanism controlling the basalt glass dissolution under field conditions. This contribution assesses the chemical evolution of fluid interacting with basalt glass in poorly drained regimes. Experiments showed a drop in kinetics of alteration (r0 = 1.7 × 10-9 mol.m−2. s−1) by two orders of magnitude in 24 days and emphasizes the onset of secondary mineral formation within 20 hr of the start of dissolution. At first, Mg starts fractionating from the solution due to brucite oversaturation and reaches undersaturation after 60th hour due to onset of other Mg-bearing minerals. From the 54th hour, montmorillonite remains oversaturated until Mg is entirely consumed by precipitation at the 164th hour. SEM-EDS investigation shows the presence of two major morphologies of secondary products: (a) honeycomb shape (smectite), with high Mg (>3 wt%) and octahedral composition similar [(Si/Al + Fe + Mg) and Al/Si] to smectite, (b) aggregate of ellipsoid and/or equant granular phases. Compositionally, elliptical and granular aggregates show affinity towards low Mg and high Fe variety of smectite amorphous precursor. The absence of pure brucite grains indicates epitaxial growth of Mg-rich, honeycomb-shaped phyllosilicate precursor on the brucite template due to well-reported structural similarity between the brucite layer and 2:1 phyllosilicate octahedral sheet. Elliptical and equant-shaped grains with or without compositional similarity with smectite phases have high Fe and low Mg, indicating their formation under a low Mg concentration stage in solution. Precipitation of the secondary phases at various stages of reaction progress affects the total reaction affinity in a closed system. Coupled dissolution and precipitation at the fluid-rock interface are responsible for lowering the kinetics of dissolution reactions in a closed system, previously explaining the slow kinetics of natural weathering regimes. The damped kinetics of dissolution and cations fractionation in secondary products within a few hours of onset of dissolution reaction can result in an overestimation (ten times) of CDR potential estimation by enhanced rock weathering (ERW) if calculations do not involve the nature of closed system evolution during basalt glass alteration.