Comparing open versus closed system weathering experiments using lithium isotopes

Chemical weathering of silicate rocks represents a critical part of the carbon cycle, and also a potential method for the artificial drawdown of atmospheric CO₂ (“enhanced weathering”). However, weathering and particularly the resulting secondary minerals (especially clays) have proven hard to measure and quantify. Here we use lithium isotopes to examine and compare two different types of laboratory weathering experiments, using the same initial water and rock, in order to determine their similarities and differences. Specifically, we compare “open system” experiments, where the reacting water drips through the rock powder, with “closed system” experiments, where the reacting water and rock are in contact in a closed beaker. The results (elemental ratios, mobility calculations, saturation indices) suggest that closed experiments, with their longer water-rock interactions times, exhibit more secondary minerals formation. This finding is confirmed by their higher “equilibrium” solution δ⁷Li values (δ⁷Li = 35.7 versus 29.5 ‰, respectively). The determination of their observed fractionation factors (α = 0.983 ± 0.004 and 0.977 ± 0.003 for the open and closed experiments, respectively) and partition coefficients between water and secondary minerals yields a test case, to determine whether the amount of clays that form during weathering can be calculated solely from Li isotope ratios, giving values similar to the amount of basalt dissolved. Based on potassium concentration (here the most mobile element), 1.9–3.2 g of rock was dissolved. Based on the Li isotopes, 1.6–2.0 g of clay was precipitated during the month-long experiment. This result agrees with other estimates from isotope systems, which suggest that clay formation is rapid, which in turn has consequences for weathering processes, and especially for the efficiency of enhanced weathering.