BRIDGING THE GAP BETWEEN LABORATORY MEASUREMENTS AND FIELD ESTIMATIONS OF SILICATE WEATHERING USING SIMPLE CALCULATIONS

Abstract

Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher than those inferred from field studies. Simple calculations show that even if the field conditions were fully simulated in standard laboratory experiments, it would be impossible to measure the slow rates of mineral dissolution that are observed in the field. As it is not possible to measure the dissolution rates under typical field conditions, one should extrapolate the available data to the field conditions. To do this, a rate law for the dissolution of plagioclase in the field was formulated by combining the far from equilibrium dissolution rate of weathered natural oligoclase at 25°C with the effect of deviation from equilibrium on dissolution rate of fresh albite at 80°C. In contrast to the common view that laboratory experiments predict dissolution rates that are faster than those in the field, the simulation based on this rate law indicates that laboratory dissolution experiments actually predict slower rates than those observed in the field. This discrepancy is explained by the effect of precipitation of secondary minerals on the degree of saturation of the primary minerals and therefore on their dissolution rate. Indeed, adding kaolinite precipitation to the simulation significantly enhances the dissolution rate of the plagioclase. Moreover, a strong coupling between oligoclase dissolution and kaolinite precipitation was observed in the simulation. We suggest that such a coupling must exist in the field as well. Therefore, any attempt to predict the dissolution rate in the field requires knowledge of the rate of the secondary mineral precipitation.