CONSTANCY OF SILICATE MINERAL WEATHERING-RATE RATIOS BETWEEN NATURAL AND EXPERIMENTAL WEATHERING - IMPLICATIONS FOR HYDROLOGIC CONTROL OF DIFFERENCES IN ABSOLUTE RATES

Inverse models which apportion watershed effluxes (geochemical mass balance) over estimated mineral surface area give weathering rates for individual silicate minerals (e.g., feldspar) 1-3 orders of magnitude slower than laboratory rates. Comparisons of rates calculated from several recent watershed geochemical mass-balance studies with experimentally determined rates reveal that the ratio of mineral weathering rates for a given silicate-mineral pair determined in an internally consistent field study equals the ratio of rates for the same minerals in any internally consistent set of laboratory experimental data. Thus, the coefficient which corrects the weathering rate for any mineral from a laboratory data-set to a field setting is identical to the corresponding coefficient for any other mineral in the same data sets. The ''correction factor'' linking experimental and natural rates must be independent of composition. Physical (e.g., hydrologic) controls, rather than compositional or chemical controls, cause the difference between weathering rates in nature and the laboratory. Because flow in natural weathering profiles is spatially heterogeneous, not all of the potentially available surface in natural systems actually participates in reactions with pore fluids.