LONG-TERM AVERAGE MINERAL WEATHERING RATES FROM WATERSHED GEOCHEMICAL MASS BALANCE METHODS: USING MINERAL MODAL ABUNDANCES TO SOLVE MORE EQUATIONS IN MORE UNKNOWNS

The number of phases for which weathering rates can be determined by watershed geochemical mass balance methods is limited by the number of equations that can be constructed from elemental flux losses. Such techniques typically use flux losses for the major cations SiO₂, Al, Na, K, Mg, and Ca, thereby limiting the number of weathering rates that can be calculated uniquely to six.

For the Brubaker Run watershed located in the Piedmont Physiographic Province of Pennsylvania (USA), long-term chemical flux losses have been determined using beryllium 10-derived total denudation rates and zirconium-normalized total chemical concentrations from bedrock and soils. For Brubaker Run eight minerals are involved in weathering. The primary minerals are epidote, ankerite, garnet, muscovite, and chlorite, and the secondary products are “weathered muscovite”, kaolinite, and gibbsite. The weathering rates of these minerals have been calculated using the six major cations, and two trace cations. Despite having the eight equations needed, geochemically reasonable weathering rates (e.g., positive primary mineral rates that reflect destruction) could not be achieved regardless of the trace elements included. This is primarily attributable to the natural heterogeneity of the trace element concentrations within the host mineral, with trace element stoichiometries in some minerals varying by as much as an order of magnitude. Furthermore, the trace elements are hosted by a relatively small number of minerals, resulting in the weathering rates of other minerals becoming very sensitive to small variations in trace cation stoichiometry.

Epidote, garnet, and ankerite of the Brubaker Run watershed together host nearly all of the Ca in the bedrock, and completely dissolve at or near the weathering front. The mole-percentage of Ca hosted by epidote, garnet, and ankerite are 49 mol %, 4 mol %, and 43 mol %, respectively. From this information the weathering rates of these minerals can be determined by distributing to each Ca-rich mineral that fraction of the total watershed Ca loss flux for which it is responsible based on its mole-percent Ca in bedrock (the “flux distribution method”). The mass balance matrix then reduces to a dimension that permits for calculation of the remaining mineral weathering rates using only major elements.