THE INFLUENCE OF AMPHIBOLITE WEATHERING ON CALCIUM BUDGETS OF THE LOCH VALE WATERSHED, ROCKY MOUNTAIN NATIONAL PARK, COLORADO

The chemical weathering of primary calc-silicate minerals is a natural mechanism by which atmospheric CO₂ is sequestered to carbonate rocks in the oceans over geologic timescales. However, determining the relative proportions of dissolved calcium in stream waters derived from silicate and carbonate minerals is often challenging. Previous studies of the Loch Vale Watershed in Colorado report that approximately three quarters of the calcium in the stream waters is attributable to calcite dissolution. The calcite is of both bedrock and atmospheric origin. The remaining stream calcium is reported to originate from the chemical weathering of sodic plagioclase (oligoclase). These calcium budget estimates do not include the preferential weathering of amphibolite schists which also host calcic plagioclase. The amphibolite schists are observed in the field to be dramatically recessed relative to the enclosing gneiss, and serve as conduits for ground water.

The relative proportion of stream calcium derived from the different calcium-bearing bedrock phases is a function of the modal abundance of each mineral lost during chemical weathering and its calcium stoichiometry. Field and petrographic observations provide evidence that the calcite found in the felsic lithologies, and the amphibole and calcic plagioclase of the amphibolite schists, completely dissolve during chemical weathering. Oligoclase of the felsic lithologies is observed to be chemically weathering and subsequently contributing calcium to stream waters. However, the oligoclase modal abundance lost during chemical weathering is difficult to constrain. If oligoclase dissolution was not occurring in the watershed, then the contribution of calcite to stream water calcium would be at a maximum of approximately 3%. In contrast, if all of the oligoclase was dissolving, then the contribution of calcite to stream water calcium would be at a minimum of approximately 1.3%. Therefore, we find that calcite contributes relatively little calcium to the Loch Vale stream waters, which differs from previous work. Nearly all of the calcium in stream waters is attributable to the weathering of the calc-silicate minerals amphibole, calcic plagioclase, and sodic plagioclase.