THE ROLE OF TRACE CALCITE IN SOLUTE MASS BALANCES FROM WEATHERING OF PLUTONIC ROCKS

Garrels and Mackenzie (1967) first recognized the presence of excess solute calcium and alkalinity exports from silicate weathering environments in their classic mass balance paper on the origin of springs and lakes in the granitic rocks of Sierra Nevada Mountains. These authors also correctly attributed this phenomenon to the dissolution of small amounts of calcite. Since that time, the role of disseminated carbonate in plutonic rocks has become recognized as important in issues such as watershed responses to acid atmospheric deposition and long-term global CO2 buffering by weathering of silicate terrains such as in the Himalayas.

The present paper makes the case for the universal presence of accessory calcite based on a global suite of granitoid rocks in which calcite, determined by acid digestion and CO2, analyses, varied between 0.01 and 20 g kg-1. Calcite occurrences include small to large isolated anhedral grains, fracture and cavity fillings and sericitized cores of plagioclase. No correlation was found between calcite concentrations and bulk chemical compositions of the granitoid rocks including CaO. Petrologic literature commonly describes calcite in plutonic rocks as being “secondary” and originating from external sources such as hydrothermal alternation. However mineral associations, in addition to in situ 87Sr/86Sr ratios determined by ion microprobe, indicate that much of the calcite is of magmatic origin, produced by late stage cooling processes. Long-term experimental dissolution of granitic rocks (> 10 years) indicates that solute Ca and alkalinity are dominated initially by calcite dissolution in proportions related to the amount of calcite initially present. With time, calcite becomes progressively exhausted and weathering becomes dominated by slower silicate weathering. This same effect is observed for natural weathering, in which recently glaciated watersheds, such as observed by Garrels and Mackenzie for the Sierra Nevada, exhibit excess Ca and alkalinity while watersheds in geomorphically older terrains exhibit no such excesses even though the fresh granitoids contain disseminated calcite.