Meta-gabbro weathering in the Georgia Piedmont, USA: implications for global silicate weathering rates

The mineral assemblages in a vertical weathering profile developed over a meta-gabbro terrain were studied for their stoichiometry and relative abundances. The site is located in the Piedmont Province in the southeast portion of the Appalachians orogenic belt in a forested residual landscape and a temperate climate. The soil is classified as a fine mixed thermic Ultic Hapludalf and defined by distinct A-, B- and C-horizons with thicknesses of 17, 62 and ∼ 250 cm, respectively. The primary mineral assemblage consists of 44% (wt.%) andesine, 40% hornblende, 6% quartz, 4% biotite, 3% ilmenite/titanomagntite/rutile/sphene grains and 3% epidote. Quantitative XRD, detailed XRD clay mineralogy and thin-section petrography revealed the incipient breakdown of the primary minerals to vermiculite (after biotite oxidation), randomly ordered mixed-layer mica/vermiculite/smectite (as grain coatings), kaolin group minerals (after mafic silicate dissolution), goethite and hematite. Vermiculite (after biotite), expandable clay coatings and all the primary minerals (with exception to quartz) are dissolved away from the A-horizon. Hydroxy-interlayered-vermiculite and minor amounts of gibbsite occur as tertiary phases in the A-horizon. Using mineral stoichiometries and average abundances for the parent rock and C-, B- and A-horizon, estimates of primary mineral mass loss were determined. The bicarbonate to silica ratio expected from the mineral loss reactions is 6 times higher than the ratio observed in average groundwater of the Georgia Piedmont. This suggests that, although weathering of mafic terrains is important to the long-term process of transferring CO2 to the oceans, the present day contribution of silicate weathering in temperate climates may be more largely influenced by the weathering of felsic terrain.