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 m eta-gabbro terrain were studied for their stoichiometry and relative abunda nces. The site is located in the Piedmont Province in the southeast portion of the Appalachians orogenic belt in a forested residual landscape and a t emperate climate. The soil is classified as a fine mixed thermic Ultic Hapl udalf and defined by distinct A-, B- and C-horizons with thicknesses of 17, 62 and similar to 250 cm, respectively. The primary mineral assemblage con sists of 44% (wt.%) andesine, 40% hornblende, 6% quartz, 4% biotite, 3% ilm enite/titanomagntite/rutile/sphene grains and 3 % epidote. Quantitative XRD , detailed XRD clay mineralogy and thin-section petrography revealed the in cipient breakdown of the primary minerals to vermiculite (after biotite oxi dation), randomly ordered mixed-layer mica/vermiculite/smectite (as grain c oatings), kaolin group minerals (after mafic silicate dissolution), goethit e and hematite. Vermiculite (after biotite), expandable clay coatings and a ll the primary minerals (with exception to quartz) are dissolved away from the A-horizon. Hydroxy-interlayered-vermiculite and minor amounts of gibbsi te occur as tertiary phases in the A-horizon. Using mineral stoichiometries and average abundances for the parent rock and C-, B- and A-horizon, estim ates of primary mineral mass loss were determined. The bicarbonate to silic a ratio expected from the mineral loss reactions is 6 times higher than the ratio observed in average groundwater of the Georgia Piedmont. This sugges ts that, although weathering of mafic terrains is important to the long-ter m process of transferring CO2 to the oceans, the present day contribution o f silicate weathering in temperate climates may be more largely influenced by the weathering of felsic terrain. (C) 2000 Elsevier Science B.V. All rig hts reserved.