MINERALOGICAL MAGAZINE, VOLUME 62A Weathering of granite under an 02- and CO2-rich atmosphere

at ~2.45 Ga on an Archaean granite. Although they have been studied by several investigators (e.g. Gay and Grandstaff, 1980; Mossman and Farrow, 1992), debates have continued as to whether or not a major loss of Fe occurred uring the soil formation. Loss or retention of Fe during weathering is crucial in models for the evolution of atmospheric oxygen. The ambiguity of the data on the Pronto palaeosols was partly due to a possibility that some rock samples collected by previous investigators from the upper parts of the palaeosol section might represent foreign soils. Therefore, we have collected a suite of rock samples, including the parental granite, palaeosols, and overlying sedimentary rocks, at Pronto, and conducted a detailed mineralogical nd geochemica