Evaluating the Magnitudes of Weathering and Potassium Metasomatism in Paleosols: Examples from Proterozoic, Cambrian, and Cretaceous Paleosols in Midcontinental Laurentia

Six Proterozoic, two Cambrian, and two Cretaceous paleosols in the Lake Superior region of midcontinental Laurentia were investigated in detail. All but the Cretaceous paleosols experienced potassium metasomatism, which resulted in the precipitation of muscovite in Proterozoic paleosols or illite and microcline in Cambrian paleosols. A comparison of the magnitude of potassium metasomatism among the paleosols is provided by depth-normalized mass flux (DNMF), where DNMF=1000×[(mass flux)/(depth of weathering)], which normalizes for different thicknesses of weathering profiles. Average DNMF values for the total addition of K2O are 0.98 ± 0.19 mol cm−3 for the Proterozoic paleosols and 1.27 ± 0.06 mol cm−3 for the Cambrian paleosols. The ages of potassium metasomatism were determined by 40Ar/39Ar isotopic dating of metasomatic muscovite in the Proterozoic McGrath, Ville Marie, and Baraboo paleosols, which yielded ages of 1742 ± 3, 1589 ± 3, and 1467 ± 11 Ma, respectively, the former being coeval with the Yavapai orogeny and the latter with the Baraboo orogeny. Metasomatic microcline in the Cambrian Trempealeau paleosol yielded a plateau age of 488.0 ± 1.0 Ma, which corresponds to the age of the Cambrian-Ordovician boundary. SiO2, CaO, and Na2O were substantially removed from the paleosols by weathering, as was K2O (before metasomatism). The average total amount of SiO2, CaO, Na2O, and K2O removed was 17.6% ± 1.9% from six Proterozoic paleosols, 28.5% ± 4.2% from two Cambrian paleosols, 36.5% ± 8.6% from two Cretaceous paleosols, and 34.2% ± 2.7% from five modern soils. The greater magnitude of weathering in the Phanerozoic weathering profiles compared with the Proterozoic ones, despite lower levels of CO2 in the Phanerozoic atmosphere, may reflect the emergence of land plants at ∼500 Ma and their profound effects on weathering.