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The 1.85 Ga paleosol at Flin Flon, Manitoba, Canada, was one of the first paleoweathering profiles taken as evidence for a dramatic rise in the oxygen content of the Paleoproterozoic atmosphere. Diagenesis and greenschist-facies metamorphism have modified the abundances of some major elements (e.g., K and Fe) in the Flin Flon paleosol. Inductively coupled plasma-mass spectrometry (ICP-MS) analyses reveal a positive Ce anomaly on both chondrite- and Amisk basalt-normalized REE patterns and marked Th/U fractionation in the uppermost maroon paleosol. The Ce anomaly is confirmed by the occurrence of cerianite and by the compositions of other REE- and U-rich accessory minerals (i.e., secondary monazite and uraninite) in the uppermost maroon paleosol. Th/U fractionation is supported by the presence of uraninite in both the paleosol and overlying Missi sedimentary rocks. Chemical ages of uraninite (1.85 to 1.0 Ga) suggest that this mineral might have formed during weathering or diagenesis but was susceptible to disturbance. Although igneous monazite and zircon are well preserved in even the uppermost maroon paleosol, primary fluorapatite exhibits variable degrees of weathering in the basal green paleosol and has been obliterated completely in the upper maroon paleosol.

american mineralogist

Cerium anomaly and Th/U fractionation in the 1.85 Ga Flin Flon Paleosol: Clues from REE- and U-rich accessory minerals and implications for paleoatmospheric reconstruction