Low-temperature illitization of smectite in the late eocene and early oligocene of the Isle of Wight (Hampshire basin), U.K.

Abstract

Variegated palaeosols, which formed from weathering of clays, silts, and brackish to freshwater limestones, are present in the late Eocene-early Oligocene Solent Group of the Hampshire Basin, southern U.K. The detrital clay mineral suite is dominated by illite and illite-smectite with minor kaolinite and chlorite. In pedogenically modified (palaeosol) and evaporitic lacustrine clay-rich sediments, the proportion of illite in the illite-smectite is greater than in the non-pedogenically modified sediments, and where alteration is most intense, kaolinite and chlorite are absent. The smectite to illite transition has been investigated in the <0.5 μm fraction by XRD analysis (powder and oriented mounts), thermogravimetry (TG), analytical SEM, and chemical analysis of Fe2+. Modeling of XRD data reveals that the illite-smectite is a mixture of compositions (overall 60–95% illite), R 0, with high rotational stacking disorder. Dehydroxylation occurs mainly at 500 °C, but also at higher temperatures, indicating heterogeneous octahedral cation composition. Analytical SEM and chemical analysis of Fe2+ indicate that the illite to smectite transition occurs through Fe reduction in octahedral sites leading to increased layer charge, coupled with K fixation. The driving mechanism for what appears to be irreversible Fe3+ reduction is wetting (reducing) and drying (oxidizing) cycles in gley soil, in which reoxidation of reduced Fe is never complete.