K-AR GEOCHRONOLOGY OF CLAYS SHOW AUTHIGENIC 1MD ILLITE FORMATION IN A GRANITE WASH SOIL, BENSON, AZ

K-Ar dating of pedogenic clays can constrain the age of paleosols and paleosurfaces. Illite age analysis (IAA) combines K-Ar geochronology and X-ray diffraction analysis of sorted clay size fractions to deconvolve the ages of two-component mixtures of high temperature 2M₁ illite and low temperature (< ~200 °C) 1M_d illite. IAA is commonly utilized to determine the timing of brittle fault movement and diagenesis in sedimentary rocks. Clay minerals also form through chemical weathering in soils, but there is uncertainty whether illite forms during this process. We use IAA of a modern soil on the late Pleistocene-age Granite Wash Member of the St. David Formation (Benson, AZ) to show that 1M_d illite in fact clearly forms during chemical weathering of granitic alluvium. The clay assemblage of the modern soil is dominated by smectite, but also contains 1M_d and 2M₁ illite as well as kaolinite. K-Ar ages decrease and 1M_d illite abundances (%) increase with smaller size fraction resulting in an endmember 1M_d illite age within uncertainty of 0 Ma. The modern age demonstrates that 1M_d illite is forming authigenically in this soil. This shows the potential promise in using IAA to date weathering environments and paleosols, as it proves that authigenic 1M_d illite forms in semi-arid soils.