A Novel Approach to Arid Soil Dating: Extraction and 40Ar/39Ar Geochronology of Pedogenic Sepiolite and Palygorskite.

In this study, we describe a promising new application of  ⁴⁰Ar*/³⁹Ar isotope geochronology to authigenic sepiolite and palygorskite in petrocalcic horizons of the ~5 Ma Mormon Mesa soil in southeastern Nevada, and the ~0.7-2 Ma La Mesa soil in southern New Mexico. We (1) test chemical methods to selectively extract sepiolite and palygorskite from indurated samples, and (2) offer preliminary ⁴⁰Ar*/³⁹Ar ages for these authigenic phyllosilicates.      

We extracted clay minerals from 13 soil samples for ⁴⁰Ar*/³⁹Ar analysis using successive treatments of sodium acetate to dissolve calcium carbonate, and tiron to remove amorphous silica. Aliquots were taken between treatments for ICP, XRD, and SEM analysis. ICP analyses of extracted clays indicate K weight percents of 0.18 to 0.99, which is sufficient for ⁴⁰Ar*/³⁹Ar analysis of 0.05 g vacuum-encapsulated samples. Our SEM-EDXS analyses strongly suggest that palygorskite and sepiolite dominate the clay-sized fraction, however, XRD data indicate that intergrades or inclusions of other minerals cannot be discounted and may account for some variability among the isotopic ages. The chemistry and mineralogy of bulk soil and clay-size mineral fractions reflect extensive, complex soil formation histories for both Mormon Mesa and La Mesa.

We conclude that ⁴⁰Ar*/³⁹Ar dating of authigenic sepiolite and palygorskite is a promising new way to determine petrocalcic soil ages. Further work is needed to constrain the range of applicability in terms of phyllosilicate composition (minimum K content), age, and chemical environment. Additional refinement of vacuum encapsulation methods may also be necessary to minimize analytical or background error. Successful dating of authigenic phyllosilicates would apply not only to extant petrocalcic soil landscapes, but also to those of the paleosol record, and will greatly improve our understanding of arid landscape processes globally.