Phyllosilicate mineralogy, argon-40/argon-39 geochronology, and genesis of petrocalcic soils in the southwest U.S.A.

摘要

Obtaining isotope ages for arid soils and paleosols, including carbonate-cemented (calcic and petrocalcic) horizons, constitutes a significant challenge for soil scientists, geomorphologists, and stratigraphers. Carbonate-cemented soils foster diverse suites of authigenic minerals with exciting potential for geochronology, however, this potential has not been realized. This dissertation describes the timing of pedogenic palygorskite and sepiolite crystallization in extremely old, extant petrocalcic soils of the southwest U.S.A., and assumes that these fibrous, high-magnesium phyllosilicates contain sufficient lattice-bound K to warrant vacuum-encapsulated 40Ar/39Ar geochronology. This three-part study evaluated the (1) selective extraction, (2) 40Ar/39 analysis, and (3) pedogenesis of palygorskite and sepiolite.;Effects of selective extraction of the phyllosilicates from carbonate and silica cements using successive buffered sodium acetate and tiron solution treatments, and particle size fractionation via centrifuge, were evaluated using scanning electron microscopy, inductively-coupled plasma spectrometry, and x-ray diffraction analysis. No adverse effects were identified, however, intergrowths and/or inclusions of detrital mineral species persisted in the samples. Moreover, Ar analysis, and 40Ar/39Ar total gas ages exceeded known relative dates by two orders of magnitude. Gas chromatograph data indicate that hydrocarbon contaminants -- derived chiefly from selective dissolution compounds, but also possibly from the natural soil environment -- caused these impossible ages. Future work is needed both to constrain argon diffusion parameters for the clays, and also to evaluate methods to remove organic contaminants.;Extensive mineralogical, chemical, and micromorphological analysis of petrocalcic soil samples from Mormon Mesa, NV, permitted conceptual modeling of mineral genesis during pluvial-interpluvial climate oscillations. These models address not only the timing and genetic relationships of palygorskite, sepiolite, calcite, and silica, but also the presence of authigenic barite (BaSO4), in the petrocalcic soil.;Isotopic dates for petrocalcic soils are sorely needed to assess rates at which arid landforms and soils have formed, stabilized, and/or degraded both in the late Holocene, and in the geologic past. Thus, the potential benefit of successful 40Ar/39Ar geochronology of pedogenic palygorskite and sepiolite, and other arid soil minerals, strongly merits further experimentation and discussion. Extensive, detailed, and targeted micromorphological analysis is an inescapable prerequisite of any study linking the genesis and geochronology of calcic and petrocalcic soil landscapes.