Jennifer M. Cole

Phosphor imaging as a tool for in situ mapping of ppm levels of uranium and thorium in rocks and minerals

Abstract Phosphor imaging is a type of digital autoradiography that has been widely used in biochemistry to examine radioactively tagged proteins. We used phosphor imaging to map in situ U and Th in polished slabs of geological materials including carbonates, phosphates, and silicate-rich rocks. We examined samples containing between 2 and >500 ppm U and ∼700 ppm Th to evaluate the applicability of the technique to geological samples. Resolution of 1 mm or better was obtained even for low concentration (∼10 ppm) samples. These analyses are routine and only require a light box, phosphor screen, and access to a phosphor imager. The technique is nondestructive, relatively inexpensive, and requires very little processing time. We used this technique to identify U- and Th-enriched carbonates and phosphates, and to find “hot spots” of U- and Th-rich minerals in a granodiorite. These high-resolution maps of U and Th allow us to effectively sample for geochronology and identify potentially interesting samples for synchrotron X-radiation studies. The maps produced by phosphor imaging also have great potential for investigating the details of adsorption of radionuclides to rocks and minerals in contaminated areas.

Using U-Pb ages of Miocene tufa for correlation in a terrestrial succession, Barstow Formation, California

Sedimentary rocks and associated fossils are the direct record of past life and its environmental context. Few methods allow direct and precise dating of this important archive, making it diffi cult to correlate climate, biologic, and tectonic events preserved in terrestrial records. We sampled lacustrine tufa calcite from the middle member of the Miocene Barstow Formation to further test the feasibility of uranium-lead dating of terrestrial carbonates. Five samples yield U-Pb ages (2σ) of 14.81 ± 0.39 Ma, 15.30 ± 0.25 Ma, 15.39 ± 0.15 Ma, 16.14 ± 0.40 Ma, and 16.24 ± 0.23 Ma. These precise ages are consistent with existing ages from K-Ar and 40 Ar/ 39 Ar dating of intercalated volcanic ashes. Our most precise U-Pb age on tufa has an uncertainty of <1% (150 ka), which is equivalent to uncertainties for K-Ar ages from the same time period. Our results indicate that U-Pb dating of carefully sampled and screened sedimentary carbonates provides much needed age constraints in purely terrestrial sequences and can be a powerful tool for correlation even in deposits with structural complexity and rapid facies changes.

Cyclic variations of uranium concentrations and oxygen isotopes in tufa from the middle Miocene Barstow Formation, Mojave Desert, California

Uranium concentrations of 42 to 169 ppm in fibrous calcite from spring-deposited tufa in the middle Miocene Barstow Formation, California, are among the highest reported for calcite. Fission-track maps of multiple bands of uniformly dull-luminescent fibrous calcite show that the concentration of U increases in the outward growth direction of the calcite of each individual band. Homogeneous dull luminescence in the fibrous calcite indicates no change in redox conditions of the fluid from which the calcite was precipitated. It is proposed that the cyclic pattern of increase in U concentration reflects a cyclical change in the U/Ca ratio in the fluid. Episodic mixing between Ca-rich spring water with a low U/Ca ratio and Ca-poor saline alkaline lake water with a high U/Ca ratio could produce the episodes of formation of fibrous calcite with increasing U concentrations. The spring water supplies the Ca for calcite precipitation, and the U concentrations increase with the decreasing fraction of spring water and increasing fraction of lake water. These cycles reflect a variable recharge of groundwater into the lake by springs. The large variation in U concentrations suggests mixing of 0% to 45% spring water with the lake, while the narrow range in δ 18 O of −6.37‰ to −6.87‰ limits the variation of the proportions of spring and lake water to 5%.