APPLICATIONS OF PHOSPHOR IMAGING TO GEOLOGIC MATERIALS: DIAGENETIC EMPLACEMENT OF URANIUM/THORIUM IN MESOZOIC PALUSTRINE CARBONATES AND CALCRETES, COLORADO PLATEAU, USA

We have applied the technique of phosphor imaging to Mesozoic palustrine carbonates, calcretes, and lacustrine stromatolites of the Colorado Plateau, U.S.A. to evaluate its potential in ancient deposits and soils, and as a quick and precise method of determining isotopic concentrations of radiogenic elements within these samples. Phosphor imaging is a type of in situ imaging process that has recently been applied to geologic materials. This variation of autoradiography, which has been widely used in the biochemistry field, is relatively cost effective, quick, and trivial in its operation. Thus, it provides a new option for enhancing chronostratigraphic studies and aids in the ability to acquire high-resolution absolute age dates. Phosphor imaging also reinforces new radiogenic dating procedures and can serve as an option for calculating concentration of radiogenic elements and adsorbtion of radionuclides and microbes in geologic samples and core. The analysis of the images produced from these samples show a strong correlation for radiogenic elements being concentrated in early diagenetic mineral phases. Early silicification, reduction spots and tubiform structures provide the densest concentrations in most of our samples, suggesting mobilization of radiogenic elements by alkaline phreatic waters during an early diagenetic phase in both palustrine carbonate and pedogenic calcretes. This trend is observable over three different rock units, the Owl Rock and Petrified Forest Member of the Upper Triassic Chinle Formation, the Upper Jurassic Morrison Formation, and the Lower Cretaceous Cedar Mountain Formation. The implications for radiogenic element concentrations in diagenetic fabrics are a potential absence of contamination via detrital ²³²Th resulting in precise measurements of uranium/thorium ratios. Compared with whole-rock analysis, phosphor imaging allows accurate location of sources of radiogenic elements in rocks, and where possible to direct micro-sampling for uranium/thorium to mitigate errors produced through authigenic stable element contamination.