AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Trace Element Distribution in Dolostones from the Permian Basin, West Texas
(1) Earth Sciences, SUNY Oswego, Oswego, NY.
(2) Bureau of Economic Geology, Jackson School, University of Texas at Austin, Austin, TX.
(3) Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL.
(4) Department of Geosciences, SUNY Stony Brook, Stony Brook, NY.
Dolomites from the San Andres and Clearfork Formations show intervals of anomalous uranium enrichment on spectral gamma ray logs. To better understand the fluids that were responsible for dolomitization and as a way of comparing the two dolomites we have used multiple analytical tools to examine elemental distribution, particularly of redox-sensitive trace metals. Phosphor imaging of the slabs shows that the uranium is excluded from the associated anhydrite and that the uranium distribution in the dolomites is heterogeneous. Using these images as a guide for more detailed analyses we took thin sections to the microprobe at X26A at the National Synchrotron Light Source. Using new flyscan technology we were able to map several millimeter-sized areas with high spatial resolution. In both sample groups the microdistribution of U appears primarily to take the form of < 0.01 mm granules and clumps. In both sets of samples elevated Pb concentrations are generally correlated with those of Fe, but areas of U-enrichment correlate with neither. Certain distinctions exist between the San Andres and Clearfork samples, however. In the San Andres formation the overall texture of elemental distribution on sub-millimeter scales is patchy. Regions of elevated Fe + Pb concentration are larger (up to 0.1 mm) and more morphologically complex. Elemental distribution in the Clearfork formation is more uniform, lacking patchiness and with few trace element enriched grains of > 0.02 mm.
Four small (100-1000 mm3) sub-samples of one Clearfork core segment from 6473 feet depth were extracted from specific parts of one thin section tablet (i.e., from within several mm of each other), digested in nitric acid and analyzed by quadrupole ICP-MS. The samples are primarily dolomite with some traces of anhydrite, which was left as an insoluble residue. These sub-samples yield restricted variability in U concentration (~30 ppm) and elemental ratios such as U/Pb (~ 40), U/Th (~2). Concentration of U is modestly correlated with redox-sensitive V (R^2 = 0.87), whereas abundances of other first row transition metals (e.g., Mn and Zn) are largely invariant relative to U. Other redox-sensitive (e.g., Mo, W) and fluid-mobile (e.g., Li) trace elements show positive but non-linear correlation with U concentrations. Sub-samples have weakly positively-sloped shale-normalized REE patterns with prominent negative Ce anomalies and flat HREE distributions.