SYNCHROTRON METHODS FOR INVESTIGATION OF SEDIMENTARY DIAGENETIC PROCESSES

The behavior of uranium in sedimentary systems during deposition and diagenesis is the product of a number of physical and chemical processes: fluid flow, diffusion, solution and surface complexation, adsorption, precipitation, co-precipitation, and, of course, redox reactions. It is difficult to understand such a complex system using macroscopic measurements, and chemical techniques that involve leaching or decomposition processes risk altering what may be very delicate states. Synchrotron techniques, however, offer a microscopic to molecular-scale view of intact, carefully-preserved samples. Element-specific X-ray absorption near-edge structure (XANES) spectroscopy provides information on oxidation state and chemical speciation. This can be used for trace elements such as uranium (down to several ppm concentration) as well as for major elements such as P, S, Fe, Mn and Ca. Specific examples will be shown, including uranium oxidation state at the L3 and M5 absorption edges, iron oxidation state, organic/inorganic sulfur speciation, and the identification of calcium, iron, and uranium phosphates using P XANES. More detailed local structural information (such as first- and second-neighbor distances and coordination numbers) can be obtained using extended X-ray absorption fine structure (EXAFS) spetroscopy, for elements at higher concentrations. Examples will include iron oxides and uranium phases. Semi-quantitative spatial distributions of elements can be imaged on the ~10-microm scale using synchrotron X-ray fluorescence microanalysis (SXRFMA). This is useful to identify mineral phases present in the sediment, and to determine with which of them the uranium may be associated. Microbeam XANES can then be employed to determine oxidation states etc. at spots of interest, and microbeam diffraction can identify crystalline materials. SXRFMA can also be employed in a microtomographic technique for 3-D structure.