A new method for the determination of Ce-III/Ce-IV ratios in geological materials; application for weathering, sedimentary and diagenetic processes

Relative abundances of rare earth elements (REE) in geological materials ar e used widely to investigate diverse geochemical issues such as the origins of igneous rocks or the degree of stratification of water columns in ocean s. One of the REE, cerium (Ce), can exist in either trivalent or tetravalen t forms depending on the redox condition. Thus, knowledge of the oxidation state of Ce in rocks and minerals could potentially be used to constrain th e redox states of past and present geological environments. However, the us e of Ce for this purpose has been hampered by an inability to measure its o xidation state directly, Here, we present a new method, employing the X-ray absorption near-edge structure, for making such determinations in samples with Ce concentrations as low as 15 ppm. By analyzing a range of diverse ge ological materials (granites, manganese nodules and charts), we showed that the method could identify significant differences in Ce speciation between materials formed under widely differing redox conditions. Used together wi th the degrees of Ce anomalies measured in REE patterns, this information h as promise for refining our knowledge of various geochemical processes such as weathering, sedimentary ard diagenetic processes that control the REE b ehavior. Such a refinement should in turn improve geological interpretation s based upon REE data. (C) 2000 Elsevier Science B.V. All rights reserved.