Cerium geochemical composition of the upper continental crust through time: Implications for tracing past surface redox conditions

Abstract

To investigate the use of cerium (Ce) in the upper continental crust (UCC) for tracing oxidative weathering, we measured the Ce anomaly (Ce/Ce*), the Ce stable isotope composition (δ142Ce), and the average Ce valence state (Ce#) in twenty-four glacial diamictite composites with depositional ages ranging from ∼2.9 to 0.3 Ga. We observe no distinguishable secular changes in Ce/Ce* (Ce/Ce* = 0.87–0.97) or δ142Ce (−0.092 ± 0.033 to 0.028 ± 0.043‰, 2 S.D.) of the diamictites. Limited elemental partitioning and isotopic fractionation of Ce can be ascribed to the immobilization of REEs, including Ce, in the diamictites during oxidative weathering. We calculated the present-day UCC δ142Ce to be −0.025 (median) ± 0.018‰ (MAD) or −0.029 (mean) ± 0.058‰ (2 S.D.). We assigned average valence state data of Ce (Ce#) in the diamictites into three age groups relative to the timing of the Great Oxidation Event (GOE): pre-GOE (2.9–2.4 Ga; Ce# = 3.09 to 3.22; average of 3.14), syn-GOE (2.4–2.2 Ga; Ce# = 3.18 to 3.73; average of 3.29), and post-GOE ages (0.8–0.3 Ga; Ce# = 3.21 to 3.68; average of 3.36). Our results show distinguishable correlations between Ce# and common weathering (major element ratios and Li isotopes) and redox tracers (e.g., Mn, Mo, V and Th/U) due to increasingly oxidative weathering with time. These results suggest that the valence state of Ce is a sensitive indicator of redox, and advise against relying solely on element- or isotope-based proxies. Combining redox proxies with valence state information can strengthen and complement the interpretation of past redox conditions.