Evidence for extreme SmNd fractionation during chemical weathering

Abstract During subaerial chemical weathering, submarine weathering and other intracrustal processes, Sm Nd chemical fractionation remains uncertain, while a little is known about the conditions, scale and magnitude, specific processes and actual mechanism for possible Sm Nd fractionation. Here, an in situ terra rossa weathering profile developed on dolostone is chosen for the study. Based on sequential extraction experiments and measurements of the mineral and geochemical compositions including Sr Nd isotopic compositions, we suggest that subaerial chemical weathering not only leads to considerable Sm Nd mobilization, redistribution and secondary enrichment, but also gives rise to extreme Sm Nd fractionation, apparent 143Nd isotope homogenization, nearly complete resetting of the 147Sm 143Nd isotope system, anomalous initial 143Nd/144Nd ratios, and meaningless 143Nd model age (TDM) for weathering products. We found that the lanthanide tetrad effect, rather than mineralogical control, is responsible for the observed substantial Sm Nd fractionation and for the other rare earth element fractionation. Under domination of the lanthanide tetrad effect, the mobility of Sm is preferential to that of Nd during their migration in the form of dissolved organic complexes. Our results also illustrate that resetting of the 147Sm 143Nd isotope system in weathering products is controlled by the combined effect of the extraneous dissolved Sm Nd input and additional Sm Nd fractionation. Finally, we argue that similar scenarios may occur in submarine weathering, diagenesis and other geological processes.