From sediments to their source rocks: Hf and Nd isotopes in recent river sediments

Unraveling continental evolution from the sedimentary record requires an understanding of time-integrated erosion laws that link sediments to their source rocks, and the extent to which erosion laws vary in different erosion systems. Detrital zircons from the Frankland River (southwestern Australia) define a continental growth curve that is strikingly similar to the Nd in shales curve for the Australian continent. This suggests that the detrital zircon data can be used as a good proxy for the sedimentary record through time. The advantage is that the age distribution of the zircons allows the contributions from different source regions to be determined for any sediment sample. Using integrated Hf and U-Pb isotopes in detrital zircons, and Nd isotope ratios of bulk recent sediments along the Frankland River, the relative contributions of different source terrains have been determined and expressed through an erosion parameter K, which relates the proportions of the material from different source rocks in the sediments to the proportions of those source rocks present in the overall catchment of the sediments analyzed. The results suggest that values of K = 4-6 are representative of mature river systems that sample large source areas, and that these should be used to reevaluate models of the evolution of the continental crust that were generally limited by the assumption of K. For the Gondwana supercontinent, K values of 4-6 indicate that at least 50% of the present-day volume of the continental crust was generated by the end of the Archean.