Which minerals control the Nd–Hf–Sr–Pb isotopic compositions of river sediments?

International audienceRiver sediments naturally sample and average large areas of eroded continental crust. They are ideal targets not only for provenance studies based on isotopic compositions, but also can be used to establish average continental crust isotopic values. In large fluvial systems, however, mineral sorting processes significantly modify the mineralogy, and thus the geochemistry of the transported sediments. We still do not know, in any quantitative way, to what extent mineral sorting affects and fractionates the isotopic compositions of river sediments. Here, we focus on this issue and try to decipher the role of each mineral species in the bulk isotopic compositions of bedloads and suspended loads sampled at the outflow of the Ganga River that drains the Himalayan mountain range.We analyzed Nd, Hf, Sr, and Pb isotopic compositions as well as trace element contents of a large number of pure mineral fractions (K-feldspar, plagioclase, muscovite, biotite, magnetite, zircon, titanite, apatite, monazite/allanite, amphibole, epidote, garnet, carbonate and clay) separated from bedload and bank sediments. We combine these data with mineral proportions typical of the Ganga sediments to perform Monte Carlo simulations that quantify the contributions of individual mineral species to the Nd, Hf, Sr, and Pb isotopic budgets of bedloads and suspended loads.The isotopic systematics of river sediments are buffered by very few minerals. Despite their extremely low proportions in sediments, zircon and monazite/allanite control Hf and Nd isotopes, respectively. Feldspars, epidote, and carbonate buffer the Sr isotopic budget while clay, feldspars, and heavy minerals dominate Pb isotopes. Hafnium, Sr, and Pb isotopic differences between bedloads and suspended loads are well explained by their different mineral compositions. This confirms that Hf, Sr and Pb isotopic compositions of sediments are strongly biased by mineral sorting processes during fluvial transport; hence they do not always constitute good proxies for provenance studies. In addition, we anticipate that fractionation of the isotopic systems continues at the river/ocean interface to deliver sediments to the deep ocean that are not necessarily similar to their crustal precursors, creating a systematic bias between the compositions of crustal sources and oceanic sediments