Fractionation of Sr and Hf isotopes by mineral sorting in Cascadia Basin terrigenous sediments.

Oceanic sediments deposited on continental margins consist mainly of erosion products of the nearby exposed continental areas. Detrital input usually dominates their geochemical budget, and the composition of these sediments should record potential changes in their continental sources. However, along margins, mineral sorting associated with transport and sedimentary processes induces significant chemical and isotopic fractionation over few tens of kilometres. The study of margin sediments should help to quantify the extent of modification of the continental terrigenous supply when it reaches deep oceans. Reported Sr, Nd, Hf and Pb isotopic compositions of fifty-seven sediments from the northernmost part of the Cascade forearc (Ocean Drilling Program, ODP, Sites 888 and 1027) suggest the involvement of two dominant end-members coming from the nearby Canadian Cordillera. Erosion products of the depleted, western part of the Cordillera dominate the detrital input, while the eastern enriched terranes of the Cordillera contribute only 10 to 28% of the input. There is no marked change of provenance of sediments during the last 3.5 Myr and they all appear unaffected by glacial-interglacial climate cycles. The average isotopic compositions of the two sites are slightly different, but are both dominated by continental signature; these values can be used in future studies to identify any subducted sediment contribution to the Cascades Arc. On a finer scale, there are differences in the isotopic signature between samples dominated by clay minerals and those with coarser lithologies. For a given Nd isotopic composition, fine sediments have more radiogenic Sr and Hf isotope ratios than sands, and we interpret the difference as resulting from mineral sorting during transport of the particles. Fine sediments concentrate minerals with radiogenic Sr and Hf such as clays and micas, while coarse-grained detritus carry the unradiogenic mineral component of a given source rock through plagioclase-epidote and zircon. ODP Site 1027 is located 100 km further away from continent than ODP Site 888 and contains more clay. As a consequence, it has significantly more radiogenic Sr and Hf bulk composition than ODP Site 888. Similar differences in isotopic signatures related to distance to continent certainly occur in other areas in the word, and will account for a large part of differences known between continental sources and deep-sea sediments. [ABSTRACT FROM AUTHOR]