Rare earth elements (REE) and yttrium in stream waters, stream sediments, and Fe-Mn oxyhydroxides: Fractionation, speciation, and controls over REE plus Y patterns in the surface environment

We have collected similar to 500 stream waters and associated bed-load sediments over an similar to 400 km(2) region of Eastern Canada and analyzed these samples for Fe, Mn, and the rare earth elements (REE + Y). In addition to analyzing the stream sediments by total digestion (multi-acid dissolution with metaborate fusion), we also leached the sediments with 0.25 M hydroxylamine hydrochloride (in 0.05 M HCl), to determine the REE + Y associated with amorphous Fe- and Mn-oxyhydroxide phases. We are thus able to partition the REE into "dissolved" (<0.45 mu m), labile (hydroxylamine) and detrital sediment fractions to investigate REE fractionation, and in particular, with respect to the development of Cc and Eu anomalies in oxygenated surface environments. Surface waters are typically LREE depleted ([La/Sm](NASC) ranges from 0.16 to 5.84, average = 0.604, n = 4 10; where the REE are normalized to the North America Shale Composite), have strongly negative Ce anomalies ([Ce/Ce*](NASC) ranges from 0.02 to 1.25, average = 0.277, n = 354), and commonly have positive Eu anomalies ([Eu/Eu*](NASC) ranges from 0.295 to 1.77, average = 0.764, n = 84). In contrast, the total sediment have flatter REE + Y patterns relative to NASC ([La/Sm](NASC) ranges from 0.352 to 1.12, average = 0.778, n = 45 1) and are slightly middle REE enriched ([Gd/Yb](NASC) ranges from 0.55 to 3.75, average = 1.42). Most total sediments have negative Ce and Eu anomalies ([Ce/Ce*]NASC ranges from 0.097 to 2.12, average = 0.799 and [Eu/Eu*](NASC) ranges from 0.39 to 1.43, average = 0.802). The partial extraction sediments are commonly less LREE depleted than the total sediments ([La/Sm](NASC) ranges from 0.24 to 3.31, average = 0.901, n = 4537), more MREE enriched ([Gd/Yb]NASC ranges from 0.765 to 6.28, average = 1.97) and Cc and Eu anomalies (negative and positive) are more pronounced. The partial extraction recovered, on average similar to 20% of the Fe in the total sediment, similar to 80% of the Mn, and 21-29% of the REEs (Ce = 19% and Y = 32%). Comparison between REEs in water., partial extraction and total sediment analyses indicates that REEs + Y in the stream sediments have two primary sources, the host lithologies (i.e., mechanical dispersion) and hydromorphically transported (the labile fraction). Furthermore, Eu appears to be more mobile than the other REE, whereas Ce is preferentially removed from solution and accumulates in the stream sediments in a less labile form than the other REEs + Y. Despite poor statistical correlations between the REEs + Y and Mn in either the total sediment or partial extractions, based on apparent distribution coefficients and the pH of the stream waters, we suggest that either sediment organic matter and/or possibly delta-MnO2/FeOOH are likely the predominant sinks for Ce, and to a lesser extent the other REE, in the stream sediments. (c) 2008 Elsevier Ltd. All rights reserved.