Sequential leachates of multiple grain size fractions from a clay-rich till, Saskatchewan, Canada: implications for controls on the rare earth element geochemistry of porewaters in an aquitard

Porewater REE concentrations vary by an order of magnitude over 45 vertical m in a thick clay-rich till aquitard in southern Saskatchewan. To address controls on aqueous REE, the till was disaggregated into seven size fractio ns (> 850, 425-850, 295-425, 180-295, 150-180, 75-150, and < 75 mu m) Each fraction was sequentially leached with water(L1); 1 M NaOAc (pH 5) (L2); 0. 25 M NH2OH . HCl (L3); 1 M NH2OH . HCl (L4); 12 M HCl + KClO3 + 4 M HNO3 (L 5); and the residue digested in Na2O2 (L6). Aqueous leachate of the < 75 mu m size fraction has a near flat REE pattern at similar to 0.1 PAAS and car ries 99% of REE, relative to the total from all L1 leached size fractions. Aqueous leachates of the coarser size fractions have flat patterns 1 to 2 o rders of magnitude lower. Leachate L2 extracts metals held electrostaticall y on inorganic or organic material: the REE also plot at similar to 0.1 PAA S with a convex up pattern and a maximum at Gd. Leachate L3 of amorphous Fe - and Mn-oxyhydroxides carries more REE than L1, L2 or L4, and Like L2, has a mildly convex up pattern. There are lower REE abundances associated with crystalline Fe- and Mn-oxides (L4) than with amorphous counterparts, and H REE are fractionated and depleted. Leaching of organic matter (L5) produces REE patterns comparable to those of LA. The residue (L6) has convex down R EE patterns at 0.1 to 0.4 PAAS with prominent positive Eu anomalies from pl agioclase feldspar, excepting the < 75 mu m size fraction. Natural porewate rs have mildly fractionated patterns with LREE and MREE depletion versus co nvex up patterns for leachates (L1, L2 and L3). LREE and MREE may readily s orb onto clays, or clay coatings, in the till, and be desorbed during leach ing. (C) 1999 Elsevier Science B.V. All rights reserved.