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 fractions (>850, 425–850, 295–425, 180–295, 150–180, 75–150, and <75 μm). 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 (L5); and the residue digested in Na2O2 (L6). Aqueous leachate of the <75 μm size fraction has a near flat REE pattern at ∼0.1 PAAS and carries 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 orders of magnitude lower. Leachate L2 extracts metals held electrostatically on inorganic or organic material: the REE also plot at ∼0.1 PAAS 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 HREE are fractionated and depleted. Leaching of organic matter (L5) produces REE patterns comparable to those of L4. The residue (L6) has convex down REE patterns at 0.1 to 0.4 PAAS with prominent positive Eu anomalies from plagioclase feldspar, excepting the <75 μm size fraction. Natural porewaters have mildly fractionated patterns with LREE and MREE depletion versus convex up patterns for leachates (L1, L2 and L3). LREE and MREE may readily sorb onto clays, or clay coatings, in the till, and be desorbed during leaching.