Geochemistry of low-temperature water–rock interaction: evidence from natural waters, andesite, and iron-oxyhydroxide precipitates at Nishiki-numa iron-spring, Hokkaido, Japan

Cold, mildly acidic, and slightly oxidizing spring water from Nishiki-numa iron-spring, Hokkaido, Japan, has been sampled in two consecutive years, and analyzed for anions and thirty cations. The chemical composition of the acid–sulphate water is independent of filtration (0.2 μm–0.8 μm), and shows only minor temporal variation. The most striking feature of the Fe-rich water is the high abundance of dissolved rare-earth elements (ΣREE: 385 nM) and Y (281 nM). Chondrite-normalized Rare-Earths and Y (REYCN; Y inserted between Dy and Ho) patterns show a strong negative EuCN anomaly, increase slightly from La to Pr, and decrease systematically from Pr to Lu. REY speciation in the spring water is dominated by sulphate complexes, preventing any significant change of the solution complexation along the REY series. Leaching experiments performed on the andesitic country-rock at Nishiki-numa allowed us to characterize the easily accessible fraction of its total element content. Alkali and alkali–earth elements are significantly less easily accessible than U and REY, the latter being the most easily accessible of all elements considered. This suggests that the often observed minor to negligible mobility of REY during alteration is almost entirely due to the formation of REY-bearing secondary minerals. Within the REY series accessibilities are constant from La to Nd and decrease from Nd to Lu, except for Eu which is considerably less easily accessible than its REY neighbours. This indicates that the easily accessible element fraction is not derived from the rock-forming minerals, but from boundary films and/or glass in the fine-grained matrix. The negative EuCN anomaly of Nishiki-numa spring water is a reflection of this anomalously low accessibility of Eu. Further comparison of water composition to results from leaching experiments shows striking similarities, suggesting that at Nishiki-numa removal by secondary phases is of minor importance for most of the elements studied. Exceptions are Ba and U, because of the formation of barite and less soluble U(IV) species, respectively. Once the spring water is expelled at the surface, elements are partially removed from solution due to scavenging by ferrihydrite precipitates. Apparent bulk coefficients for `partitioningʹ of REY between the ferrihydrite and the mildly acidic spring water, appKREYD, give a sigmoidal pattern with a strong negative deviation from the general trend for Y, indicating strong fractionation of Y from its geochemical twin Ho. Considering that speciation (solution complexation) is similar for all REY, the differences between the individual appKREYD values must be related to complexation with functional groups on the ferrihydrite surface. Our data show that Y–Ho fractionation is not confined to the mildly alkaline marine environment, but the typical result of interaction of dissolved REY with iron-oxyhydroxides.