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 compo sition of the acid-sulphate water is independent of filtration (0.2 mu m-0.8 mu m), and shows only minor temporal variation. The most striki ng feature of the Fe-rich water is the high abundance of dissolved rar e-earth elements (Sigma REE: 385 nM) and Y (281 nM). Chondrite-normali zed Rare-Earths and Y (REYCN; Y inserted between Dy and Ho) patterns s how a strong negative Eu-CN anomaly, increase slightly from La to Pr, and decrease systematically from Pr to Lu. REY speciation in the sprin g water is dominated by sulphate complexes, preventing any significant change of the solution complexation along the REY series. Leaching ex periments performed on the andesitic country-rock at Nishiki-numa allo wed us to characterize the easily accessible fraction of its total ele ment content. Alkali and alkali-earth elements are significantly less easily accessible than U and REY, the latter being the most easily acc essible of all elements considered. This suggests that the often obser ved minor to negligible mobility of REY during alteration is almost en tirely 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 access ible than its REY neighbours. This indicates that the easily accessibl e element fraction is not derived from the rock-forming minerals, but from boundary films and/or glass in the fine-grained matrix. The negat ive Eu,, anomaly of Nishiki-numa spring water is a reflection of this anomalously low accessibility of Eu. Further comparison of water compo sition to results from leaching experiments shows striking similaritie s, suggesting that at Nishiki-numa removal by secondary phases is of m inor importance for most of the elements studied. Exceptions are Pa an d U, because of the formation of barite and less soluble U(IV) species , respectively. Once the spring water is expelled at the surface, elem ents are partially removed from solution due to scavenging by ferrihyd rite precipitates. Apparent bulk coefficients for 'partitioning' of RE Y between the ferrihydrite and the mildly acidic spring water, K-app(D )REY, give a sigmoidal pattern with a strong negative deviation from t he general trend for Y, indicating strong fractionation of Y from its geochemical twin Ho. Considering that speciation (solution complexatio n) is similar for all REY, the differences between the individual K-ap p(D)REY 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 typica l result of interaction of dissolved REY with iron-oxyhydroxides. (C) 1998 Elsevier Science B.V. All rights reserved.