COMPARATIVE-STUDY OF YTTRIUM AND RARE-EARTH ELEMENT BEHAVIORS IN FLUORINE-RICH HYDROTHERMAL FLUIDS

The mineral 'fluorite' is utilized as a probe to investigate the behav iour of the pseudolanthanide yttrium with respect to the lanthanides ( rare-earth elements, REE) in fluorine-rich hydrothermal solutions. Hyd rothermal vein fluorites are characterized by the close association of Y and REE, but in contrast to igneous and elastic rocks they show var iable and nonchondritic Y/Ho ratios of up to 200. This suggests that Y and Ho, although similar in charge and size, may be fractionated in f luorine-rich medium-temperature aqueous fluids. In such solutions Y ac ts as a pseudolanthanide heavier than Lu. Y/Ho ratios of hydrothermal siderites are slightly below those of chondrites, suggesting that in ( bi)carbonate-rich siderite-precipitating solutions Y may act as a Sm-l ike light pseudolanthanide. This indicates that Y-Ho fractionation is not a source-related phenomenon but depends on fluid composition. Base d on these results it is strongly recommended that discussions of norm alized REE patterns in general should be extended to normalized Rare-E arth-and-Yttrium (REY) patterns (Y inserted between Dy and Ho), becaus e the slightly variable behaviour of the pseudolanthanide yttrium with respect to the REE may provide additional geochemical information. Av ailable thermodynamic data suggest a negative correlation between Y/Ho and La/Ho during migration of a fluorite-precipitating hydrothermal s olution. Cogenetic fluorites, therefore, should display either similar Y/Ho and similar La/Ho ratios, or a negative correlation between thes e ratios. This criterion may help to choose samples suitable for Sm-Nd isotopic studies prior to isotope analysis. However, in cogenetic hyd rothermal vein fluorites the range of Y/Ho ratios is often almost negl igible compared to the range of La/Ho ratios. This may be explained by modification of REE distributions by post-precipitation processes inv olving (partial) loss of a separate LREE-enriched phase. The presence of variable amounts of such an accessory phase in most fluorite sample s is revealed by experiments employing stepwise incomplete fluorite de composition. Fluorites derived from and deposited near to igneous rock s apparently display chondritic Y/Ho ratios close to those of their ig neous source-rocks. However, a positive Y-SN anomaly is likely to deve lop as the distance between sites of REY mobilization and deposition i ncreases.