ALBERT

Notes: Abstract The mineral ‘fluorite’ is utilized as a probe to investigate the behaviour of the pseudolanthanide yttrium with respect to the lanthanides (rare-earth elements, REE) in fluorine-rich hydrothermal solutions. Hydrothermal vein fluorites are characterized by the close association of Y and REE, but in contrast to igneous and clastic rocks they show variable 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 fluorine-rich medium-temperature aqueous fluids. In such solutions Y acts 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-like light pseudolanthanide. This indicates that Y-Ho fractionation is not a sourcerelated phenomenon but depends on fluid composition. Based on these results it is strongly recommended that discussions of normalized REE patterns in general should be extended to normalized Rare-Earth-and-Yttrium (REY) patterns (Y inserted between Dy and Ho), because the slightly variable behaviour of the pseudolanthanide yttrium with respect to the REE may provide additional geochemical information. Available thermodynamic data suggest a negative correlation between Y/Ho and La/Ho during migration of a fluoriteprecipitating hydrothermal solution. Cogenetic fluorites, therefore, should display either similar Y/Ho and similar La/Ho ratios, or a negative correlation between these ratios. This criterion may help to choose samples suitable for Sm-Nd isotopic studies prior to isotope analysis. However, in cogenetic hydrothermal vein fluorites the range of Y/Ho ratios is often almost negligible compared to the range of La/Ho ratios. This may be explained by modification of REE distributions by post-precipitation processes involving (partial) loss of a separate LREE-enriched phase. The presence of variable amounts of such an accessory phase in most fluorite samples is revealed by experiments employing stepwise incomplete fluorite decomposition. Fluorites derived from and deposited near to igneous rocks apparently display chondritic Y/Ho ratios close to those of their igneous source-rocks. However, a positive YSN anomaly is likely to develop as the distance between sites of REY mobilization and deposition increases.

Notes: Abstract  The parameters which control the behaviour of isovalent trace elements in magmatic and aqueous systems have been investigated by studying the distribution of yttrium, rare-earth elements (REEs), zirconium, and hafnium. If a geochemical system is characterized by CHArge-and-RAdius-Controlled (CHARAC) trace element behaviour, elements of similar charge and radius, such as the Y-Ho and Zr-Hf twin pairs, should display extremely coherent behaviour, and retain their respective chondritic ratio. Moreover, normalized patterns of REE(III) should be smooth functions of ionic radius and atomic number. Basic to intermediate igneous rocks show Y/Ho and Zr/Hf ratios which are close to the chondritic ratios, indicating CHARAC behaviour of these elements in pure silicate melts. In contrast, aqueous solutions and their precipitates show non-chondritic Y/Ho and Zr/Hf ratios. An important process that causes trace element fractionation in aqueous media is chemical complexation. The complexation behaviour of a trace element, however, does not exclusively depend on its ionic charge and radius, but is additionally controlled by its electron configuration and by the type of complexing ligand, since the latter two determine the character of the chemical bonding (covalent vs electrostatic) in the various complexes. Hence, in contrast to pure melt systems, aqueous systems are characterized by non-CHARAC trace element behaviour, and electron structure must be considered as an important additional parameter. Unlike other magmatic rocks, highly evolved magmas rich in components such as H2O, Li, B, F, P, and/or Cl often show non-chondritic Y/Ho and Zr/Hf ratios, and “irregular” REE patterns which are sub-divided into four concave-upward segments referred to as “tetrads”. The combination of non-chondritic Y/Ho and Zr/Hf ratios and lanthanide tetrad effect, which cannot be adequately modelled with current mineral/melt partition coefficients which are smooth functions of ionic radius, reveals that non-CHARAC trace element behaviour prevails in highly evolved magmatic systems. The behaviour of high field strength elements in this environment is distinctly different from that in basic to intermediate magmas (i.e. pure silicate melts), but closely resembles trace element behaviour in aqueous media. “Anomalous” behaviour of Y and REEs, and of Zr and Hf, which are hosted by different minerals, and the fact that these minerals show “anomalous” trace element distributions only if they crystallized from highly evolved magmas, indicate that non-CHARAC behaviour is a reflection of specific physicochemical properties of the magma. This supports models which suggest that high-silica magmatic systems which are rich in H2O, Li, B, F, P, and/or Cl, are transitional between pure silicate melts and hydrothermal fluids. In such a transitional system non-CHARAC behaviour of high field strength elements may be due to chemical complexation with a wide variety of ligands such as non-bridging oxygen, F, B, P, etc., leading to absolute and relative mineral/melt or mineral/aqueous-fluid partition coefficients that are extremely sensitive to the composition and structure of this magma. Hence, any petrogenetic modelling of such magmatic rocks, which utilizes partition coefficients that have not been determined for the specific igneous suite under investigation, may be questionable. But Y/Ho and Zr/Hf ratios provide information on whether or not the evolution of felsic igneous rocks can be quantitatively modelled: samples showing non-chondritic Y/Ho and Zr/Hf ratios or even the lanthanide tetrad effect should not be considered for modelling. However, the most important result of this study is that Y/Ho and Zr/Hf ratios may be used to verify whether Y, REEs, Zr, and Hf in rocks or minerals have been deposited from or modified by silicate melts or aqueous fluids.