CONTROLS ON THE FRACTIONATION OF ISOVALENT TRACE-ELEMENTS IN MAGMATICAND AQUEOUS SYSTEMS - EVIDENCE FROM Y HO, ZR/HF, AND LANTHANIDE TETRAD EFFECT/

The parameters which control the behaviour of isovalent trace elements in magmatic and aqueous systems have been investigated by studying th e 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 char ge and radius, such as the Y-Ho and Zr-Hf twin pairs, should display e xtremely coherent behaviour, and retain their respective chondritic ra tio. Moreover, normalized patterns of REE(III) should be smooth functi ons of ionic radius and atomic number. Basic to intermediate igneous r ocks show Y/Ho and Zr/Hf ratios which are close to the chondritic rati os, indicating CHARAC behaviour of these elements in pure silicate mel ts. In contrast, aqueous solutions and their precipitates: show non-ch ondritic Y/Ho and Zx/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 exclusive ly depend on its ionic charge and radius, but is additionally controll ed 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 contra st to pure melt systems, aqueous systems are characterized by non-CHAR AC trace element behaviour, and electron structure must be considered as an important additional parameter. Unlike other magmatic rocks, hig hly 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 s mooth 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 diff erent from that in basic to intermediate magmas (i.e. pure silicate me lts), but closely resembles trace element behaviour in aqueous media. ''Anomalous'' behaviour of Y and REEs, and of Zr and Hf, which are hos ted by different minerals, and the fact that these minerals show ''ano malous'' trace element distributions only if they crystallized from hi ghly evolved magmas, indicate that non-CHARAC behaviour is a reflectio n of specific physicochemical properties of the magma. This supports m odels which suggest that high-silica magmatic systems which are rich i n H2O, Li, B, F, P, and/or Cl, are transitional between pure silicate melts and hydrothermal fluids. In such a transitional system non-CHARA C behaviour of high field strength elements may be due to chemical com plexation with a wide variety of ligands such as non-bridging oxygen, F, B, P, etc., leading to absolute and relative mineral/melt or minera l/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 coefficient s that have not been determined for the specific igneous suite under i nvestigation, may be questionable. But Y/Ho and Zr/Hf ratios provide i nformation on whether or not the evolution of felsic igneous rocks can be quantitatively modelled: samples showing non-chondritic Y/Ho and Z r/Hf ratios or even the lanthanide tetrad effect should riot be consid ered for modelling. However, the most important result of this study i s 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.