PARTITIONING OF HIGH FIELD-STRENGTH AND RARE-EARTH ELEMENTS BETWEEN AMPHIBOLE AND QUARTZ-DIORITIC TO TONALITIC MELTS: AN EXPERIMENTAL STUDY

Abstract

The knowledge of rare-earth element (REE) and high field-strength element (HFSE) partitioning between minerals such as amphibole, pyroxenes or garnet and tonalitic liquids is essential to understand where and how tonalitic melts are generated. In this paper we present the results of trace-element partitioning studies between amphibole and quartz-dioritic to tonalitic liquids which have been conducted at 1 GPa and temperatures of 800°C, 850°C, and 900°C. Amphiboles crystallized from tonalitic liquids are nearly homogeneous, indicating that equilibrium has been closely attained. Henry's Law behaviour was confirmed for trace-element concentrations ranging from a few ppm as in natural systems up to doping levels of 0.5 wt.%. Partition coefficients (D) for the REE on the M4-site and of Ti, Zr and Hf on the M2-site of the amphibole structure can be fitted to parabolas, using the equation of . Parabolas for all isovalent series of cations have the same shape in the temperature range investigated. The maxima of the parabolas for the REE defining the size of the unstrained site r0 lie at Dy. At 1 GPa partition coefficients DAmph/L for Dy range from 1.77 at 900°C to 2.68 at 800°C. DAmph/L partition coefficients of Nb and Ta are identical within error limits. From this observation, together with the coincidence of their ionic radii with fitted radii of r0 for the M2-site, it is concluded that partition coefficients of Nb and Ta plot close to the peak of the parabola for the pentavalent cations. From a comparison of the fitted parabola parameters of the REE for amphibole and clinopyroxene it is inferred that at fixed conditions the amphibole/clinopyroxene partition coefficients DCpx/Amph for all REE have to be constant.The partition coefficients of the cations investigated in this study correlate negatively with the degree of depolymerisation in the melt expressed as the ratio of nonbridging oxygens to the number of tetrahedrons (NBO/T). Logarithms of partition coefficients normalized to the degree of depolymerisation define a linear function when plotted versus reciprocal temperature. By means of the fitted parabola parameters, straight-line relationships between DREE and DCa as well as DREE and DTi can be calculated. The predicted correlations agree very well with the experimentally determined data.