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Partition coefficients were determined in near-liquidus experiments using three different rocks as starting materials (tonalites from Tonale and from Val Melirolo/Italian Alps, trondhjemite from Bay of Islands/Newfoundland). They cover a range of SiO2-, Al2O3-, CaO-, MgO-, and K2O-contents from 58-66 wt.%, 15-17 wt.%, 4-7 wt.%, 1-4 wt.%, 0.4-2.2 wt.%, respectively. In order to promote amphibole crystallization, 7 wt.% water were added to all samples. The run conditions covered a temperature range from 800°C to 950¯ C and a pressure range from 1 GPa to 1.5 GPa. Most samples were doped with two trace element oxides in concentrations of 0.5 wt.% each. Run products were analysed with an electron microprobe, using the technique of McKay et al. (1986). Reversal runs were carried out in order to ensure that equilibrium was reached. Results of the experiments with the tonalite from Val Melirolo are shown in Fig. 1. For the entire temperature range [100°C] the REE distribution coefficient patterns are subparallel revealing distinct maxima at Dy and a small negative Eu-anomaly, which very likely indicates that part of the Eu is divalent. As also observed by Brenan et al. (1995) and LaTourette et al. (1995), the partition coefficients of Hf are significantly higher than those of Zr for all run conditions although the two elements have similar ionic radii. Nb and Ta show a similar fractionation. These observations may be relevant to account for variations in Nb/Ta- and Zr/Hf-ratios observed in the continental crust (Green, 1995). As known from clinopyroxenes (Gallahan and Nielsen, 1992), the partition coefficients of the REE show a good positive correlation with those of Ti. They also show an increase with the degree of polymerisation of the melt, expressed by NBO/T. Partition coefficients, if normalized to the polymerisation of the melt, decrease with increasing temperature even if the amphiboles remain identical in composition. This observation thus indicates that these partition coefficients are directly temperature-dependent. The data suggest that amphibole does not play a major role in tonalite genesis.
Brenan, J. M. et al., Earth Planet. Sci. Letts. 135, 1-11 (1995).
Gallahan, W. E., Nielsen, R. L., Geochim. Cosmochim. Acta. 56, 2387-2404 (1992).
Green, T. H., Chem. Geol. 120, 347-359 (1995).
LaTourette ,T. et al., Earth Planet. Sci. L. 135, 13-30 (1995).
McKay, G. et al., Geochim. Cosmochim. Acta 50, 927-937 (1986).
Fig. 1. Partition coefficients of amphiboles for selected temperatures as a function of the ionic radii of the pertinent elements.