The dependence of the partitioning of iron and europium between plagioclase and hydrous tonalitic melt on oxygen fugacity

The dependence of iron and europium partitioning between plagioclase and me lt on oxygen fugacity was studied in the system SiO2(Qz)-NaA1Si(3)O(8)(Ab)- CaA1(2)Si(2)O(8)(An)-H2O. Experiments were performed at 500 MPa and 850 deg rees C/750 degrees C under water saturated conditions. The oxygen fugacity was varied in the log f(O2) -range from -7.27 to -15.78. To work at the mos t reducing conditions the classical double-capsule technique was modified. The sample and a c-O-H bearing sensor capsule were placed next to each othe r within a BN jacket to minimise loss of hydrogen to the vessel atmosphere. By this setup redox conditions slightly more reducing than the FeO-Fe3O4 b uffer could be maintained even in 96 h runs. Raman spectra showed that the BN was modified by reaction with hydrogen resulting in a low hydrogen perme ability. The partition coefficients determined for Eu at 850 degrees C and 500 MPa vary from 0.095 at conditions of the Cu-Cu2O buffer to 1.81 at the most reducing conditions (C-O-H sensor). In the same f(O2), interval the pa rtition coefficient for Fe varies from 0.55 at oxidising conditions to 0.08 at the most reducing conditions. The partitioning of Sm, which was added a s a reference for a trivalent REE, does not vary with the oxygen fugacity, yielding an average value for D = 0.07. Lowering the temperature to 750 OC for a given f(O2) decreases the partition coefficient of Eu and increases t hat of Fe. Comparison with published data at 1 atm and at higher temperatur es shows that both temperature and composition of the melt have strong effe cts on the partitioning behaviour. As the change of the partition coefficie nts in the geologically relevant f(O2), range is quite strong, element part itioning of Eu and Fe might be used to estimate redox conditions for the ge nesis of igneous rocks. Furthermore, by modelling the partitioning data it is possible to extract information about the redox state of the melt. Resul ting ferric-ferrous ratios show significant differences from those predicte d by empirical models.