76.
The dependence of iron and europium partitioning between plagioclase and melt on oxygen fugacity was studied in the system
SiO
2(Qz)—NaAlSi
3O
8(Ab)—CaAl
2Si
2O
8(An)—H
2O. Experiments were performed at 500 MPa and 850 °C/750 °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 most reducing conditions the classical double-capsule technique was modified.
The sample and a C—O—H bearing sensor capsule were placed next to each other within a BN jacket to minimise loss of hydrogen
to the vessel atmosphere. By this setup redox conditions slightly more reducing than the FeO—Fe
3O
4 buffer 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 permeability. The partition coefficients determined for Eu at 850 °C and 500 MPa vary from 0.095 at conditions
of the Cu—Cu
2O buffer to 1.81 at the most reducing conditions (C—O—H sensor). In the same
f
O2 interval the partition 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 as 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 °C for a given
f
O2 decreases the partition coefficient of Eu and increases that of Fe. Comparison with published data at 1 atm and at higher
temperatures shows that both temperature and composition of the melt have strong effects on the partitioning behaviour. As
the change of the partition coefficients in the geologically relevant
f
O2 range is quite strong, element partitioning of Eu and Fe might be used to estimate redox conditions for the genesis of igneous
rocks. Furthermore, by modelling the partitioning data it is possible to extract information about the redox state of the
melt. Resulting ferric-ferrous ratios show significant differences from those predicted by empirical models.
Received: 14 October 1998 / Received: 5 March 1999
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