Phase relations of three samples of the Laacher See Tephra (LST) have been determined experimentally as a function of temperature (760 to 880 degreesC), pressure (200, 300 and 400 MPa), water content of the melt and oxygen fugacity (fO(2)). The crystallization experiments were carried out at fO(2) = NNO buffer and at NNO = +2.3 log units. The melt water contents varied between 6 and more than 8 wt% H2O, corresponding to water-undersaturated and water saturated conditions respectively. The synthetic products are compared to the natural phases to constrain pre-eruptive conditions in the Laacher See magma chamber. The major phases occurring in the LST have been reproduced. The stability of hauyne is favoured at high fO(2) (approximate to NNO + 2.3). The CaO contents in melt and plagioclase synthesized under water-saturated conditions are significantly higher than in the natural phases, implying that most of the differentiation of the phonolites took place under water-undersaturated conditions. However, this does not exclude the presence of a S-, Cl- and CO2-rich fluid phase in the upper parts of the magma chamber. The phase relationships and the TiO2 contents of melts show that the temperature was lower than 760 degreesC in the upper part of the magma column (probably down to 720 degreesC in the most differentiated levels) and that temperatures above 840-860 degreesC prevailed in the lower part. The variation of the X-Mg of ferromagnesian minerals observed in both natural and experimental phases reflects the strong variations in fO(2) in the lower magma chamber just prior to eruption (probably variation of about 2 log units). The most probable explanation for these fO(2) variations is the injection of an oxidized alkali-rich magma, containing Mg-rich phenocrysts, at the base of a chemically zoned and more reduced magma column prior to eruption. Although the amount of injected magma may not have been very important, it was sufficient to change the fO(2) conditions locally, explaining the heterogeneous X-Mg Of ferromagnesian minerals and the formation of hauyne at the base of the chamber.
