Experimental constraints on storage conditions in the chemically zoned phonolitic magma chamber of the Laacher See volcano

Abstract

Phase relations of three samples of the Laacher See Tephra (LST) have been determined experimentally as a function of temperature (760 to 880 °C), pressure (200, 300 and 400 MPa), water content of the melt and oxygen fugacity (ƒO₂). The crystallization experiments were carried out at ƒO₂=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 ƒO₂ (≈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 °C in the upper part of the magma column (probably down to 720 °C in the most differentiated levels) and that temperatures above 840–860 °C 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 ƒO₂ in the lower magma chamber just prior to eruption (probably variation of about 2 log units). The most probable explanation for these ƒO₂ 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 ƒO₂ conditions locally, explaining the heterogeneous X _Mg of ferromagnesian minerals and the formation of hauyne at the base of the chamber.