Phase equilibria of the Lower Laacher See Tephra (East Eifel, Germany): constraints on pre-eruptive storage conditions of a phonolitic magma reservoir

Laacher See volcano erupted about 12,900 years ago, resulting in a tephra blanket widely dispersed in central Europe. The eruption tapped a chemically and mineralogically strongly zoned phonolitic magma reservoir. In order to better constrain the pre-eruptive storage of that magma, hydrothermal phase equilibria experiments have been performed on a powdered sample from the highly evolved phonolite of the Lower Laacher See Tephra (LLST). Experiments were run at water pressures of 75–175 MPa and temperatures of 725–800 °C. Stability fields of sanidine and amphibole, the most common crystal phases in LLST, suggest that the magma last equilibrated at temperatures of 750–760 °C and pressures of 115–145 MPa. Those conditions suggest the magma should have contained between 5 and 6 wt.% water, which agrees well with water contents of 5.7 wt.% found in glass inclusions. That agreement, along with the observation that Cl and S vary with the composition of the inclusions, suggest that the Laacher See magma was saturated at depth with a water-rich fluid, and thus was stored at pressures near those estimated from our experiments. Such pressures roughly correspond to depths of 5–6 km. Clinopyroxene and plagioclase did not occur in the experimental runs, although present in the natural pumice sample. It seems likely that these phases are not inherent to the LLST. They were formed in other parts of the chemically stratified column and later redistributed. A recalculation of the magma density, together with a new estimate of the temperature and pressure range suggests a steep density gradient and a thin withdrawal layer, which is typical for volatile-enriched magma bodies and confirms earlier models of convective fractionation in the Laacher See magma reservoir.