Preparatory experiments to investigate the vesicle formation of hydrous lower Laacher See phonolite at near liquidus conditions

The Laacher See volcano in the Eifel region of western Germany caused one of the largest eruptions in Central Europe within the last 100,000 years. The Plinian eruptions lasted days to weeks and ejected about 6.3 km³ phonolitic magma. The ascent and subsequent discharge of magma is greatly influenced by the amount of dissolved H₂O. During the magma ascent the solubility of H₂O in the melt decreases while simultaneously the supersaturation of the melt increases. This is the prerequisite for phase separation, which leads to the formation of volatile bubbles.

A set of high temperature decompression experiments with hydrous lower Laacher See phonolite (LLST) has recently been conducted at high temperature superliquidus conditions of 1323 K and starting pressure of 200 MPa showing spontaneous phase separation (Marks and Nowak 2024) at a final pressure of 80 MPa. In order to investigate the temperature dependence of vesicle formation, preparatory phase relation experiments were conducted in the cold seal pressure vessel at low temperature near liquidus conditions. Synthetic glass cylinders of the LLST composition were hydrated for 11 days with excess H₂O at 1123 K and 200, 90, and 70 MPa to obtain H₂O solubility data and pressure dependent liquidus conditions.

The hydrated and quenched sample at 200 MPa is a crystal-free homogeneous glass with residual H₂O fluid. The H₂O content of the glass was determined with FTIR-spectroscopy. The measured H₂O content of ~5.7 wt.% at 200 MPa and 1123 K is in good agreement with previous results of Schmidt and Behrens (2008).

In contrast, the samples hydrated at P <200 MPa are partially crystallized. Needle shaped crystals grew from the capsule walls towards the sample center indicating subliquidus conditions. Based on optical microscopy and Raman spectroscopy, the crystal phases are indicative of feldspar. For a detailed phase identification, the samples will be investigated with the electron beam microprobe.

The preparatory samples provide important information on planned low temperature decompression experiments: (1) At 1123 K, 200 MPa, and H₂O saturated conditions the melt is superliquidus, which is a prerequisite for homogeneous vesicle formation.  (2) Pressures conditions between 90 and 70 MPa are subliquidus, resulting in partial crystallization of the hydrous melt. This might induce crystal formation during decompression. However, (3) the planned decompression rates of 0.17 and 1.7 MPa·s^-1 and experimental run times <15 min are expected to be fast enough to inhibit partial crystallization (Brugger and Hammer, 2010). Therefore, we suggest that the determination of the low temperature near liquidus H₂O phase separation mechanism of the lower Laacher See phonolite will be successful.

Marks P. L. and Nowak M., 2024. Decoding the H2O phase separation mechanism as the trigger for the explosive eruption of the Lower Laacher See phonolite. EGU24-7723.

Schmidt, B.C., Behrens, H., 2008. Water solubility in phonolite melts: influence of melt composition and temperature. Chem. Geol. 256, 259–268.

Brugger, C. R. & Hammer, J. E., 2010 Crystal size distribution analysis of plagioclase in experimentally decompressed hydrous rhyodacite magma. Earth Planet. Sci. Lett. 300, 246–254.