Decoding the H2O phase separation mechanism as the trigger for the explosive eruption of the Lower Laacher See phonolite

The Laacher See volcano is one of the youngest volcanoes in Germany with its last eruption 13,006 9 years BP(1). About 6.3 km of hydrous phonolitic magma was explosively erupted by phreatomagmatic and plinian eruptions in less than 10 days(2). The eruption behavior of such volcanic systems is determined by the phase separation mechanism of H2O fluid from the supersaturated hydrous silicate melt during ascent. The number of fluid vesicles per unit volume of silicate melt (VND in mm-3) is a standard parameter used to quantify the efficiency of fluid-melt separation and thus the acceleration of magma ascent. According to the nucleation theory, the VND increases strongly with decompression rate(3) and is thus a suitable parameter for quantifying magma ascent velocity. Recently and specifically for phonolitic melts, the process of spinodal decomposition has been proposed, which manifests in the independence of VND from the decompression rate(4). To characterize the degassing behavior of the lower Laacher See phonolite, systematic decompression experiments were conducted in the internally heated pressure vessel. The melts were hydrated with 5.7 or 5.0 wt% H2O at 200 MPa and 1523 K for 96 h and then continuously decompressed at 1323 K with 0.064 1.7 MPa/s to final pressures between 110 MPa and 30 MPa. By reaching the final pressure, the samples were rapidly quenched to room temperature to preserve the vesicle textures and the residual H2O contents in the melts and to minimize vesicle shrinkage during cooling. The VNDs and the spatial distribution of the vesicles, as well as the H2O contents in the decompressed melts were analyzed with quantitative image analysis, transmission light microscopy, and FTIR-spectroscopy. Upon reaching sufficient supersaturation pressure of >80 MPa, all samples exhibit homogeneously dispersed and micrometer-sized vesicles in the sample center (Fig. 1). The results consistently show that the initial VND is independent of the decompression rate, with high logVNDs of 4.1 to 5.6 causing very fast near equilibrium adjustment of H2O concentration of the melt by degassing. Further decompression of the vesiculated melts leads to vesicle coalescence, resulting in a significant reduction of VND by orders of magnitude. These observations are consistent with that of Allabar and Nowak (2018), who determined a decompression rate independent logVND of ~5.2 0.5 for hydrous phonolitic melt of the AD79 Vesuvius white pumice composition. From this, a trend emerges that at least for hydrated phonolitic melts, spinodal decomposition plays a crucial role in the H2O degassing behavior of the melt and thus in the explosive eruption behavior of the volcanic systems. (1)Allabar A., Nowak M. (2018) Message in a bottle: Spontaneous phase separation of hydrous Vesuvius melt even at low decompression rates. EPSL, 501, 192-201. (2)Reinig F., et al. (2021) Precise date for the Laacher See eruption synchronizes the Younger Dryas. Nature, 595, 66-69. (3)Toramaru A. (2006) BND (bubble number density) decompression rate meter for explosive volcanic eruptions. J. Volcanol. Geotherm. Res., 154, 303-316. (4)Wrner G., Schmincke H.-U. (1984) Petrogenesis of the Zoned Laacher See Tephra. J. Petrol., 25, 805-835.