H2O-vesicle formation in the hybrid region of a bimodal melt system. An experimental progress.

The eruption behavior of magmas depends strongly on the volatile content of the melt. Dissolved H₂O significantly affects magma ascent and the nature of volcanic eruptions. The formation and subsequent growth of fluid vesicles increases the magma volume and thus the internal pressure of the magma chamber. Consequently, the tensile strength of the overlying bedrock may be exceeded, triggering an eruption⁽¹⁾. Vesicle formation may be enhanced in volatile-rich bimodal magmatic systems, such as the Askja eruption in Iceland in 1875⁽²⁾ and the 16.5 Ma Yellowstone eruption⁽³⁾.

A bimodal magmatic system can result from a basaltic melt entering a volatile-rich rhyolitic magma chamber, leading to magma mixing, magma mingling, and magma ascent. Experimentally decompressed bimodal hydrous melts show a depletion of alkalis, especially Na₂O, in the hybrid zone⁽⁴⁾. This amplifies supersaturation of H₂O and subsequently the enhanced formation of H₂O vesicles in the hybrid zone, as H₂O solubility closely correlates with the alkali content of a silicate melt⁽⁵⁾.

In general, nucleation is considered as the driving mechanism for vesicle formation of volatiles in silicate melts⁽⁶⁾. Nucleation describes the process of the formation of a critical vesicle in the thermodynamically metastable range, which can increase in volume due to diffusion processes, thereby achieving near equilibrium conditions⁽⁶⁾. However, decompression rate independent vesicle number densities observed in experimentally decompressed phonolitic melts contradict the results of nucleation theory⁽⁷⁾. Instead, the phase separation of H₂O from the silicate melt may proceed in the thermodynamically unstable range, in which spontaneous spinodal decomposition is the controlling mechanism.

For further detailed investigation of the mechanisms behind enhanced H₂O vesicle formation in the hybrid zone of bimodal melts, we synthesized glass with the hybrid melt composition given in (4).  Subsequently, H₂O solubility experiments were conducted in the internally heated argon pressure vessel (IHPV). For this purpose, the hybrid melt was hydrated with H₂O excess for 96 h at 1523 K and 60, 80, 100 or 200 MPa, further equilibrated at 1323 K for 0.5 h and then isobarically quenched with 16 or 97 K·s^-1. The resulting solubility data are essential to conduct decompression experiments of initially slightly H₂O undersaturated melts at rates of 1.7-0.17 MPa·s^-1 to the final pressures of 60-100 MPa, followed by the analysis of the H₂O vesicle number density, spatial distribution and H₂O contents in the decompressed melts with quantitative image analysis and FTIR-spectroscopy and the calculation of the equilibrium porosity. A comparison of the data with the bimodal decompression experiments⁽⁴⁾ could provide decisive information on the melt degassing mechanism of hybrid melt zones.

 

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