Dissolved H₂O significantly governs the eruptive behavior of magmas, as the formation and growth of fluid vesicles increase magma volume, potentially triggering an eruption. Some explosive eruptions, such as the 1875 Askja eruption in Iceland and the 16.5 Ma Yellowstone volcanic system, result from basaltic magma injection into a volatile-rich rhyolitic magma chamber, suggesting enhanced vesicle formation in these systems.

A previous experimental investigation demonstrated that decompression of bimodal hydrated rhyolitic and basaltic melts leads to enhanced vesicle formation within the developed hybrid melt zone(1). This zone is characterized by alkali depletion, particularly Na₂O, in the rhyolitic-dominated part. As H₂O solubility is decisively influenced by the alkali content of silicate melts(2), its depletion amplifies H₂O supersaturation, further enhancing vesicle formation in the hybrid zone.

Expanding this experimental approach, we synthesized glass with the hybrid melt composition(1), and conducted H₂O solubility experiments using an internally heated argon pressure vessel. The hybrid melt was hydrated with H₂O excess for 96 h at 1523 K and pressures of 60, 80, 100 or 200 MPa, then equilibrated at 1323 K for 0.5 h before being isobarically quenched at rates of 16–97 K/s. The resulting solubility data are essential for decompression experiments of initially slightly H₂O-undersaturated melts, conducted at rates of 1.7-0.17 MPa/s to final pressures of 60–100 MPa. These experiments are followed by quantitative image analysis and FTIR-spectroscopy to assess H₂O vesicle number density, spatial distribution, porosity, and H₂O content in the decompressed melts quenched to glasses. Comparing these data with the bimodal decompression experiments(1) will provide decisive insights into the vesicle formation and degassing mechanism of hybrid rhyolitic-basaltic melt zones.

(1) Marks, P. L. et al. (2023) J. Mineral., 35(4), 613-633.

(2) Allabar A. et al. (2022) Mineral. Petr., 177(52).