Experimental Constraints on H2O Vesiculation in the Hybrid Zone of a Bimodal Rhyolitic-Basaltic Melt System.

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⁽¹⁾, result from the injection of basaltic magma into a volatile-rich rhyolitic magma chamber, suggesting enhanced vesicle formation in these systems.

A recent experimental study⁽²⁾ investigated the melt injection of a hydrous basaltic melt into a hydrous rhyolitic melt. The authors demonstrated that continuous decompression of bimodal hydrated rhyolitic and basaltic melts enhances vesicle formation within the evolving, alkali depleted rhyolitic hybrid melt. Expanding their experimental approach, we synthesized a glass with the composition of the highly vesiculated hybrid melt and conducted H₂O solubility experiments using an internally heated argon pressure vessel (IHPV). For the subsequent combined hydration and decompression experiments, we hydrated the hybrid melts with 5.7 wt. % H₂O in an IHPV for 96 h at 1523 K and 200 MPa and equilibrated at 1323 K for 1 h prior to continuous decompression at 0.17 and 1.7 MPa·s^-1 to final pressures of 60–100 MPa.

The decompression rate-dependent vesicle number densities (VND) and vesicle sizes, together with polymodal vesicle size distributions indicate that H₂O-phase separation proceeds in the thermodynamic field of metastability via nucleation. Although the present study confirms the general mode of H₂O-phase separation observed in the melt injection-study⁽²⁾, pronounced textural deviations occur between the homogeneous hybrid melt and a hybrid melt generated during magma mixing. In comparison, the hybrid melts produced during melt injection require substantially reduced supersaturation pressures for the onset of vesiculation and smaller pressure intervals to obtain high VND. These differences demonstrate that chemical disequilibrium and diffusion processes during melt injection substantially enhance vesiculation. Consequently, the comparison of the two experimental series shows that H₂O degassing depends not only on the melt composition, but also on geological processes operating within the magma reservoirs, in this case magma mixing and the associated shift out of equilibrium, generated by rapid depletion of the alkali components Na₂O and K₂O. As H₂O solubility of rhyolitic melts is decisively controlled by the alkali content⁽³⁾, its depletion amplifies H₂O supersaturation, further enhancing vesicle formation in the hybrid zone.

(1) Sparks, R. S. J. (1978) Geoth. Res., 3(1-2), 1-37.

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

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