The injection of mafic magma into a hydrous felsic magma chamber is a potential trigger mechanism for bimodal explosive volcanism. Contact of hot mafic magma with cooler, H2O-rich rhyolitic magma causes a decrease in H2O solubility <300 MPa to trigger vesicle formation. Mixing processes at the interface may reinforce vesicle formation (Caricchi et al. 2021).
Bimodal decompression and reference experiments were performed with basaltic and rhyolitic melts. Pre-hydrated rhyolite- and basalt-cylinders were perfectly contacted, heated, and equilibrated to experimental conditions. The initial sample properties were determined by a bimodal reference experiment, quenched immediately after equilibration. To simulate the magma ascent, three bimodal samples, and a rhyolite-rhyolite decompression experiment for testing the experimental setup were decompressed to the final pressure and then quenched.
The rhyolite reference experiment showed homogeneously distributed H2O vesicles throughout the sample, verifying the two-cylinder design. In all bimodal samples, a hybrid zone formed between the end-members, due to diffusion-induced mixing processes. An enhanced vesiculated zone developed in the rhyolite-dominated hybrid zone compared to the unmixed endmembers. This results from the rapid diffusive loss of alkalis from the mildly peralkaline rhyolitic melt into the basaltic melt. The decreased Na2O concentration reduces the H2O solubility (Allabar et al. 2022) and promotes H2O supersaturation in the depleted rhyolitic melt during decompression.
This suggests that injection of a basaltic melt into a hydrated peralkaline rhyolitic melt reservoir can lead to significantly enhanced H2O vesicle formation in the hybrid zone. Increased degassing and the associated triggering of explosive eruptions can be expected.