H2O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach
Authors: Marks Patricia Louisa , Allabar Anja , Nowak Marcus 
Published: August 2023

Abstract. The injection of mafic magma into a hydrous felsic magma chamber is a
potential trigger mechanism for bimodal explosive volcanism. As H2O is
the most abundant volatile component in magmas, the interaction and the
degassing behavior of mildly peralkaline hydrous rhyolitic melt in contact
with hydrous basaltic melt were investigated by decompression experiments.
Preparatory hydration experiments and bimodal magma decompression
experiments, as well as reference experiments, were carried out in an
internally heated argon pressure vessel. Pre-hydrated rhyolite and
basalt cylinders were perfectly contacted together in a precious-metal
capsule, heated to 1348 K at 210 MPa, and thermally equilibrated for 10 min.
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 decompression experiment with two
contacted rhyolite cylinders for testing the experimental setup were
decompressed with 0.17 or 1.7 MPa s−1 to the final pressure of 100 MPa and then quenched. All decompression experiments resulted in vesiculated samples. The H2O
vesicles observed in the decompressed sample of the monomodal
rhyolite–rhyolite reference experiment are homogeneously distributed
throughout the sample. The former interface between the contacted glass
cylinders is invisible after decompression and quench. This reference
experiment proves that the two-cylinder design does not influence the
degassing behavior of the hydrous melt, e.g., an increased formation of
vesicles at possible nucleation sites at the contact plane of the cylinders. The undecompressed bimodal rhyolite–basalt sample shows crystal-free
rhyolitic glass, whereas 3 µm sized idiomorphic magnetite crystals
coexist with glass in the basaltic part of the sample. Within the 10 min run
time, a ∼ 300 µm wide hybrid composition zone developed
between the hydrous rhyolitic and basaltic endmembers, caused by
diffusion-induced mixing processes. Decompression and quenching of the bimodal melts resulted in vesiculated
glass samples. A ∼ 100 µm wide zone of alkali-depleted
rhyolitic glass as part of the ∼ 300–560 µm wide
hybrid zone is covered with an enhanced number of H2O vesicles
compared to the pristine rhyolitic and basaltic glass volumes. We suggest
that this enhanced vesiculated zone forms by a rapid diffusional loss of
alkalis from the mildly peralkaline rhyolitic melt into the basaltic melt of
the sample. The reduced alkali concentration significantly reduces the
H2O solubility of the rhyolitic melt. This process enhances the
H2O supersaturation necessary for vesicle formation during
decompression. In summary, the new findings imply that convective magma ascent driven by
the injection of hot basaltic magma into a hydrous peralkaline rhyolitic
melt reservoir leads to enhanced H2O vesicle formation near the melt
interface and thus to efficient degassing. This in turn can accelerate
buoyancy-driven magma ascent and mingling and mixing processes that
induce further degassing and potentially trigger explosive volcanic
eruptions.