Search

Fluids exsolved from mafic melts are thought to be dominantly CO2-H2O ± S fluids. Curiously, although CO2 vapor occurs in bubbles of mafic melt inclusions (MI) at room temperature (T), the expected accompanying vapor and liquid H2O have not been found. We reheated olivine-hosted MI from Mt. Somma-Vesuvius, Italy, and quenched the MI to a bubble-bearing glassy state. Using Raman spectroscopy, we show that the volatiles exsolved after quenching include liquid H2O at room T and vapor H2O at 150 °C. We hypothesize that H2O initially present in the MI bubbles was lost to adjacent glass during local, sub-micrometer-scale devitrification prior to sample collection. During MI heating experiments, the H2O is redissolved into the vapor in the bubble, where it remains after quenching, at least on the relatively short time scales of our observations. These results indicate that (1) a significant amount of H2O may be stored in the vapor bubble of bubble-bearing MI and (2) the composition of magmatic fluids directly exsolving from mafic melts at Mt. Somma-Vesuvius may contain up to 29 wt% H2O.

The 2018 lower East Rift Zone (LERZ) eruption and the accompanying collapse of the summit caldera marked the most destructive episode of activity at Kı̄lauea Volcano in the last 200 years. The eruption was extremely well‐monitored, with extensive real‐time lava sampling as well as continuous geodetic data capturing the caldera collapse. This multi‐parameter dataset provides an exceptional opportunity to determine the reservoir geometry and magma transport paths supplying Kı̄lauea’s LERZ. The forsterite contents of olivine crystals, together with the degree of major element disequilibrium with carrier melts, indicates that two distinct crystal populations were erupted from Fissure 8 (termed High‐ and Low‐Fo). Melt inclusion entrapment pressures reveal that Low‐Fo olivines (close to equilibrium with their carrier melts) crystallized within the Halema’uma’u reservoir ( ∼2 km depth), while many High‐Fo olivines ( > Fo81.5; far from equilibrium with their carrier melts) crystallized within the South Caldera reservoir ( ∼3–5 km depth). Melt inclusions in High‐Fo olivines experienced extensive post‐entrapment crystallization following their incorporation into cooler, more evolved melts. This favoured the growth of a CO2 ‐rich vapor bubble, containing up to 99% of the total melt inclusion CO2 budget (median=93%). If this CO2 ‐rich bubble is not accounted for, entrapment depths are significantly underestimated. Conversely, reconstructions using equation of state methods rather than direct measurements of vapor bubbles overestimate entrapment depths. Overall, we show that direct measurements of melts and vapor bubbles by SIMS and Raman Spectroscopy, combined with a suitable H2 O −CO2 solubility model, is a powerful tool to identify the magma storage reservoirs supplying volcanic eruptions.

The 2018 lower East Rift Zone (LERZ) eruption and the accompanying collapse of the summit caldera marked the most destructive episode of activity at Kı̄lauea Volcano in the last 200 years. The eruption was extremely well‐monitored, with extensive real‐time lava sampling as well as continuous geodetic data capturing the caldera collapse. This multi‐parameter dataset provides an exceptional opportunity to determine the reservoir geometry and magma transport paths supplying Kı̄lauea’s LERZ. The forsterite contents of olivine crystals, together with the degree of major element disequilibrium with carrier melts, indicates that two distinct crystal populations were erupted from Fissure 8 (termed High‐ and Low‐Fo). Melt inclusion entrapment pressures reveal that Low‐Fo olivines (close to equilibrium with their carrier melts) crystallized within the Halema’uma’u reservoir ( ∼2 km depth), while many High‐Fo olivines ( > Fo81.5; far from equilibrium with their carrier melts) crystallized within the South Caldera reservoir ( ∼3–5 km depth). Melt inclusions in High‐Fo olivines experienced extensive post‐entrapment crystallization following their incorporation into cooler, more evolved melts. This favoured the growth of a CO2 ‐rich vapor bubble, containing up to 99% of the total melt inclusion CO2 budget (median=93%). If this CO2 ‐rich bubble is not accounted for, entrapment depths are significantly underestimated. Conversely, reconstructions using equation of state methods rather than direct measurements of vapor bubbles overestimate entrapment depths. Overall, we show that direct measurements of melts and vapor bubbles by SIMS and Raman Spectroscopy, combined with a suitable H2 O −CO2 solubility model, is a powerful tool to identify the magma storage reservoirs supplying volcanic eruptions.