New Estimates of Volatiles in Melt Inclusions

During volcanic eruptions, magma rises from beneath Earth’s surface. As it does so it cools, crystallizes, and exsolves magmatic fluids prevalently containing water, carbon dioxde, and sulfur.

During a volcanic eruption, these exsolved magmatic fluids return to the atmosphere. Sometimes, tiny portions of silicate melt (also known as melt inclusions) get trapped during mineral crystal growth. Host crystals of melt inclusions ideally act as a pressure capsule, preventing the total exsolution and “escape” of volatile from the trapped melt, and thus hold valuable information about Earth’s volatile budget. In addition, melt inclusions represent the tool that Mother Nature provided to investigate the type of magmatic fluid exsolving from natural melts at sub-volcanic conditions.

Melt inclusions, however, are technically very challenging to work with. In a new paper published in the journal American Mineralogist, DCO’s Rosario Esposito (University of California Los Angeles, USA; previously at University of Naples “Federico II,” Italy) and colleagues carefully analyze melt inclusions in samples from Mt. Somma-Vesuvius, Italy, to address the behavior of carbon dioxide, water, and sulfur in melt at sub-volcanic conditions [1].






After trapping, melt inclusions can crystallize minerals and form bubble(s) and these modifications can result in an optically dark inclusion difficult to study at room temperature in the lab. By heating the samples, Esposito and colleagues were able to reproduce a melt phase plus a bubble, and then quench these two phases to room temperature and track the behavior of trapped volatiles to study the type of magmatic fluid exsolving from these natural melts. They found that liquid H2O in bubbles of melt inclusions is often isolated as a thin film around the edge of the bubble, and thus can escape detection in standard fluid/melt inclusion analyses.

“When I had the idea of this study, I pictured the melt inclusion as a magma chamber and the host as the country rock surrounding the magma chamber,” said Esposito. “We hypothesized that if we could recreate or approximate in the laboratory the conditions for which silicate melt and the exsolved fluid phase (a mixture prevalently of CO2, H2O and S) are in thermal equilibrium, then we could study the magmatic fluid phase directly developed from these natural melts.”

 “We can, in fact, collect a sample erupted on the Earth’s surface and find crystals containing small and deep droplet of melts,” added Esposito. “As far as we know, our study proved for the first time that H2O is a volatile component present in the fluid exsolving directly from melt inclusion hosted in olivine minerals. Knowing the specific composition of magmatic fluid exsolved from magmas is important because these fluids exert fundamental controls on explosivity of volcanic eruptions, global cycle of volatiles, magma dynamics, heat and mass transport, and ore formation.”

Image: Melt inclusions hosted in a phenocryst from Mt. Somma-Vesuvius. A) Photomicrograph of melt inclusions (dark rounded objects) before heating experiments. B) Photomicrograph of the same melt inclusions after quenching from high temperature. Note that melt inclusions are glass plus bubble(s).   

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