From Magma to Atmosphere: Halogen Fluxes During Volcanic Eruptions

Volcanic eruptions release significant quantities of volatile elements into the atmosphere and stratosphere. Plinian eruptions are particularly efficient at dispersing these elements due to their large magma volumes and high eruption columns. Halogens (F, Cl, Br) are among the most reactive volatile species and can strongly affect the atmospheric composition. However, their fluxes from magma to the atmosphere remain poorly constrained because of their low concentrations in volcanic products, which makes direct measurement challenging. This study presents direct microanalytical measurements (electron and ion microprobe analysis) performed on melt inclusions trapped in minerals—representing pre‐eruptive conditions—and bulk measurements on emitted pyroclasts—reflecting post‐eruptive compositions. We investigated the degassing behavior of halogens (F, Cl, Br) alongside major volatiles (H 2 O, CO 2 , S) in a series of Plinian eruptions from various latitudes and with diverse magma compositions. Our results indicate that H 2 O is the dominant volatile component (16–29 Mt/km 3 ), followed by CO 2 (0.07–2.7 Mt/km 3 ). Estimated halogen fluxes to the stratosphere are approximately 0.1 Mt/km 3 for F, 1 Mt/km 3 for Cl, and 0.01 Mt/km 3 for Br, while sulfur (S) varies but is generally below 0.5 Mt/km 3 , except in basaltic eruptions. These represent minimum values, as an additional fluid phase (vapor, brine) may have been exsolved in the reservoir prior to eruption and contributed to the total volatile release. Cl/Br ratios in magmas are not constant and vary with magma source. Consequently, indirect estimates of Br fluxes based on Cl data can lead to substantial biases in Br concentration (over or underestimation).