Rapid pre-eruptive mush reorganisation and atmospheric volatile emissions from the 12.9 ka Laacher See eruption, determined using apatite

Magma is commonly thought to be stored as a crystal-rich mush within vertically extensive, crustal storage regions, but how to remobilise and erupt such crystal-rich material is a key problem in volcanology. An important question is whether the growth of gas bubbles within the mush could promote remobilisation. In order to investigate this, we need improved constraints on the timing of volatile saturation in magmas. The mineral apatite represents a potentially useful record of pre-eruptive magmatic volatiles, but data interpretation is complex because exchange reactions control the volatile partitioning. Here we present a numerical forward modelling program that allows the identification of trends in populations of apatite crystals. The program also includes a sensitivity analysis, that helps to identify alternative sets of starting parameters that can reproduce the trends. The model is applied to a new dataset of volatiles in apatite from the 12.9 ka Laacher See eruption, Eifel volcanic region, Germany. The results indicate that the magma was initially strongly volatile-undersaturated and became saturated (reached second boiling) through progressive crystal fractionation. Lack of apatite-melt equilibrium indicates dispersal of crystals into different chemical environments, and consideration of apatite diffusivity suggests that this reorganisation occurred shortly before eruption. The model results also allow us to constrain directly the amount of pre-eruptive magmatic vapour that was emitted during the explosive eruption, highlighting the importance of considering the behaviour of halogens during magma storage. Overall, our approach confirms the value of measuring apatite volatile contents and highlights the potential of this method to provide quantitative constraints on magmatic evolution and storage conditions.