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To testmechanisms of basalticmagma degassing,\ud continuous decompressions of volatile-bearing (2.7–3.8 wt%\ud H2O, 600–1,300 ppm CO2) Stromboli melts were performed\ud from 250–200 to 50–25 MPa at 1,180–1,140 C.Ascent rates\ud were varied from 0.25 to *1.5 m/s. Glasses after decompression\ud show a wide range of textures, from totally bubblefree\ud to bubble-rich, the latter with bubble number densities\ud from 104 to 106 cm-3, similar to Stromboli pumices. Vesicularities\ud range from 0 to *20 vol%. Final melt H2O concentrations\ud are homogeneous and always close to solubilities.\ud In contrast, the rate of vesiculation controls the finalmelt CO2\ud concentration. High vesicularity charges have glass CO2\ud concentrations that follow theoretical equilibrium degassing\ud paths, whereas glasses from low vesicularity charges show\ud marked deviations from equilibrium, with CO2 concentrations\ud up to one order of magnitude higher than solubilities.\ud FTIR profiles and maps reveal glass CO2 concentration gradients\ud near the gas–melt interface. Our results stress the\ud importance of bubble nucleation and growth, and of volatile\ud diffusivities, for basaltic melt degassing. Two characteristic\ud distances, the gas interface distance (distance either between\ud bubbles or to gas–melt interfaces) and the volatile diffusion\ud distance, control the degassing process. Melts containing\ud numerous and large bubbles have gas interface distances\ud shorter than volatile diffusion distances, and degassing proceeds\ud by equilibrium partitioning of CO2 and H2O between\ud melt and gas bubbles. For melts where either bubble nucleation\ud is inhibited or bubble growth is limited, gas interface\ud distances are longer than volatile diffusion distances.\ud Degassing proceeds by diffusive volatile transfer at the gas–\ud melt interface and is kinetically limited by the diffusivities of\ud volatiles in the melt. Our experiments show that CO2-oversaturated\ud melts can be generated as a result of magma\ud decompression. They provide a new explanation for the\ud occurrence of CO2-rich natural basaltic glasses and open new\ud perspectives for understanding explosive basaltic volcanism

Magma crystallisation is a fundamental process driving eruptions and controlling the style of volcanic activity. Crystal nucleation delay, heterogeneous and homogeneous nucleation and crystal growth are all time-dependent processes, however, there is a paucity of real-time experimental data on crystal nucleation and growth kinetics, particularly at the beginning of crystallisation when conditions are far from equilibrium. Here, we reveal the first in situ 3D time-dependent observations of crystal nucleation and growth kinetics in a natural magma, reproducing the crystallisation occurring in real-time during a lava flow, by combining a bespoke high-temperature environmental cell with fast synchrotron X-ray microtomography. We find that both crystal nucleation and growth occur in pulses, with the first crystallisation wave producing a relatively low volume fraction of crystals and hence negligible influence on magma viscosity. This result explains why some lava flows cover kilometres in a few hours from eruption inception, highlighting the hazard posed by fast-moving lava flows. We use our observations to quantify disequilibrium crystallisation in basaltic magmas using an empirical model. Our results demonstrate the potential of in situ 3D time-dependent experiments and have fundamental implications for the rheological evolution of basaltic lava flows, aiding flow modelling, eruption forecasting and hazard management.