Magma Ascent Timescales in Basaltic Explosive Eruptions: Constraints from Decompression Experiments

International audienceWe performed high pressure and temperature decompression experiments to constrain the conditions of ascent and degassing of volatile-bearing basaltic melts prepared from Stromboli pumice. Experiments were conducted in an internally heated pressure vessel between NNO-1.4 and +4.3, and so different amounts of Fe-Ti oxides were present. Melts with volatile (H2O, CO2, S) contents in the range of Stromboli melt inclusions were synthesized at 1200°C and 200 MPa, continuously decompressed between 200 and 25 MPa at 39 and 78 kPa/s (or 1.5 and 3 m/s), and rapidly quenched. Run products were characterized both texturally (by X-ray microtomography and scanning electron microscopy) and chemically (by IR spectroscopy and electron microprobe analysis), and then compared with Stromboli pumice products. In CO2-bearing basaltic melts, bubbles start to nucleate heterogeneously on Fe sulfides and Fe-Ti oxides for supersaturation pressures ΔPHeN ≤ 1-100 MPa and to nucleate homogeneously for ΔPHoN ≤ 50-100 MPa (ΔPHeN and ΔPHoN are the difference between the saturation pressure and the pressure at which heterogeneous and homogeneous bubble nucleation are observed, respectively). Limited bubble growth, coalescence and outgassing occur in addition to continuous bubble nucleation, which is driven by the generation of CO2 supersaturated melts. The results demonstrate that heterogeneous bubble nucleation is limited in common basaltic melts and that bubble nucleation should be for the most part homogeneous. Additionally, they have implications for the interpretation of textural (bubble number densities, shapes, sizes, and distributions) and chemical data (residual volatile concentrations, volcanic gases) on explosive basaltic volcanoes. This has allowed to estimate ascent timescales from 43 to 128 min for the Stromboli magmas emitted as pumices during paroxysms, corresponding to ascent rates of 1-3 m/s