Bubble nucleation in basaltic melts: results of high temperature decompression experiments
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Abstract
VUELCO Conference, 2013, Barcelona (Spain)
Previous experiments have demonstrated that mechanisms of basalt degassing are strongly contrasted when gas bubbles are present or when they are absent. Consequently, experimental information on the kinetics of bubble nucleation in basaltic melts is needed, and high temperature decompression experiments have been investigated.
All experiments used basaltic pumice from Stromboli as starting material. The sample was fused in air at 1400°C for 3h and then cylinders were cored. Synthesis experiments were performed to produce the volatile-bearing melts to be used in the decompression experiments. Glass cores, distilled H2O and Ag2C2O4 were loaded in AuPd capsules (3 different dissolved H2O/CO2 were introduced). The synthesis experiments were ran at 1200°C during about 40h in an internally heated pressure vessel. At the end, glasses were cut in 2 parts: one for the decompression experiments and the other for the analysis. Decompression experiments were conducted at a fast rate of 39kPa/s and 1200°C, from an initial pressure (Pin) of 200 MPa and to final pressures (Pf) of 200, 150, 100, 50 and 25 MPa. Charges were rapid-quenched immediately after attainment of Pf. Textures were analyzed by X-ray microtomography, and volatile concentrations and spatial distributions in pre- and post-decompression glasses were determined by FTIR.
Pre-decompression glasses have homogeneous volatile contents and distributions: group #1 (average H2O content = 4.82 wt.%, average CO2 content = 0 ppm), group #2 (2.15 wt.%, 883 ppm), group #3 (0.82 wt.%, 849 ppm).
Textural characteristics (vesicularities, bubble numbers, densities and sizes) suggest that homogeneous bubble nucleation occurs between 50 and 25 MPa.
Decompression experiments show that equilibrium (groups #1 and 3) and disequilibrium degassing paths (group #2) can be obtained and that final glass compositions can be CO2 supersaturated.
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