Homogeneous bubble nucleation in rhyolitic magmas:: An experimental study of the effect of H2O and CO2 -: art. no. 2066

[1] Rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal-free rhyolitic liquid at 800 degreesC. Bubble nucleation was produced by lowering the pressure at 1-10 MPa s(-1) from an initial value between 200 and 295 MPa to a final value below the volatile saturation pressure P-Sat. Six volatile compositions with 4.1-7.7 wt % H2O and 10-1200 ppm CO2 were investigated. For each composition we determined the critical pressure P-HoN below which homogeneous nucleation can proceed. The samples quenched below PHoN showed a nucleated core with a large number of uniformly spaced bubbles. With decreasing pressure, bubble number densities increased from <10(11) m(-3) (for samples quenched just below P-HoN) to >10(15) m(-3). The degree of supersaturation required for homogeneous nucleation, DeltaP(HoN) (= P-Sat P-HoN), increased with decreasing H2O content or increasing CO2 content. Huge values of DeltaP(HoN), greater than or equal to135 to 310 MPa, were measured in the H2O-poor compositions (4.1-4.6 wt % H2O; 50-1100 ppm CO2); much lower values from approximate to60 to 160 MPa were obtained in the H2O-rich compositions (7.0-7.7 wt % H2O; 10-630 ppm CO2). The high DeltaP(HoN) in liquids with 4-5 wt % H2O should result in the buildup of large degrees of supersaturation during magma ascent, a very late nucleation event, and a rapid (explosive) vesiculation. By contrast, rhyolitic liquids with much larger water contents have higher saturation pressures and much lower DeltaP(HoN) : bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near-equilibrium degassing.