Homogeneous bubble nucleation in rhyolitic magmas: An experimental study of the effect of H 2 O and CO 2

Rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal‐free rhyolitic liquid at 800°C. 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 % H 2 O and 10–1200 ppm CO 2 were investigated. For each composition we determined the critical pressure P HoN below which homogeneous nucleation can proceed. The samples quenched below P HoN 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, Δ P HoN (= P Sat − P HoN ), increased with decreasing H 2 O content or increasing CO 2 content. Huge values of Δ P HoN , ≥135 to 310 MPa, were measured in the H 2 O‐poor compositions (4.1–4.6 wt % H 2 O; 50–1100 ppm CO 2 ); much lower values from ≈60 to 160 MPa were obtained in the H 2 O‐rich compositions (7.0–7.7 wt % H 2 O; 10–630 ppm CO 2 ). The high Δ P HoN in liquids with 4–5 wt % H 2 O 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 Δ P HoN : bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near‐equilibrium degassing.