We report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release. Two rhyolitic obsidians, one containing less than 1% of microlites of Fe-Ti oxides and the other about 20% of various crystals were hydrated at 150 MPa and 780-850-degrees-C. After saturation was reached (5.3-5.5 wt% water), pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time, before the final, rapid quenching of the experiments. The results demonstrate the importance of heterogeneous nucleation. Microlites of Fe-Ti oxides are very efficient as sites for bubble nucleation. In their presence, modest nucleation was observed even after decompression by < 1 MPa, and decompression of more than 5 MPa produced extensive nucleation (10(6)-10(8) bubbles cm-3). In the absence of microlites, no nucleation occurred at DELTAP < 10 MPa. At DELTAP > 10 MPa, bubbles also nucleated on crystals of biotite, zircon and apatite. Modest nucleation (10(3) - 10(5) cm-3) took place even in crystal-free samples, but it was still heterogeneous. When DELTAP exceeded 80 MPa, nucleation in crystal-free samples became extensive (10(5)-10(7) cm-3). The lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous. Nucleation rates were controlled mainly by the availability of sites. Rates were faster than 10(6) cm-3 s-1 when microlites were present, and faster than 10(5) cm-3 s-1 in the absence of microlites at DELTAP > 70 MPa. Narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop. The experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation. When a large number of efficient sites (e.g., Fe-Ti oxide) are present, bubble nucleation requires very little supersaturation, and to a good approximation, gas and magma are in equilibrium. In magmas that are crystal-free or contain crystals that are inefficient at nucleating bubbles, very high degrees of supersaturation are required in order to initiate nucleation. These two modes of exsolution may lead to contrasting styles of convection, pressure build up and eruption.
