Kinetics of heterogeneous bubble nucleation in rhyolitic melts: implications for the number density of bubbles in volcanic conduits and for pumice textures

Abstract

International audienceWe performed decompression experiments to simulate the ascent of a phenocryst-bearing rhyolitic magma in a volcanic conduit. The starting materials were bubble-free rhyolites water-saturated at 200 MPa–800°C under oxidizing conditions: they contained 6.0 wt% dissolved H2O and a dense population of hematite crystals (8.7 ± 2 9 105 mm-3). Pressure was decreased from the saturation value to a final value ranging from 99 to 20 MPa, at constant temperature (800C); the rate of decompression was either 1,000 or 27.8 kPa/s. In all experiments, we observed a single event of heterogeneous bubble nucleation beginning at a pressure PN equal to 63 ± 3 MPa in the 1,000 kPa/s series, and to 69 ± 1 MPa in the 27.8 kPa/s series. Below PN, the degree of water supersaturation in the liquid rapidly decreased to a few 0.1 wt%, the nucleation rate dropped, and the bubble number density (BND) stabilized to a value strongly sensitive to decompression rate: 80 mm-3 at 27.8 kPa/s vs. 5,900 mm-3 at 1,000 kPa/s. This behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite-H2O system and in numerical simulations of vesiculation in ascending magmas. Similar degrees of water supersaturation were measured at 27.8 and 1,000 kPa/s, implying that a faster decompression rate does not result in a larger departure from equilibrium. Our experimental results imply that BNDs in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate jdP=dtj and the number density of phenocrysts, especially the number density of magnetite microphenocrysts (1–100 mm-3), which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation. Very low BNDs (&1 mm-3) are predicted in the case of effusive eruptions (jdP=dtj & 0.1 kPa/s). High BNDs (up to 107 mm-3) and bimodal bubble size distributions are expected in the case of explosive eruptions: (1) a relatively small number density of bubbles (1–100 mm-3) will first nucleate in the lower part of the conduit (jdP=dtj & 10 kPa/s), either at high pressure on magnetite or at lower pressure on quartz and feldspar (or by homogeneous nucleation in the liquid) and (2) then, extreme decompression rates near the fragmentation level (jdP=dtj & 103 kPa/s) will trigger a major nucleation event leading to the multitude of small bubbles, typically a few micrometers to a few tens of micrometers in diameter, which characterizes most silicic pumices

Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography

“…Moreover, the vesicle concentration textures we recorded around relict sedimentary fragments and crystals are virtually identical to textures that have been derived from experiments where quartz was heated to magmatic temperatures. In these experimental studies, quartz degassing created a halo of vesicles that attached to the crystal's solid boundaries (Vasiloi et al 1985;Kendrick et al 2006;Cluzel et al 2008), matching our observations on El Hierro xeno-pumice (Figs. 2c and 3c).…”

Volatiles in subduction zone magmatism

“…Exsolution of volatiles requires nucleation of bubbles, and it has been shown previously on the basis of theory and experiment that magnetite crystals constitute prime bubble nucleation sites for exsolving gases (e.g. Hurwitz & Navon 1994;Cluzel et al 2008). In this volume, using mafic enclaves from Montserrat in the Lesser Antilles as an example, Edmonds et al (2014a) for the first time confirm that this process is indeed relevant in natural systems.…”

Trace elements in magnetite from massive iron oxide-apatite deposits indicate a combined formation by igneous and magmatic-hydrothermal processes

“…Due to surface energy reduction, exsolving magmatichydrothermal fluid prefers to nucleate bubbles initially on mineral surfaces, and thus crystallizing magnetite promotes water supersaturation (Hurwitz and Navon, 1994). Owing to larger wetting angles (Ψ) between fluid and oxides (Ψ=45-50°) compared to fluid and silicate minerals (Ψ=5-25°) (Gualda and Ghiorso, 2007) the attachment of bubbles is energetically favored on magnetite microlites (Hurwitz et al 1994;Gardner and Denis, 2004;Cluzel et al 2008), which generates magnetite-bubble pairs (Fig. 12a).…”