Abstract
We 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 × 105 mm−3). Pressure was decreased from the saturation value to a final value ranging from 99 to 20 MPa, at constant temperature (800°C); 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 P N 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 P N, 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 \( \left| {\left. {{\text{d}}P/{\text{d}}t} \right|} \right. \) 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 (\( \left| {\left. {{\text{d}}P/{\text{d}}t} \right|} \right. \) ≈ 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 (\( \left| {\left. {{\text{d}}P/{\text{d}}t} \right|} \right. \) ≈ 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 (\( \left| {\left. {{\text{d}}P/{\text{d}}t} \right|} \right. \) ≈ 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.
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Acknowledgments
Our research program on bubble nucleation was supported by grants from the Institut National des Sciences de l’Univers (ACI “Dynamique éruptive des volcans antillais” 2004–2006) and from the Agence Nationale de la Recherche (ANR-EXPLANT, contract No. ANR-05-CATT-003 to C. Martel). It benefited from discussions with Hélène Massol, Morihisa Hamada, Tim Druitt, Thomas Giachetti, and Caroline Martel. We are grateful to Jean-Luc Devidal for technical assistance with the electron microprobe, Jean-Marc Hénot for assistance with the scanning electron microscope, and Nathalie Bolfan-Casanova and Ken Koga for assistance with the FTIR spectrometer.
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Cluzel, N., Laporte, D., Provost, A. et al. Kinetics of heterogeneous bubble nucleation in rhyolitic melts: implications for the number density of bubbles in volcanic conduits and for pumice textures. Contrib Mineral Petrol 156, 745–763 (2008). https://doi.org/10.1007/s00410-008-0313-1
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DOI: https://doi.org/10.1007/s00410-008-0313-1