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| Titel |
Laboratory evidence of fragmentation during slow decompression of a magma analogue containing volatiles and solid particles. |
| VerfasserIn |
Eleonora Rivalta, Karen Pascal, Jeremy Phillips, Alessandro Bonaccorso |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035872
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| Zusammenfassung |
| With the motivation of studying switches in the eruption regime at Stromboli volcano, we
performed a series of shock-tube experiments decompressing Gum Rosin dissolved in
acetone (GRA mixture) as a volatile-bearing analog of magma, obtaining for the
first time evidence of fragmentation of a natural system containing solved volatiles
and solid particles during slow decompression. We exposed the magma analog to
sudden decompression and to slow decompressions of the order of about 100-400 Pa
s-1 from atmospheric pressure patm down to different pressures pL. We used five
different concentrations of acetone in the mixture: 15%, 23%, 30%, 35%, 40%. This
corresponds to about 2 - 6% H2O in magma. We use our sudden decompression
experiments to draw a phase diagram of our mixture. During fast decompression, we
first observe bubble nucleation for pL - 25 kPa. For 13 kPa < pL < 18 kPa we
observe progressively more intense boiling of acetone in the mixture. If the mixture is
decompressed down to pL = 5 - 12 kPa, we observe slow expansion of foam
(velocity of the order of a few mm-cm per second). If pL < 5 kPa, we observe
fragmentation (velocity of expansion is tens of meters per second). Mixtures of different
concentrations show somewhat different values of the maximum pressure at which
fragmentation is observed. During slow decompression, in general we observe a similar
behavior, albeit with slightly different threshold values of pL, except that in general
fragmentation does not occur at all. However, in some cases we surprisingly do
not observe any bubble nucleating around the boiling point of acetone. In those
cases, fragmentation occurs when pressure reaches about pL = 8 - 10 kPa. The
mixture apparently becomes supersaturated even if small rosin particles, which
should ease nucleation, populate the mixture, as we could ascertain observing the
samples at the microscope. Fragmentation events during slow decompression occurred
only - but not always -when decompressing the mixtures at 35% and 40%, while
we never observed it for mixtures of 30% concentration or below. Using the 40%
mixture, fragmentation occurred about in half of the cases (4 times out of 8), for the
35% it occurred in one case out of four. In conclusion, fragmentation during slow
decompression occurs when the gas content is high (or when the content in particles is
relatively low). A pressure threshold exists, which is about half of the pressure of the
acetone’s boiling point and might mark the onset of homogeneous nucleation. This
process seems to be highly non-linear since different runs of the experiment in
approximately the same conditions lead to different results. Further studies are necessary
in order to constrain better the conditions promoting fragmentation during slow
decompression and to make inferences on the dynamics of the plumbing system at Stromboli
volcano. |
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