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| Titel |
Dissolved H2O distribution in vesicular magmatic glass records quench history and challenges emerging paradigm |
| VerfasserIn |
Iona McIntosh, Ed Llewellin, Madeleine Humphreys, Alain Burgisser, Ian Schipper, Alex Nichols |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250074644
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| Zusammenfassung |
| Volcanic eruptions are driven by the nucleation and growth of bubbles in magma. Bubbles
grow as volatile species in the melt, of which water is volumetrically the most important,
diffuse down a concentration gradient towards and across the bubble wall. On cooling, the
melt quenches to glass, preserving the spatial distribution of water concentration around
the bubbles (now vesicles). We use Backscatter Scanning Electron Microscopy
(BSEM), Secondary Ion Mass Spectrometry (SIMS) and Fourier Transform Infra-Red
spectroscopy (FTIR) to measure the spatial distribution of water around vesicles
in naturally vesicular pyroclasts and in experimentally-vesiculated samples, with
unprecedented spatial resolution. We find that, contrary to expectation, the water
concentration increases (by up to 3wt.%) in the ~30 microns closest to the vesicle
wall.
Our samples, both natural and experimental, record significant resorption of water back
into the melt around bubbles during the quench process. We propose that the observed
resorption profiles result from the increase in the equilibrium solubility of water as
temperature decreases during the quench to glass, and that the resorption locally
overprints any pre-existing concentration profile resulting from bubble growth during
eruption/decompression. Our experimental samples demonstrate that the bulk of the
resorption occurs above the glass transition, while the melt is still plastic; consequently,
resorption may reduce bubble volumes and sample porosities by as much as a factor of two.
Experimental samples are quenched too rapidly (1-5 seconds) for observed resorption
profiles to be generated by diffusion of ‘total’ water (H2Ot). Speciation data showing
molecular (H2Om) and hydroxyl (OH) water concentrations around vesicles in these
samples reveal that quench resorption is driven by rapid, disequilibrium diffusion of
H2Om.
Our work lays the foundations for a new tool for the interpretation of the
pressure-temperature history of natural pyroclasts, and challenges the conclusions of two
recent studies which interpret similar features as evidence of re-pressurization of
magma in the shallow subsurface, prior to eruption. Our data also demonstrate that
the diffusion of water in magmatic glass can, under the appropriate conditions, be
orders-of-magnitude faster than is usually assumed, and that the failure of previous
experimental studies of bubble growth to account for resorption may have led to incorrect
conclusions. |
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