Publication Date:
2017-04-04
Description:
Fluorine and chlorine diffusion were measured in two natural phonolitic melts, from Vesuvius (Italy) and from
Laacher See (Germany), at 0.5 and 1.0 GPa, between 1250 and 1450 °C at anhydrous conditions and with about 2
and 5 wt.% of dissolvedwater. The two different startingmaterials allowus to investigate the alkali effect,Na vs. K,
on halogen diffusion.One compositionwas a K-rich (~10wt.%) phonoliticmelt corresponding to thewhite pumice
phase of the 79ADeruption of Vesuvius, and the other aNa-rich (~10 wt.%) phonoliticmelt corresponding tomost
differentiated melt of the 12,000 BC eruption of Laacher See. The diffusion-couple technique in a piston cylinder
was used for the experiments. Experiments were performed with only one halogen diffusing and with the
simultaneous diffusion of a halogenmixture (F, Cl, Br) in order to evaluate the interactions between the halogens
during diffusion. Diffusion coefficients for F range between 2×10−11m2/s at 1250 °C and 7×10−11m2/s at 1450 °C
for the Na-rich melt and between 1×10−11 m2/s at 1250 °C and 8×10−11 m2/s at 1450 °C for the K-rich melt at
anhydrous conditions. Diffusion coefficients for Cl range between 2×10−12 m2/s at 1250 °C and 1×10−11 m2/s at
1450 °C for theNa-richmelt and between 7×10−12m2/s at 1250 °C and 2×10−11m2/s at 1450 °C for the K-richmelt
at anhydrous conditions. Fluorine diffusivity is higher than Cl in the Na-rich-phonolitic melt by one order of
magnitude,whereas in the K-rich-phonoliticmelt F and Cl diffusivities are similar. The effect ofwater is significant
for Cl in both Na-rich and K-rich melts: the addition of water enhances Cl diffusivity by up to one order of
magnitude, butwater does not significantly affect F diffusion. F and Cl diffusivities always differ fromone another
in the same phonoliticmelt composition. F diffusivities are similar in both compositions. Conversely, Cl diffusion
depends upon the dominant alkali. These results evidence that halogen diffusivitymay represent a limiting factor
for their degassing during rapid syneruptive decompression and vesiculation of H2O-rich-phonolitic melts. The
contrasting volatile diffusivities of F and Cl in silicate melts duringmagma vesiculation may be a key, controlling
factor of the composition of the vapour phase (bubbles) produced. Such diffusion controlled degassingmodelmay
explain the absence of F and Cl degassing observed during the 79AD eruption of Vesuvius.
Description:
IPGP contribution: 2364; Geotop contribution:
2008-0029.
Description:
In press
Description:
2.3. TTC - Laboratori di chimica e fisica delle rocce
Description:
JCR Journal
Description:
reserved
Keywords:
Diffusio
;
Halogenes
;
04. Solid Earth::04.08. Volcanology::04.08.02. Experimental volcanism
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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