Publication Date:
2017-04-04
Description:
We report results from geochemical and isotopic analyses conducted on nine samples collected from the vertical
section of a pahoehoe lava flow unit from Mt. Etna volcano. Textural observations in the field show that,
during lava emplacement, volatile exsolution and degassing allowed the nucleation and growth of bubbles,
which subsequently rose from the base towards the uppermost lava crust. The amount of phenocrysts is
low (about 12 vol.%) and their compositions are comparable to intratelluric crystals analyzed for from historical
and recent products. In contrast, from the basal zone to the uppermost lava crust, groundmass microlites
are characterized by progressively more primitive compositions, i.e., olivines and clinopyroxenes show increasing
Mg#, plagioclases are enriched in anorthite, and the ulvospinel content in titanomagnetites increases.
Calculations based on thermodynamic models, thermometers and oxygen barometers indicate
that, during post-eruption conditions, crystals formed at higher crystallization temperatures with increasing
vertical height. The redox state of the melt progressively increased from the base towards the uppermost
crust of the lava flow as is attested by increasing Fe2O3/FeO ratios in clinopyroxene and titanomagnetite.
The lowest fO2 is recorded at the basal zone and suggests that this part of the lava was inaccessible to diffusion
of atmospheric oxygen at the time of emplacement; whereas, the highest fO2 measured for the uppermost
lava crust testifies to post-eruptive hydrogen loss, mainly transported by carrier gas phases.
Whole-rock analyses of lava samples also indicate higher Fe2O3/FeO ratios with increasing vertical height
as well as substantial enrichments in MgO and transitional elements. Results from both mass balance and
Rayleigh fractionation calculations show that about 6 vol.% of titanomagnetite and olivine microlites accumulated
in the upper parts of the lava flow during emplacement. At the same time, due to progressive volatile
exsolution, concentrations of Cl, F and Cs in whole rock decreased from the base towards the uppermost
lava crust; indeed, oxygen isotopes indicate a substantial 22 wt.% of CO2 degassing. From this, we conclude
that even short-term exposure to post-eruptive conditions results in significant local compositional changes
for lava flows at Mt. Etna volcano.
Description:
Published
Description:
115-127
Description:
2R. Laboratori sperimentali e analitici
Description:
JCR Journal
Description:
restricted
Keywords:
Degassing
;
04. Solid Earth::04.08. Volcanology::04.08.03. Magmas
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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