Volcanic gases and the reaction of sulfur dioxide with aluminosilicate glasses
Date
2018
Authors
Renggli, Christian Josef
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Canberra, ACT : The Australian National University
Abstract
Volcanic gases are an important part of the volatile cycle in
active planetary systems and contribute significantly to the
mobilization and transport of metals to planetary surfaces. On
Earth, Venus, Mars and Io, SO2 is the most abundant corrosive
species in volcanic gases, and basalts are ubiquitous on these
planetary bodies. The reaction between SO2 and silicate rocks
forms oxidized sulfate and reduced sulfide. This reaction is a
key process in the formation of porphyry deposits. In volcanic
eruption plumes SO2 reacts with volcanic ash and is scavenged
onto the surface of the ash particles. Knowledge of the reaction
mechanisms between volcanic gas and rocks, minerals and glasses,
and processes controlling the metal mobilization and transport in
volcanic gas can constrain models of volatile and metal budgets
of planetary crusts and surfaces.
Using thermochemical modelling, I present a new model for the
composition of volcanic gas on the Moon and compare it to a
terrestrial volcanic gas from Erta Ale volcano (Ethiopia). The
main species in lunar volcanic gas are H2, S2 and CO. This
finding is in contrast to previous studies which suggested that
CO was the sole driver of explosive volcanic eruptions on the
Moon. This lunar volcanic gas has a lower capacity for metal
transport compared to the Cl- and H2O-rich volcanic gas from Erta
Ale volcano.
To identify how SO2-glass reactions occur at high temperature and
to investigate what might promote and limit these reactions, I
present results from an experimental study. Pure SO2 was reacted
with silicate glasses in the system anorthite-diopside-albite and
with Fe-bearing natural basaltic glasses.
The sulfate reaction products are relatively enriched in Ca
compared to the silicate glass composition, in particular in
experiments with Fe-free anorthite-diopside glasses. On these
Fe-free glasses CaSO4 is the sole observed phase in the coatings
at 800 °C, whereas at 600 °C minor amounts of MgSO4 were
detected. At 800 °C, the flux of Ca from the silicate glass to
the surface exceeds that of Mg by a factor of up to 330, whereas
at 600 °C this factor is only 3. The rate of reaction is not
constant, decreasing by an order of magnitude from 1 h to 24 h at
800 °C.
The reaction of SO2 with tholeiitic basalt glasses produces
coatings of CaSO4, MgSO4, Na2SO4 and oxides including Fe2O3 and
TiO2. In addition, the reaction modifies the basalt glass because
Ca, Mg and Na are lost to the coating. This results in the
nucleation of crystalline spherulites and needles including SiO2,
Al2O3, as well as Fe-Na-rich and Mg-rich pyroxenes.
VIII
The results suggest that the structural properties of the
silicate glass substrate control the diffusive transport of Ca,
Na, Mg, Fe and Ti to the surface which in turn controls the
overall reaction rate and the formation of sulfates, oxides and
silicates. These findings can be applied to predicting reactions
on planetary surfaces and at shallow levels within their crusts.
Description
Keywords
Volcanism, volcanic gas, experimental petrology, sulfur dioxide, SO2, gas-solid reactions, Moon, lunar volcanism, volatiles, metal transport, Erta Ale, planetary science
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DOI
10.25911/5d5142f0d9852