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| Titel |
Volcanic controls on ash iron solubility: thermodynamic modeling of gas-ash interaction in the hot core of volcanic plumes |
| VerfasserIn |
G. Hoshyaripour, M. Hort, B. Langmann |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250059830
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| Zusammenfassung |
| Recently it has been shown that volcanic ash can act as a fertilizer for phytoplankton bloom
by injecting bio-available iron into the surface ocean. However, it is also well known
that iron in volcanic ash at least at its generation point (i.e. magma) is mostly in
insoluble form, i.e. not bio-available. Although different volcanic and atmospheric
processes are assumed to contribute to the transformation of insoluble iron into soluble
salts, the causes of iron mobilization in volcanic ash are poorly constrained. Here
we explore the volcanic control on the mobilization of iron in volcanic ash in the
hot core of volcanic plumes (T>600Ë C) based on thermodynamic equilibrium
considerations. A conceptual box model is considered for the hot core in which 1000Ë C
magmatic gas, ash and 25Ë C ambient air are mixed. The initial composition of
volcanic gas and ash are parameterized based on three types of tectonic settings
(convergent plate, divergent plate, and hot spot) and basaltic to rhyolitic magmas.
The effect of the initial oxidation state is also considered by changing the oxygen
fugacity.
First, magmatic oxides (i.e. SiO2, FeO, MgO etc) are titrated into the magmatic gas at
constant temperature and fugacity in order to generate the initial iron carrying minerals. Since
the alteration of ash composition is mainly diffusion controlled, we assume that inside the hot
core of the volcanic plume the Fe speciation is only affected at or near to the ash surface.
Results show that the main initial iron carrying minerals are usually ilmenite and fayalite with
some addition of pyhrrotite at reduced conditions in divergent plate and hot spot
settings.
Then the 1000Ë C magmatic gas-ash mixture is mixed with the 25Ë C air (N2 79%, O2
21%) until a temperature of 600Ë C is reached. Results demonstrate that the hot core
functions as an oxidizing reactor for the ash surface transforming the whole Fe2+ minerals to
Fe3+ species while being cooled to 600Ë C. However, in reduced scenarios in divergent plate
and hot spot volcanoes the iron remains in Fe2+ form (i.e. fayalite, pyhrrotite). Under such
conditions bio-available iron production appears more likely because iron is in a more soluble
oxidation state (i.e. Fe2+). The role of the “ash iron” in sulfur and halide scavenging is
negligible but alkali metals and also Ca can scavenge up to 37% of the erupted
sulfur.
Albeit the hot core does not produce any bio-available iron directly (e.g. chloride,
fluoride, sulfate), the oxidation state of the iron at or in the vicinity of the surface of volcanic
ash can significantly control mobilization processes in the colder parts of the volcanic plume.
Divergent plate and hot spot settings seem to be more favorable for the iron fertilization but
the atmospheric controls need to be studied too. |
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