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| Titel |
Effect of fO2 on phase relationship in basaltic andesites during magmatic differentiation: Control of fO2 and sulphur speciation in piston cylinder experiments. |
| VerfasserIn |
Vladimir Matjuschkin, Brian Tattitch, Jonathan D. Blundy, Susanne Skora |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094699
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| Publikation (Nr.) |
 EGU/EGU2014-10128.pdf |
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| Zusammenfassung |
| Within the mantle wedge above subduction zones, oxidation reaction take place by
interaction of reduced mantle rocks with more oxidized, hydrous fluids, which can cause a
local drop of the solidus, resulting in partial melting (2,6,7). The resultant melts are more
oxidized that their ocean floor counterparts, which has implications for their subsequent
differentiation paths, the speciation of multivalent elements and the solubility and transport of
chemical compounds in magmatic systems (1,4,5).
We present a series of sulphur-doped high-pressure experiments conducted to investigate
the effect of oxygen fugacity on phase relationships and the behaviour of sulphur in silicate
melts. Natural aphyric andesite (FM37) erupted from Laguna del Maule volcano, Chile (3)
was selected as a starting composition. Experiments were carried out at 5kbar, 950-1150°C
and variable oxygen fugacity conditions. New experiments buffered at Co-CoO
and Ni-NiO buffer conditions have been performed using a new "MTB capsule
design" developed in order to accurately control fO2 by means of a double capsule
containing metal-oxide buffers and a pyrex sleeve to minimise H2 diffusion. This new
design constrains oxygen fugacity to within ±0.1-0.2logfO2 units of the target value.
Before conducting these experiments, the assemblage was tested multiple times
at 10kbar, 1000°C over 24-48 hours and demonstrated consistent, accurate fO2
control.
Analyses of the preliminary experimental run products, from a related Chilean basaltic
andesite starting composition, demonstrate a clear effect of fO2 on phase relationships and
the proportion of melt generated during experiments. Under oxidized conditions, as
temperature decreased from 1150°C to 1050°C, the amount of melt decreased from 100%
to ~80%, due to the formation of orthopyroxene, anhydrite and plagioclase. In contrast, in
reduced runs the system remains nearly liquid (~5% crystals) down to 950°C due to the
change in sulphur speciation and onset of orthopyroxene precipitation at much lower
temperatures. The change in temperature from 1150 to 950°C resulted in a drop in S content
from ~2500ppm to ~1000ppm in the melt for oxidized experiments, whereas S slightly
increased from ~3000 to ~3500ppm in the reduced experiments. Quantitative control over
fO2 will allow for more precise determination of phase relations and control of sulphur
specification, offering a possibility of detailed reconstruction of metals enrichment in silicate
melts.
Cited references:
[1] Botcharnikov et al. (2011) Nature 4:217-230, [2] Foley (2011) J Petrol 52:1363-1391,
[3] Frey et al. (1984) CMP 88:133-149, [4] Jenner et al. (2010) J Petrol 51:2445-2464,
[5] Jugo et al. (2010) GCA 74:5926-5938, [6] Rohrbach et al. (2007) Nature
449:456-458,
[7] Taylor and Green (1988) Nature 332:349-352 |
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