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| Titel |
Experimental calibration of a new oxybarometer for silicic magmas based on the partitioning of vanadium between magnetite and silicate melt |
| VerfasserIn |
Róbert Arató, Andreas Audétat |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250132330
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| Publikation (Nr.) |
 EGU/EGU2016-12829.pdf |
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| Zusammenfassung |
| Oxygen fugacity is an important parameter in magmatic systems that affects the stability of
mineral phases and fluid species. However, there is no well-established method to
reconstruct the oxygen fugacity of slowly cooled magmas such as granite, for example,
because existing oxybarometers (e.g., magnetite-ilmenite method) are susceptible to
re-equilibration processes during slow cooling and thus lead to erroneous results when
applied for granitic rocks. In this study, we aim at developing an oxybarometer
that is based on the partitioning of vanadium (a redox-sensitive element) between
magnetite inclusions and silicate melt inclusions preserved in quartz phenocrysts, where
they were protected from subsolidus alteration and can be measured as entities by
LA-ICP-MS.
In the first – experimental – part of this study we investigated the effects of temperature
(800-950 ˚ C), pressure (1-2 kbar), oxygen fugacity (from ΔFMQ+0.7 to ΔFMQ+4.0),
magnetite composition, and melt composition on the partition coefficient of vanadium
between magnetite and melt (DVmgt-melt). The experiments were carried out in cold-seal
pressure vessels and the starting material was a mixture of V-doped haplogranite glasses or
natural obsidian powder with variable aluminum saturation index (ASI), and synthetic,
V-free magnetite of 10-20 μm grain size. The vanadium partition coefficient was
found to depend strongly on oxygen fugacity, and to lesser (but still considerable)
degrees on melt composition and temperature. A more than 1.5 log unit decrease in
DVmgt-melt values with increasing oxygen fugacity can be explained by a change of the
dominant valence state of V in the silicate melt. For a given oxygen fugacity buffer
DVmgt-melt decreases with increasing temperature, but this reflects mostly the
change in absolute fO2 values while the net temperature effect is in fact positive.
DVmgt-melt depends significantly on melt composition, resulting in higher D-values with
increasing aluminum saturation index (ASI). This seems to reflect less favorable
incorporation of V into peraluminous melts compared to depolymerized, peralkaline
melts. Changing pressure from 1 to 2 kbar had an effect only at NNO, causing
0.3 log unit increase in D, whereas the Ti-content of magnetite turned out to have
negligible effect on the V partitioning. In summary, the dependence of DVmgt-melt on
temperature, ASI and oxygen fugacity can be described by the following regression
equation:
logD(V)mgt∕melt=–1.22+0.31*10^5/T(˚ K) +1.73*ASI –0.49*ΔFMQ
First tests of the equation on natural samples were carried out on rapidly cooled tuffs and
vitrophyres from variable tectonic settings, for which fO2 could be constrained
independently by the magnetite-ilmenite method. All calculated fO2 values fall within ±
0.75 log unit within those suggested by the Fe-Ti oxybarometer, whereas 12 out of 16
samples agree within 0.5 log units . |
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