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| Titel |
Apatite: a new redox proxy for silicic magmas? |
| VerfasserIn |
Andrew Miles, Colin Graham, Chris Hawkesworth, Martin Gillespie, Geoff Bromiley, Richard Hinton |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104337
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| Publikation (Nr.) |
 EGU/EGU2015-3759.pdf |
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| Zusammenfassung |
| The oxidation states of magmas provide valuable information about the release and speciation
of volatile elements during volcanic eruptions, metallogenesis, source rock compositions,
open system magmatic processes, tectonic settings and potentially titanium (Ti) activity in
chemical systems used for Ti-dependent geothermometers and geobarometers. In this
presentation we explore the use of Mn in apatite as an oxybarometer in intermediate and
silicic igneous rocks. Increased Mn concentrations in apatite in granitic rocks from the
zoned Criffell granitic pluton (southern Scotland) correlate with decreasing Fe2O3
(Fe3+) and Mn in the whole-rock and likely reflect increased Mn2+/Mn3+and greater
compatibility of Mn2+ relative to Mn3+ in apatite under reduced conditions. Fe3+/Fe2+
ratios in biotites have previously been used to calculate oxygen fugacities (fO2)
in the outer zone granodiorites and inner zone granites where redox conditions
have been shown to change from close to the magnetite-hematite buffer to close to
the nickel-nickel oxide buffer respectively[1]. This trend is apparent in apatite Mn
concentrations from a range of intermediate to silicic volcanic rocks that exhibit
varying redox states and are shown to vary linearly and negatively with log fO2, such
that
logfO2=-0.0022(±0.0003)Mn(ppm)-9.75(±0.46)
Variations in the Mn concentration of apatites appear to be largely independent of
differences in the Mn concentration of the melt. Apatite Mn concentrations may therefore
provide an independent oxybarometer that is amenable to experimental calibration, with
major relevance to studies on detrital mineral suites, particularly those containing a record
of early Earth redox conditions, and on the climatic impact of historic volcanic
eruptions[2].
[1] Stephens, W. E., Whitley, J. E., Thirlwall, M. F. and Halliday, A. N. (1985) The
Criffell zoned pluton: correlated behaviour of rare earth element abundances with isotopic
systems. Contributions to Mineralogy and Petrology, 89, 226–38.
[2] Miles, A.J., Graham, C.M., Hawkesworth, C.J., Gillespie, M.R., Hinton, R.W., 2014,
Mn in apatite: A new redox proxy for silicic magmas?: Geochimica et Cosmochimica Acta v.
132, p. 101-119. |
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