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| Titel |
Aegirine-melt element partitioning and implications for the formation of nepheline syenite REE deposits |
| VerfasserIn |
Charles Beard, Vincent van Hinsberg, John Stix, Max Wilke |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250146274
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| Publikation (Nr.) |
 EGU/EGU2017-10291.pdf |
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| Zusammenfassung |
| Sodic clinopyroxene is a key fractionating phase in alkaline magmatic systems but its impact
on metal enrichment processes, and the formation of REE + HFSE mineralizations in
particular, is not fully understood. Sodic pyroxenes appear to more readily incorporate REE
than their calcic equivalents1. Despite this, melts in evolved alkaline systems can attain high
REE contents, even up to economic levels (e.g. the Nechalacho layered suite in
Canada2).
To constrain the control of pyroxene on REE + HFSE behaviour in alkaline magmas, a
series of internally heated pressure vessel experiments was performed to determine
pyroxene-melt element partitioning systematics. Synthetic trachy-andesite to phonolite
compositions were run water saturated at 650–825ºC with fO2 buffered by ca. 1 bar of H2
(QFM + 1) or by Hm-Mt (QFM +5). Fluorine was added to selected experiments (0.3 to 2.5
wt %) to ascertain its effect on element partitioning. Run products were analysed by EMP for
major elements and LA-ICP-MS for trace elements. Mineral and glass compositions bracket
the compositions of natural alkaline systems, allowing for direct application of our
experimental results to nature.
Our results indicate that REE partitioning systematics vary strongly with pyroxene
composition: Diopside-rich pyroxenes (Aeg5−25) prefer the MREE, medium aegirine
pyroxenes (Aeg25−50) preferentially incorporate the LREE, whereas high aegirine pyroxenes
(Aeg55−95) strongly prefer HREE. REE partitioning coefficients are 0.3–40, typically 2–6,
with minima for high aegirine pyroxenes. Melt composition (e.g. (Na+K)/Al) also impacts
partitioning although to a lesser extent, except for the F-content, which shows no impact at
all. The composition of fractionating pyroxene has a major impact on the REE pattern of the
residual melt, and thus on the ability of a system to develop economic concentrations of the
REE. Element partitioning systematics suggest that late-crystallising aegirine-rich cumulates
would be HREE-rich, in accord with the composition of mineralised intrusions, such as
Nechalacho2.
1 – Marks, M., Halama, R., Wenzel, T. & Markl, G., 2004. Chem. Geol. 211,
185–215.
2 – Möller, V. & Williams-Jones, A. E., 2016. J. Petrology 57, 229-276. |
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