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| Titel |
The replacement of plagioclase by albite in hydrothermal experiments: the replacement mechanism and element mobilisation |
| VerfasserIn |
J. Hövelmann, A. Putnis, T. Geisler, B. C. Schmidt, U. Golla-Schindler |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250021282
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| Zusammenfassung |
| Regional scale albitisation of feldspars due to metasomatism is a relatively common
phenomenon in the Earth‘s upper crust. The spatial relationship between large albitite bodies
and ore mineral accumulations, which has often been recognized in nature, suggests that
albitising fluids are conducive to remobilising elements and are capable of transporting them
through crustal rock units.
Hydrothermal experiments on plagioclase (600˚ C, 2 kbars, 14-21 days) were performed
to further develop our understanding of the albitisation process and associated secondary
mineralisation processes. Re-equilibration of oligoclase (An22) and labradorite (An58)
crystals in an alkaline, sodium and silica-bearing solution results in reaction rims of albite
that are up to 50 μm thick. Scanning electron microscopy (SEM), transmission electron
microscopy (TEM) and electron probe microanalysis (EPMA) reveal that the replacement is
pseudomorphic and characterised by a sharp chemical interface which moves through the
crystal while preserving the crystallographic orientation. Raman spectroscopy on oligoclase
crystals that were allowed to re-equilibrate in an 18O-enriched solution demonstrates that the
oxygen isotopes within the feldspar framework structure are redistributed during albitisation.
The observed features are similar to naturally albitised plagioclase and all indicative to
an interface-coupled dissolution-reprecipitation mechanism (Putnis and Putnis,
2007). Chemical analyses of the reaction products by electron microprobe and
LA-ICP-mass spectrometry demonstrate that the albitisation is accompanied by the
mobilisation of major, minor and trace elements also including elements such as Al, Ti
and rare earth elements (REE) which are commonly regarded as immobile during
hydrothermal alteration. Our results suggest that albitising fluids that are hot and
alkaline have a high capability for mobilisation and transport of a wide variety of trace
elements.
References
Putnis A. and Putnis C.V. (2007), J. Solid State Chem., 180, 1783-1786 |
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