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| Titel |
Metasomatic alteration of monazite: constraints on fluid chemistry and the dating of mass transport |
| VerfasserIn |
D. E. Harlov, C. J. Hetherington |
| 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 |
250024118
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| Zusammenfassung |
| Monazite [(Ce,La,Nd,Th,Ca)(P,Si)O4] is a common Th-bearing LREE orthophosphate
mineral in both igneous and metamorphic rocks. Thorium enrichment or depletion in
monazite as ThSiO4 and/or CaTh(PO4)2 can occur in the form of overgrowths, as magmatic
zoning, or as patchy, curvilinear intergrowths with sharp compositional boundaries, both
along the monazite grain rim as well as in the grain interior. Recent advances in the dating of
monazite using the electron microprobe have indicated that these overgrowths and
intergrowths, whether enriched or depleted in Th, can give ages younger than the original
monazite. The morphology of the intergrowths suggests that the original monazite grain
could have been partly metasomatised by a Th-Si-Ca-bearing fluid. This hypothesis has been
tested in the piston-cylinder apparatus at 1000 MPa and 900 °C utilizing unzoned,
homogeneous, Th-bearing monazite-(Ce) grains plus a series of alkali fluids including 2N
NaOH, 2N KOH, and H2O + Na2Si2O5. In each experiment a subset of the monazite grains
acquired ThSiO4-enriched intergrowths with sharp compositional boundaries that show
no evidence of being overgrowths. These experiments support the hypothesis that
Th-enriched and Th-depleted intergrowths observed in natural monazite can be
metasomatically induced via dissolution-reprecipitation yielding information concerning
the nature of the fluid present under a variety of P-T conditions. Subsequently,
these intergrowths could also be used to date multiple metasomatic events assuming
that all the original Pb is removed during metasomatic alteration. The results of
these experiments also seriously question the use of monazite-based ceramics as
repositories for radioactive waste since under high pH conditions they would be
susceptible to partial dissolution and the subsequent release of Th into local aquifers. |
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