Equilibration and disequilibration between monazite and garnet: indication from phase-composition and quantitative texture analysis

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State: Public
Version: Final published version
Serval ID
serval:BIB_FAD1A0B8F5FE
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
Equilibration and disequilibration between monazite and garnet: indication from phase-composition and quantitative texture analysis
Journal
Journal of Metamorphic Geology
Author(s)
Berger A., Scherrer N.C., Bussy F.
ISSN-L
0263-4929
Publication state
Published
Issued date
2005
Peer-reviewed
Oui
Volume
23
Pages
865-880
Language
english
Abstract
The integration of information which can be gained from accessory
[i.e. age (t)] and rock-forming minerals [i.e. temperature (T) and
pressure (P)] requires a more profound understanding of the
equilibration kinetics during metamorphic processes. This paper presents
an approach comparing conventional P-T estimate from equilibrated
assemblages of rock-forming minerals with temperature data derived from
yttrium-garnet-monazite (YGM) and yttrium-garnet-xenotime (YGX)
geothermometry. Such a comparison provides an initial indication on
differences between equilibration of major and trace elements. Regarding
this purpose, two migmatites, two polycyclic and one monocyclic gneiss
from the Central Alps (Switzerland, northern Italy) were investigated.
While the polycyclic samples exhibit trace-element equilibration between
monazite and garnet grains assigned to the same metamorphic event, there
are relics of monazite and garnet obviously surviving independent of
their textural position. These observations suggest that surface
processes dominate transport processes during equilibration of those
samples. The monocyclic gneiss, on the contrary, displays rare isolated
monazite with equilibration of all elements, despite comparably large
transport distances. With a nearly linear crystal-size distribution of
the garnet grain population, growth kinetics, related to the major
elements, were likely surface-controlled in this sample. In contrast to
these completely equilibrated examples, the migmatites indicate
disequilibrium between garnet and monazite with a change in REE patterns
on garnet transects. The cause for this disequilibrium may be related to
a potential disequilibrium initiated by a changing bulk chemistry during
melt segregation. While migmatite environments are expected to support
high transport rates (i.e. high temperatures and melt presence), the
evolution of equilibration in migmatites is additionaly related to
change in chemistry. As a key finding, surface-controlled equilibration
kinetics seem to dominate transport-controlled processes in the
investigated samples. This may be decisive information towards the
understanding of age data derived from monazite.
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01/10/2012 19:07
Last modification date
20/08/2019 16:26
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