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| Titel |
Garnet and clinopyroxene pseudomorphs: example of local mass balance in the Caledonides of western Norway. |
| VerfasserIn |
Stephen Centrella, Håkon Austrheim, Andrew Putnis |
| 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 |
250103795
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| Publikation (Nr.) |
 EGU/EGU2015-3211.pdf |
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| Zusammenfassung |
| The Precambrian granulite facies rocks of Lindås Nappe, Bergen Arcs, Caledonides of
W.Norway are partially hydrated at amphibolites and eclogite facies conditions. The Lindås
Nappe outcrop over an area of ca 1000 km2 where relict granulite facies lenses make up only
ca 10%. At Hillandsvatnet, garnetite displays sharp hydration fronts across which the
granulite facies assemblage composed of garnet (70%) and clinopyroxene (30%)
is replaced by an amphibolite facies mineralogy defined by chlorite, epidote and
amphibole. This setting allows us to assess the mechanism of fluid transport through an
initially low permeability rock and how this induces changes of texture and element
transport.
The replacement of garnet and clinopyroxene is pseudomorphic so that the grain shapes
of the garnet and clinopyroxene are preserved even if when they are completely
replaced.
This requires that the reactive fluids must pass through the solid crystal grains and this
can be achieved by an interface coupled dissolution-precipitation mechanism. Porosity
generation is a key feature of this mechanism (Putnis and Austrheim 2012). The porosity is
not only a consequence of reduction in solid molar volume but depends on the relative
solubilities of parent and product phases in the reactive fluid. Putnis et al. 2007 and Xia et al.
2009 have shown that even in pseudomorphic reactions where the molar volume increases,
porosity may still be generated by the reaction. This is fundamental in understanding the
element mobility and the mass transfer in a low permeability rock even more when the
bulk rock composition of these two rocks stay unchanged; except a gain in water
during amphibolitisation. The textural evolution during the replacement of garnet by
pargasite, epidote and chlorite and pyroxene by hornblende and quartz in our rock
sample conforms to that expected by a coupled dissolution-precipitation mechanism.
SEM and Microprobe analysis coupled with the software XMapTools V 1.06.1
.(Lanari et al., 2014) were used to quantify the local mass transfer required during
the replacement processes and to identify the importance of fluid in metamorphic
reactions.
Lanari, P., Vidal, O., Andrade, V. de, Dubacq, B., Lewin, E., Grosch, E.G., and Schwartz,
S., 2014, XMapTools: A MATLAB©-based program for electron microprobe X-ray
image processing and geothermobarometry. In: Computers & Geosciences, v. 62, p.
227–240.
Putnis A, Austrheim H (2012) Mechanisms of metasomatism and metamorphism on the
local mineral scale: The role of dissolution-reprecipitation during mineral re-equilibration. In:
Metasomatism and the chemical transformation of rock; the role of fluids in terrestrial and
extraterrestrial processes, Springer pp 141-170.
Putnis A, Putnis CV (2007) The mechanism of reequilibration of solids in the presence of
a fluid phase. J Solid State Chem 180: 1783-1786.
Xia F, Brugger J, Chen G, Ngothai Y, O’Neill B, Putnis A, Pring A (2009)
Mechanism and kinetics of pseudomorphic mineral replacement reactions: a case
study of the replacement of pentlandite by violarite, Geochim Cosmochim Acta 73:
1945-1969.
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