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| Titel |
Fluid infiltration, porosity generation and mass transport during the replacement of garnet and pyroxene in the Caledonides of western Norway. |
| VerfasserIn |
Stephen Centrella, Håkon Austhreim, Andrew Putnis |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091757
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| Publikation (Nr.) |
 EGU/EGU2014-6066.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.
There is no visible porosity within the garnets and clinopyroxenes but all new minerals
formed are porous. This 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. The textural evolution during the replacement of
garnet and pyroxene in our rock sample conforms to that expected by a coupled
dissolution-precipitation mechanism. Microprobe analysis and SEM were used to quantify
this mass transfer and to observe the evolution of porosity during chloritisation and
amphibolitisation.
Putnis A, Austrheim H (2012) Mechanisms of metasomatism and metamorphism on the
local mineral scale: The role of dissolution-reprecipitation during mineral re-equilibration.
Metasomatism and the chemicaltransformation of rock; the role of fluids in terrestrial and
extraterrestrial processes, 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 (2009a) 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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