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| Titel |
Orthopyroxene-amphibole bi-phase corona from the Arnøya metagabbro, Norway: an example of open system disequilibrium microstructure |
| VerfasserIn |
Pritam Nasipuri, Holger Stünitz, Erling J. Krogh Ravna, Luca Menegon |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049485
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| Zusammenfassung |
| Corona textures involving olivine-plagioclase are common in metamorphosed mafic rocks.
Although the precursors of corona assemblages are olivine and plagioclase, a significant
variation in the newly formed mineral species and zonal sequences may be develop
through reaction and diffusion between the reactants (olivine and plagioclase) . In this
contribution, we describe the sequence of mineral development in bi-phase coronas at the
olivine-plagioclase interface from undeformed gabbroic rock, Arnøya, Norway. The
metagabbroic rock from Arnoya contains magmatic olivine (Ol), plagioclase (Pl) and
clinopyroxene (Cpx). Ilmenite occurs as exolution phase within clinopyroxene. The
orthopyroxene (Opx)-amphibole (Amph) bi-phase corona develops extensively at the contact
between olivine and plagioclase.
The undeformed part of the rock contains olivine, plagioclase and clinopyroxene,
demonstrating the cumulus nature of the rock. Olivine grains are usually fractured and have a
wavy grain boundary with adjacent plagioclase grains. Magmatic plagioclase grains (up to
1Â mm long) occur either as individual crystals or as polycrystalline aggregates. Most of the
plagiclase grains are commonly twinned and locally includes olivine suggesting that
olivine predated plagioclase. Grains of plagioclase and olivine are mostly armored by
pyroxene.
The metamorphic mineral sequence develops exclusively at the contact between olivine
and plagioclase. The sequence of mineral development is as follows: Ol (matrix) > Opx ±
Spl > Amph ± Ky/Sill ± calcic Pl > Pl (matrix). Metamorphic orthopyroxene are extremely
small in size and the individual grains occur at high angle to the rounded olivine grain
boundary. Calcic amphibole is colorless to pale green between the Ol-Pl domains. A very thin
zone of amphibole (II) and calcic plagioclase develops at the outer edge of the amphibole
corona. Occasionally, blebs of sillimanite/kyanite grains are observed in the amphibole
layer.
Except for olivine (XMg = 0.82), the entire mineral assemblage shows a steep
compositional zoning. XMg of orthopyroxene varies from 0.85 to 0.82 towards the amphibole
zone. XMgin amphibole (ferrian tshermakite) varies from 0.82 to 0.79 towards the
plagioclase zone. In the symplectite zone amphibole (ferrian tshermakite) becomes slightly
less magnesian (XMg = 0.77). Plagioclase composition changes from XAn= 0.87 near the
symplectite zone to XAn= 0.58 towards the unreacted domain. Geothermometers using local
amphibole-plagioclase equilibrium estimate 700-780 0C at 5 kbar for the development of
amphibole corona.
The corona microstructure could be explained through the following reactions: a) Ol + Pl
+ H2O -> Opx + Spl + Amph; b) Pl + Opx + H2O -> Amph + Ky/Sill. However, the
presence of a chemical gradients in the orthopyroxene (XMg = 0.85-0.82) and amphibole
layers (XMg= 0.82-0.77) suggest that diffusion processes have controlled the rate
of the reaction. The mineral layering described above could be explained as an
open system reaction that removes mainly MgO and produce orthopyroxene (Ol ->
Opx + MgO + FeO), spinel and calcic amphibole (Pl + H2O + MgO + FeO ->
Amph + Spl + CaO; Ol + H2O + CaO -> Amph + MgO + FeO) at the expense of
olivine and plagioclase. Removal of MgO by diffusive mass transfer is responsible
in destabilizing the olivine plagioclase interface and the presence or absence of
fluid phase at the interface will cause the formation of different types of reaction
rims.
Spl: Spinel; Ky: Kyanite; Sill: Sillimanite |
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