Relationship between fluid-bearing and fluid-absent invariant points and a petrogenetic grid for a greenschist facies assemblage in the system CaO-MgO-Al2O3-SiO2-CO2-H2O |
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Authors: | B J Watts |
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Institution: | (1) N.S.W. Geological Survey, Sydney |
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Abstract: | In the 6 component system CaO-MgO-Al2O3-SiO2-CO2-H2 with 9 solid phases (quartz, plagioclase, epidote, tremolite, talc, chlorite, magnesite, calcite, dolomite) and a fluid phase, all 17 possible fluid-absent reactions have been set up and balanced. Using molar entropy and volume data for the solid phases, these reactions are arranged in P-T space about the 8 possible fluid-absent invariant points after the method of Schreinemakers. Field observations in Ordovician greenschist facies basic volcanics at Sofala N.S.W., indicate that neither talc+epidote nor magnesite+calcite are stable under the conditions of metamorphism. Assuming these conditions to apply to the theoretical study here, the fluid-absent invariant points are arranged in a relative fashion with fluid-absent reactions subdividing P-T space into smaller areas.A scheme which permits a fluid of composition
(i.e. a fluid containing CO2 and H2O together with other components), is modeled by treating H2O as a mobile component independent of CO2, and by allowing
values that lie off the
locus of binary H2O-CO2. Taking into account that neither talc+epidote nor magnesite +calcite is to be permitted, the fluid scheme is used to set up and balance all 39 possible fluid-bearing reactions. These are then arranged about 20 valid fluid-bearing invariant points in
space after the method of Korzhinskii and Sehreinemakers.A characteristic solid phase assemblage is defined for each P-T area using chemographic relations inherent from the fluid-absent boundary reactions. The fluid-bearing invariant points that have a solid assemblage compatible with the characteristic assemblage in a particular P-T area are stable within the P-T regime of that area. When these stable fluidbearing invariant points are arranged in a relative fashion in
space, they outline a fluid grid which can be used to study the possible effects of local variation in X
fluid over the particular P-T regime.Symbols Used
U
chemical potential
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S
entropy
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V
molar volume
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n
coefficient of a phase in a reaction
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X
mole fraction
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T
temperature
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P
pressure
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F
number of degrees of freedom
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C
number of components
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p
number of phases
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s
solid
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slope of reaction
- 1
quartz
- 2
plagioclase
- 3
epidote
- 4
tremolite
- 5
talc
- 6
chlorite
- 7
dolomite
- 8
magnesite
- 9
calcite |
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Keywords: | |
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