Barrovian Facies Metamorphism of Carbonate Rocks

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The Barrovian facies series, named for George Barrow, who described them around 1900, is the "normal" metamorphic facies series. These are the rocks formed in a typical geothermal gradient of about 25 C per km. The typical pressures under which these rocks form are 5-10 kb (mid-crustal depths)

Low Temperature

carb100.gif (1701 bytes) The normal sedimentary assemblage in carbonate rocks.

400 C 

Carb400.gif (1890 bytes) Around 400 C, talc forms via the reaction
3Mag + 4Qz + H2O = Tc + 3CO2.

450 C

Carb450.gif (1823 bytes) Around 450 C, dolomite ceases to coexist with quartz:
3Dol + 4Qz + H2O = Tc + 3Cal + 3CO2.

This is the same reaction as above, except this one extracts the magnesium component from dolomite.

500-550 C

Carb500.gif (2083 bytes) At about 500 C, talc, calcite and quartz react to form tremolite. Calcite can coexist with talc only under extremely restricted conditions.

Greenschist Facies

Carb510.gif (2020 bytes) The marginal compatibility of calcite and talc ends completely as calcite plus talc react to give tremolite plus dolomite.
CARB520.GIF (2224 bytes) About 520 C diopside begins to form:
Tr + 3Cal + Qz = 5Di + 3CO2 + H2O
CARB525.GIF (2311 bytes) Forsterite appears:
Tc + 5Mag  = 4Fo + 5CO2 + H2O. Note that talc is becoming progressively less compatible with carbonates.
CARB530.GIF (2230 bytes) Dolomite and talc can no longer coexist as dolomite plus talc react to give tremolite plus forsterite. Note that talc now cannot coexist with any carbonates.
CARB540.GIF (2210 bytes) Around 540 C
Tr + 11Dol = 8Fo + 13Cal + 9CO2 + H2O

Note how the stability fields of dolomite are shrinking with each reaction.

600-700 C

CARB600.GIF (2144 bytes) Around 600 C
3Tr + 5 Cal = 11Di + 2Fo +  5CO2 + 3H2O
CARB640.GIF (2188 bytes) About 640 C at 1 kb, or 680 at 2kb, wollastonite forms:
Cal + Qz = Wo + CO2

Since the reaction involves liberation of CO2, it is inhibited by high pressures.

700 C

CARB740.GIF (2179 bytes) At about 740 C, magnesite breaks down to periclase (MgO):
Mag  = Per + CO2
carb780.gif (2076 bytes) At about 780 C, dolomite breaks down:
Dol  = Cal + Per + CO2

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Created 22 Sept 1997, Last Update 22 Sept 1997

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