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). All temperatures are approximate and dependent on pressure.
|The normal sedimentary assemblage in carbonate rocks.|
|Around 400 C, talc forms via the reaction
3Mag + 4Qz + H2O = Tc + 3CO2.
|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.
|At about 500 C, talc, calcite and quartz react to form tremolite. Calcite can coexist with talc only under extremely restricted conditions.|
|The marginal compatibility of calcite and talc ends completely as calcite plus talc react to give tremolite plus dolomite.|
|About 520 C diopside begins to form:
Tr + 3Cal + Qz = 5Di + 3CO2 + H2O
Tc + 5Mag = 4Fo + 5CO2 + H2O. Note that talc is becoming progressively less compatible with carbonates.
|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.|
|Around 540 C
Tr + 11Dol = 8Fo + 13Cal + 9CO2 + H2O
Note how the stability fields of dolomite are shrinking with each reaction.
|Around 600 C
3Tr + 5 Cal = 11Di + 2Fo + 5CO2 + 3H2O
|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.
|At about 740 C, magnesite breaks down to periclase (MgO):
Mag = Per + CO2
|At about 780 C, dolomite breaks down:
Dol = Cal + Per + CO2
Created 22 Sept 1997, Last Update 22 Sept 1997
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