Talc carbonate

In a general sense, talc carbonate metamorphic assemblages are diagnostic of the magnesium content of the ultramafic protolith.

• Lower-magnesian ultramafic rocks (12-18% MgO as a rule of thumb) tend to favor talc-chlorite assemblages
• Medium-MgO rocks (15-25% MgO) tend to produce talc-amphibole assemblages.
• High-MgO rocks with in excess of 25% MgO tend to form true talc-magnesite metamorphic assemblages.

Thus, the MgO content of a metamorphosed ultramafic rock can be estimated by understanding the mineral assemblage of the rock. Magnesium content determines the proportion of talc and/or magnesite and aluminium-calcium-sodium content determines the proportion of amphibole and/or chlorite.

Several minerals are diagnostic of talc carbonated ultramafic rocks;

• Talc
• Chlorite, generally magnesian bluish-green
• Tremolite-cummingtonite-grunerite amphiboles in greenschist facies rocks
• Anthophyllite-cummingtonite amphibole in weakly carbonated serpentinite at greenschist facies or very rarely, uncarbonated amphibolite facies serpentinites
• Magnesite, and rarely dolomite in association with amphibolitic compositions

At amphibolite facies, the diopside-in isograd is reached (dependent on carbon dioxide partial pressure) and metamorphic assemblages trend toward talc-pyroxene and eventually toward metamorphic olivine.

Serpentinisation of olivine:

forsterite2Mg₂SiO₄ + 3H₂O → serpentineMg₃Si₂O₅(OH)₄ + bruciteMg(OH)₂

Or by addition of aqueous silica:

3Mg₂SiO₄ + 4H₂O + SiO_2,aq → 2Mg₃Si₂O₅(OH)₄

Carbonation of serpentine to form talc-magnesite

2Mg₃Si₂O₅(OH)₄ + 3CO₂ → Mg₃Si₄O₁₀(OH)₂ + 3MgCO₃ + 3H₂O

Because carbon dioxide is such a common component of metamorphic fluids, talc-carbonated ultramafics are relatively common. However, the degree of talc-carbonation is usually somewhere between the two end-member compositions of pure serpentinite and pure talc-carbonate. It is common to see serpentinites which contain talc, amphibole and chloritic minerals in small proportions which infer the presence of carbon dioxide in the metamorphic fluid.

Talc carbonate is present in many of the ultramafic bodies of the Archaean Yilgarn craton, Western Australia. Notably, the Widgiemooltha Komatiite shows pure talc-carbonation on the eastern flank of the Widgeimooltha Dome, and almost pure serpentinite metamorphism on the western flank.