| Summary: | The mantle section of the Advocate ophiolite (Newfoundland, Canada) contains unique outcrops of listvenite (magnesite-quartz), antigorite- and quartz-bearing talc-magnesite rock, and carbonated antigorite-serpentinite. This lithological sequence records the sequential carbonation of serpentinite by CO2-rich hydrothermal fluids. High Cr and Ni contents and preservation of Cr-spinel with a composition similar to that of Atg-serpentinite (molar Mg/Mg + Fe = 0.50–0.65; Cr/Cr + Al = 0.50–0.70), show that the Advocate listvenite and talc-magnesite rocks formed by carbonation of variably serpentinized mantle harzburgite. Replacement of lizardite by magnesite coeval with the breakdown of lizardite to antigorite + brucite and the lack of prograde olivine and magnetite in antigorite serpentinite and talc-magnesite rocks constrain the temperature of carbonation between c. 280 °C and 420 °C. Thermodynamic modelling of carbonation of serpentinite at 300 °C and 0.2–0.5 GPa accounts for the sequence of carbonated rocks in the Advocate complex. Phase relations and petrological observations indicate that the aqueous aSiO2 and aCO2 of the infiltrating CO2-rich fluid were buffered at the Atg-Tlc-Mgs and Qtz-Tlc-Mgs pseudo-invariant points, forming dominantly three-phase rocks by variable extents of carbonation at these pseudo-invariant points. Listvenites formed at large fluid-rock ratio when quartz became saturated in the fluid and precipitated along magnesite grain boundaries and in variably sized tensional veins. The whole rock Fe3+/Fetotal ratio of the Advocate carbonate-bearing sequence decreases with increasing whole rock carbon content, from 0.65–0.80 in brucite-bearing antigorite serpentinite to 0.10–0.30 in talc-magnesite rocks and listvenite. The whole rock iron reduction is associated with an increase in the ferrous iron content of magnesite and the formation of hematite and goethite, indicating a concomitant increase of the fluid oxygen fugacity. The sequence of carbonation reactions is uniquely preserved in three main ...
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