COH serpentinites and metasomatic rocks from Cogne (Aosta Valley, Western Italian Alps): Insights into seafloor fluid-rock interactions

The serpentinites of Cogne contain magnetite, serpentine, calcite, graphite, brucite, chlorite, diopside, and andradite/hydroandradite garnet. These mineralogical assemblages result from oceanic serpentinization and metasomatic processes, mediated by hydrothermal-vent fluids on the Tethyian seafloor. Fluid speciation and redox potential are strictly linked to each other in carbon-saturated COH (GCOH) fluids. If the redox buffer capacity of rocks can be overwhelmed, as in the case of high fluid/rock ratios, COH fluids can control externally the redox potential of rocks. The speciation of the COH fluid and therefore the redox conditions in force during the oceanic serpentinization and metasomatism recorded by the Cogne rocks can be unravelled by the mineral assemblages observed. Thermodynamic modelling of the garnet-forming reaction magnetite + calcite + lizardite + O2 + H2O = andradite + brucite + CO2 suggests that the formation of andradite at T ranging from 250 to 400°C is possible only for very low XCO2 [=CO2/(H2O+CO2)] in the fluid (log XCO2 = -5 to -4), which is comparable to the present-day seawater.  Thermodynamic modelling of GCOH fluids at 500 bar and 350°C shows that this CO2 content is consistent with fO2 conditions broadly approaching the FMQ buffer, which well agrees with estimates concerning oceanic hydrothermal vents and with the sulphide parageneses found at Cogne. The constraints to the fluid speciation allowed the construction of a fluid-saturated, isobaric (500 bar) T-X diagram, ranging from two representative bulk-rock compositions: (1) a typical magnetite- and sulphide-bearing serpentinite and (2) an andradite-, diopside- and calcite-bearing metasomatic rock. At composition (1), T-X diagram represents a model for the serpentinization of a peridotite in a pure FMS+COH system. When oceanic peridotite undergoes hydration, it becomes a magnetite± brucite-bearing serpentinite. Prograde forsterite is expected to grow at the expenses of brucite and lizardite above ~ 400 °C. In fact, we observed olivine (Fo98) neoblasts developing after brucite. Towards composition (2), at T below 400 °C and for low degrees of metasomatism, the expected association should contain clinochlore and calcite in addition to lizardite, magnetite and brucite. Higher degrees of metasomatism will stabilize andradite at T < 360°C and diopside at T > 360°C in brucite free rocks. Eventually, for (2) andradite and diopside coexist even at T < 360°C, and calcite disappears. At T < 300°C, diopside should be replaced by amphibole, never observed in the investigated samples of Cogne. In conclusion, thermodynamic modelling suggests that mineral assemblages observed at Cogne match a process of seafloor serpentinization and Ca (±Al) metasomatism at 300-360°C conveyed by carbon-saturated COH hydrothermal vent fluids characterized by CO2 contents comparable to present-day seawater, capable to fix the redox potential of rocks close to the FMQ buffer.