Petrogenesis of impure marbles from the UHP Brossasco-Isasca Unit (Dora-Maira Massif, Western Alps): evidence for Alpine equilibration in the diamond-stability field and evaluation of pre-Alpine vs Alpine X(CO2) fluid evolution

Two impure ultrahigh-pressure (UHP) marbles, a calcite marble with the peak assemblage Grt + Phe + Cpx + Rt + (Arg) and a dolomite marble with the peak assemblage Crn + Chl + Rt + Dol (±Arg), from the same lens from the polymetamorphic complex of the Brossasco-Isasca Unit (BIU) (southern Dora-Maira Massif) have been petrologically investigated and modelled by calculating P–T phase-diagram projections for H2O–CO2 mixed-volatile systems. Thermobarometric data obtained from the calcite marble suggest Alpine peak conditions in the diamond stability field (4.0 GPa at 730°C), and allow reconstruction of the earlier portion of the Alpine retrograde P–T path, which is characterized by a significant decompression coupled with a moderate and continuous cooling to 650°C at 2.50 GPa. The modelled fluid compositions at peak conditions point to 0.025 < X(CO2) < 0.10 and X(CO2) < 0.0012 in the calcite marble and dolomite marble, respectively, suggesting fluid heterogeneity at the local scale and an internally buffered fluid evolution of the studied impure marbles. The lack of micro-diamond in the BIU marbles is explained by the very-low X(CO2) values, which favoured relatively high fO2-conditions, preventing the formation of diamond at the UHP peak metamorphic conditions.