Dolomite evolution from HP to UHP conditions: the impure Cal-Dol marbles from Dora-Maira Massif (Italian Western Alps).

During deep and cold subduction, carbon dissolution is a relevant mechanism for carbon transfer from the slab into the mantle. The ultra-high pressure (UHP; 730 °C and 4.0-4.5 GPa) impure Cal-Dol-marbles from the Dora-Maira Massif are studied to investigate the poorly known evolution of dolomite during deep subduction. Dolomite constitutes pre-kinematic porphyroclasts showing four stages of growth, coupled with chemical variations and distinct included mineral assemblages, depending on bulk-rock composition: I) a pre-Alpine inner core; II) a HP prograde-Alpine outer core, concentrically overgrowing the inner core; III) an UHP inner rim asymmetrically overgrowing the partly resorbed core; IV) an UHP early-retrograde outer rim that asymmetrically overgrows the inner rim in sharp discontinuity. To explain the growth of dolomite through prograde, peak, and early retrograde metamorphism, a chemically simple marble (Cal, Dol, Cpx, Ol, and retrograde Atg, Tr, Mg-Chl) has been studied in detail. To predict the production- consumption of dolomite by metamorphic reactions along the P-T path, mixed-volatile P-T grids modelled in the simple CMS-H 2 O-CO 2 system have been calculated. T/P-X(CO 2 ) petrogenetic grids and pseudosections, and mixed-volatile P- T grids predict the prograde (1.7 GPa, 550 °C) growth of Dol II in equilibrium with Cpx and Ol through the breakdown of Atg+Arg. The subsequent HP-UHP evolution is predicted in the Cpx+Fo+Dol+Arg stability field in equilibrium with a dominantly aqueous COH fluid (0.0003 < X(CO 2 ) < 0.0008). According to thermodynamics, only HP prograde growth of Dol II is due to metamorphic reactions, whereas growth of peak Dol III and early-retrograde Dol IV cannot be induced by simple isochemical metamorphic reactions. A possible explanation for these microstructures comes from the abundant primary H 2 O+chloride±Cal+Dol+Tlc+Tr fluid inclusions present in the Alpine Cpx. The relatively-broad Raman peaks of minerals, the presence of molecular water in the crystallographic structure of Tlc, and the constant occurrence of Cl in the hydrous minerals indicate that these minerals have a poor crystallinity due to their precipitation from a saline (salinity > 26.3 wt.% of NaCleq) aqueous solution. The growth of Dol III and Dol IV is interpreted as due to dissolution/precipitation episodes in saline aqueous fluids. Kinetics of Dol dissolution in aqueous fluids is poorly known, and experimental and thermodynamic data under HP conditions are still lacking. Data on calcite indicate that dissolution at HP is enhanced by a prograde increase in both P and T, by high salinity in aqueous fluids, and/or low pH conditions. In the studied marble, the prograde P-T path and the occurrence of free high-saline fluids represent favourable conditions i) for the inferred dissolution-precipitation processes of the stable dolomite in a closed system; ii) for possible migration of the dissolved carbonate, if the system would have been open during subduction.