Following dolomite fate from HP to UHP conditions: the impure Cal-Dol marbles from Dora-Maira Massif (western Alps).

Polymetamorphic impure Cal-Dol-marbles from the UHP (730°C at 4.0–4.5 GPa) Brossasco-Isasca Unit (BIU; Dora-Maira Massif) are promising rocks to investigate the poorly known fate of dolomite within the Earth’s interior. Dolomite occurs as pre-kinematic porphyroclasts with curved, irregular or lobed margins. Under both CL and SEM, dolomite usually shows at least 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) an 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. A chemically simple marble consisting of Cal, Dol, Cpx, Ol, and retrograde Atg, Tr and Mg-Chl has been studied in detail. Alpine Cpx includes abundant primary, crystallographically oriented H2O+chloride ± carbonate fluid inclusions, and local intergrowths of Dol+Cal. T/P-X(CO2) petrogenetic grids and pseudosections, and mixed-volatile P-T grids modelled in the CMS-H2O-CO2 system predict the first, and only, prograde (ca. 1.7 GPa, 550°C) growth of Dol in equilibrium with Cpx and Ol through the breakdown of Atg+Arg. In a fluid saturated system, the subsequent HP-UHP prograde and early retrograde P-T evolution is thermodynamically predicted in the Cpx+Fo+Dol+Arg stability field in equilibrium with a dominantly aqueous COH fluid (0.0003<X(CO2)<0.0008). This implies that, though prograde growth of Dol II can be explained by the Atg dehydration, the growth of Dol III and Dol IV cannot have been induced by simple isochemical metamorphic reactions. In Dol, re-entrant irregular grain boundaries are interpreted as due to dissolution/precipitation episodes in saline aqueous fluids. They cannot be ascribed only to local fluid-assisted grain-boundary migration processes, since the presence of abundant crystallographically-oriented (i.e., primary) fluid inclusions in Alpine Cpx coexisting with Dol indicates migration of carbonate- and chloride-bearing aqueous fluids at least at the sample scale. Kinetics of Dol dissolution in aqueous fluids is poorly known, and experimental and thermodynamic data under HP conditions are still lacking. However, data on calcite indicate that dissolution at HP during subduction is enhanced by following a prograde increase in both P and T, by high salinity in aqueous fluids, and/or low pH conditions. In a closed system, variations in one or more of these parameters should promote carbonate dissolution and/or precipitation processes. 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.