Lawsonite eclogites represent fossil records of processes occurring during subduction of cold and fast-subducting slabs, and provide the opportunity to investigate the thermal and metamorphic evolution of palaeosubduction zones. Occurrences of lawsonite eclogites are rare because lawsonite is often replaced during exhumation. We report here, for the first time, the occurrence of a lawsonite eclogite from the Monviso metaophiolite (Western Alps), representing the product of the Alpine metamorphism of a FeTi-oxide gabbro. The prograde metamorphic evolution of this lawsonite eclogite has been investigated using the petrologic approach of pseudosections. The reconstruction of its prograde P-T evolution allowed us to monitor changes in chemical and physical properties (mineral assemblages and compositions, density, H 2 O content, oxygen fugacity) during subduction. The pseudosection modelling suggests peak metamorphic conditions of T ≥ 550°C, P = 25-26 kbar, and a decrease in the thermal gradient during subduction from about 9°C/km to less than 7°C/km, that may be interpreted as related to an increase in the subduction rate. During its prograde evolution, at a depth of about 65-70 km, the eclogite-facies metagabbro experienced a significant dehydration passing from the Grt 1 +Omp 1 +Lws+Chl+Qtz+Rt assemblage (stage I) to the Grt 2 +Omp 2 +Tlc+Qtz+Rt assemblage (stage II). The breakdown of lawsonite and chlorite caused the release of up to 3 wt% of H 2 O and also of oxygen, as modelled by the redox-equilibrium Lws+Qtz+Chl+Omp 1 =Grt+Omp 2 +H 2 O+O 2 . This redox-equilibrium represents the boundary between an earlier, more-oxidized assemblage (stage I), stable at lower T, and a later, more-reduced assemblage (stage II), stable at higher T. These results have possible implications for the understanding of the complex interactions between crust and mantle in the subduction zones, especially in clarifying the processes locally involved in the oxidation of the mantle wedge overlying the subducting slab.

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 H 2 O-CO 2 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(CO 2 ) £ 0.10 and X(CO 2 ) £ 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(CO 2 ) values, which favoured relatively high fO 2 -conditions, preventing the formation of diamond at the UHP peak metamorphic conditions.

The Zhu‐Jia‐Chong quartz eclogites occur within two mica–epidote–garnet–plagioclase gneiss as lenses up to 200 m thick and more than 1 km long. The eclogite is medium‐grained and consists of omphacite, garnet, clinozoisite, minor kyanite and quartz. No evidence for the former presence of coesite was found. Locally, poikiloblastic amphibole occurs, which is zoned from glaucophane in the core to Mg–cummingtonite in the rim. In some localities, clinozoisite occurs as polycrystalline aggregates, interpreted as pseudomorphs after former porphyroblasts of lawsonite. The eclogite is cut by irregular metamorphic veins up to 20 cm thick and several meters long, consisting of kyanite ± zoisite ± omphacite ± rutile ± apatite ± clinozoisite in a quartz matrix. Locally, vein minerals attain pegmatite‐like grain size. Geothermobarometric calculations and mineral compatibilities of peak and retrograde minerals of both eclogite and veins indicate a clockwise P–T path, characterized by initial eclogitic conditions prograding from the lawsonite to the epidote stability field. The veins are considered to have formed before peak metamorphic conditions by prograde dehydration of lawsonite to kyanite + zoisite/clinozoisite + quartz + fluid; the fluid released from the reaction favored the growth of coarse‐grained vein minerals. The eclogitic peak, estimated at ∼ 24 kbar and 700 °C, is followed by a two‐stage retrograde evolution: the earlier stage, characterized by a quasi‐adiabatic decompression from eclogite‐ to the amphibolite‐facies conditions, implies a rapid exhumation from depths of ∼ 90 km to mid‐crustal levels; the later stage, characterized by slower exhumation rates, led to the final exposure of eclogites and country rocks. The role of lawsonite as storage of fluid in subducting plates is therefore discussed.

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