Constraining P–T Paths in Eclogitic Ultramafic Massifs Using Humite-Group Minerals and Associated Lithologies (Mt. Avic, W. Alps)

Metaserpentinites are recognized to play an important role in subduction processes, through their impact on element cycling, and rocks mechanical properties. At high-pressure conditions, most of the metamorphic reactions that affect metaserpentinites are temperature-dependent, preventing accurate thermobarometric estimates in these rocks and thus limiting the ability of relating natural observations to subduction processes. Using the example of the Mt. Avic in the Western Alps, we herein attempt to constrain a P–T path in an eclogite-facies ultramafic massif using a twofold approach: (i) by conducting thermobarometric estimates in associated rock types (metamafic rocks, metasediments) and (ii) by implementing humite-group minerals in thermodynamic databases to enhance P–T estimates in metaserpentinites. Petrographic observations indicate that all three rock types are characterized by well-preserved eclogite-facies assemblages, notably highlighted by the remarkable presence of large humite-group minerals (titanian-clinohumite and titanian-chondrodite) in metaserpentinites. Thermobarometric estimates (Raman spectroscopy on carbonaceous material thermometry, thermodynamic modeling) highlight the homogeneity of peak P–T conditions recorded by the three rock types (mostly in the range 2.3–2.6 GPa and 510–580°C), and across the study area. The widespread formation of humite-group minerals in the Western Alps therefore allows to constrain the evolution of the Liguro-Piemont oceanic lithosphere from titanian-clinohumite crystallization during prograde burial, to titanian-chondrodite crystallization at peak pressure conditions and to humite minerals breakdown and subsequent reserpentinization during exhumation. The widespread occurrence of titanium-bearing minerals and enrichment in titanium compared to mantle protoliths highlight titanium mobility at high-pressure conditions during subduction.