Olivine dis-equilibrium growth in the Val Malenco contact aureole (Northern Italy), sparks before the fireworks?

Dehydration reactions are metamorphic reactions that release water (aqueous fluids). They are crucial for the dynamics and chemical recycling in subduction zones. The dehydration of serpentinites is of particular importance as it represents the main source of volatiles in subduction zones and it occurs in a narrow temperature interval. It is generally assumed that dehydration reactions proceed at near-equilibrium. In such situation, the pressure of the fluid produced by the reaction approaches the lithostatic pressure. Deviations from the equilibrium model have been invoked for antigorite dehydration based on natural observations of anisotropic olivine grain growth at high-pressure conditions (1.5-2.0 GPa) [1,2]. Here we show another occurrence of texturally non-equilibrated olivine growth at low pressure (0.4 GPa) in olivine-talc veins crosscutting serpentinites in the Bergell intrusion contact aureole at Alpe Zocca (Malenco Unit, Northern Italy) [3].

The dehydration reactions, which resulted in the replacement of serpentinites by olivine-talc metaperidotites, occurred under quasi-static conditions. The main reaction front, which defines the equilibrium ol + tlc isograd, is a 150 m wide zone outcropping at ~750 m from the Bergell intrusion. In the metaperidotites, olivine has a crystallographic preferred orientation (CPO) correlated with the precursor antigorite CPO, with [010]_Ol axes parallel to [001]_Atg. These CPOs are accompanied by shape-preferred orientations (SPO) that mark the foliation in both rock types. Talc crystals are also oriented and often bent around olivine crystals, suggesting local compaction. We interpret the foliated metaperidotites as formed at near equilibrium conditions, with pervasive fluid extraction from the metaperidotite by viscous metamorphic compaction.

However, downstream (<100 m) of the equilibrium dehydration reaction front, ol + tlc bearing veins with variable width and shapes are common. These veins are often surrounded by centimeter- to decimeter-scale dehydration reaction zones that propagate into the serpentinite wall-rock. Olivine crystals in dehydration veins also have a strong SPO and CPO that define a jackstraw texture within the plane of the vein. They are elongated (reaching up to 50 cm) parallel to [001] within the vein plane and have their [010] axes normal to the plane of the vein. We interpret the olivine–talc assemblage in the veins and in the nearby reaction zones as resulting from premature dehydration reactions at lower temperature than the equilibrium conditions owing to effective fluid extraction from the wall-rock into the veins. The jackstraw texture indicates fast kinetics, with the crystal orientation controlled by anisotropic growth under a fluid pressure gradient. These textures record local displacement of the reactions towards lower temperature conditions owing to the formation of extensional veins, which acted as high permeability channels allowing for effective draining of the system. These brittle ‘sparkles’ during or before the massive pervasive dehydration seem to have little impact on the overall reaction. 

[1] Padrón-Navarta et al. (2011). Journal of Petrology 52, 2047-2078. [2] Dilissen et al. (2018). Lithos 320-321, 470-489. [3] Clément, M., Padrόn-Navarta, J. A. & Tommasi, A. (2019). Interplay between fluid extraction mechanisms and antigorite dehydration reactions (Val Malenco, Italian Alps). Journal of Petrology, in press.