Interplay between Fluid Extraction Mechanisms and Antigorite Dehydration Reactions (Val Malenco, Italian Alps)

Abstract

Abstract We investigate the feedback between antigorite dehydration reactions and fluid extraction mechanisms through the analysis of field-scale structures and microtextures formed by antigorite dehydration reactions producing olivine and talc in the Bergell intrusion contact aureole at Alpe Zocca (Malenco Unit, Northern Italy). The reactions, which resulted in the replacement of serpentinites by metaperidotites, occurred under quasi-static conditions. The main reaction front, which defines the equilibrium isograd, is an ∼150 m wide zone composed of variably reacted rocks with an irregular distribution at scales ranging from a few centimeters to a few meters. Veins composed of the prograde mineral assemblage occur downstream (<100 m) of this front. They are often surrounded by centimeter- to decimeter-scale dehydration reaction zones that propagate into the serpentinite wall-rock. Olivine in the metaperidotites and partially reacted serpentinites has a crystallographic preferred orientation (CPO) correlated with the antigorite CPO, with [010]Ol axes parallel to [001]Atg. This CPO is accompanied by a shape-preferred orientation (SPO) that marks the foliation in both rock types. Olivine crystals in Ol–Tlc dehydration veins also have SPO and CPO that define a jackstraw texture within the plane of the vein. They are elongated parallel to [001] within the vein plane and have their [010]Ol axes normal to the vein. We interpret the olivine–talc assemblage in the veins as resulting from dehydration reactions at lower temperature than the equilibrium conditions owing to 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. We interpret the foliated metaperidotites as formed at near equilibrium conditions, with pervasive fluid extraction from the metaperidotite by viscous metamorphic compaction. Olivine CPO in these rocks may result from topotaxy, oriented growth in the presence of an anisotropic (foliation-controlled) fluid flow, and/or solid reorientation of the anisometric olivine crystals accommodated by the deformation of the weaker talc (± chlorite) matrix during compaction.