On the potential role of reactive flow precipitation due to fluid-pressure gradients for the genesis of olivine veins in subducted metaserpentinite

Serpentinite-derived fluids are released at different P and T conditions through several quasi-discontinuous dehydration reactions, such as the breakdown of brucite and antigorite forming olivine at forearc depths, and the terminal breakdown of antigorite to olivine and orthopyroxene at subarc depths. In subduction-related metamorphic terranes, the record of the colder and shallower brucite-breakdown reaction (< 1.0 GPa and < 450 ºC) in serpentinite occurs as locally olivine veining, whereas the deeper and hotter high-pressure terminal antigorite dehydration (> 1.5 GPa and ca. 660 ºC) shows pervasive replacement patterns with varied textures. Previous works have provided important constraints on the contrasting fluid flow mechanisms associated with these dehydration reactions, but the potential role of the stress field in controlling the geometry of the structures and eventually dictating the fluid pathways remains poorly understood.

Here we report the results from field observations from Cerro del Almirez (Nevado-Filábride Complex, Betic Cordillera, S. Spain) that records the formation of olivine-rich veins in prograde serpentinite at temperatures lower than the terminal antigorite dehydration. We show the existence of two generations of abundant olivine-rich veins formed as open, mixed-mode and shear fractures during prograde metamorphism. Type I veins were likely synchronous with the development of the serpentinite main foliation, whereas Type II veins postdate the foliation indicating that Atg-serpentinites experienced punctuated brittle behavior events during subduction. Type I veins were formed by the fluid overpressure developed during the brucite breakdown reaction, whereas Type II were potentially formed by continuous compositional and structural changes in antigorite that released subordinate amounts of fluids. Type II olivine-rich veins were formed by brittle failure in a well-defined triaxial stress field and were not significantly deformed after their formation.

We interpret olivine-rich veins as due to the replacement of antigorite by olivine at the walls of the crack due to reactive fluid-flow dissolution and precipitation. The ultimate driving force for the dissolution and precipitation is the low and contrasting solubility of SiO₂ and MgO in the aqueous fluid in combination with fluctuations in the fluid pressure relative to the lithostatic pressure. Equilibria under lower fluid pressure in the crack caused the nucleation and growth of olivine at the expense of antigorite dissolution. Comparison of the principal stress orientation inferred from Type II veins with those formed at peak metamorphic conditions in the ultramafic rocks at Cerro del Almirez shows a relative switch in the orientation of the maximum and minimum principal stress. These relative changes can be attributed to the cyclic evolution of shear stress, fluid pressure and fault-fracture permeability allowing for stress reversal.

FUNDING: This work is part of the project DESTINE (PID2019-105192GB-I00) funded by MICIN/AEI/10.13039/501100011033  and the  FEDER  program  “Una manera de hacer Europa”. J.A.P.N. acknowledges a Ramón y Cajal contract (RYC2018-024363-I) funded by 452MICIN/AEI/10.13039/501100011033 and the FSE program “FSE invierte en tu futuro”.