Mechanisms of serpentinite Dehydration in subduction zones: Constraints from the almirez exhumed Metamorphic terrane

Abstract

Subduction zones are the main sites of water recycling on Earth. At intermediate depth of subducting slabs, metamorphic devolatilization reactions are the principal source of fluids. A key devolatilization reaction is the discontinuous dehydration of antigorite serpentinite (Atg-serpentinite) that releases substantial amounts of water at the intermediate-depth of subducting slabs. The main aim of this Ph.D. Thesis is to constrain the dynamics and mechanisms of serpentinite dehydration and associated fluid release from observations in the exhumed Almirez ultramafic massif (Betic Cordillera, SE Spain). This massif uniquely preserves the dehydration front of the Atg-serpentinite reaction to chlorite harzburgite (Chl-harzburgite) at subarc depth of a subducting slab. The combination of field, petrological, microstructural and geochemical data sheds new light on the episodic nature and dynamics of antigorite dehydration and their relation to stress orientation and kinematics of subducting slabs, and the role of kinetics and fluid dynamics in metamorphic crystallization. This thesis presents strong natural evidence supporting the episodic release of fluids during Atg-serpentinite dehydration to Chl-harzburgite. Detailed mapping of the Almirez reveals the alternation of Chl-harzburgite lenses with granofels and spinifex textures. The lenses have alike thicknesses, volumes and calculated time-integrated water volumes. Their precursor Atg-serpentinite lenses have thicknesses that agree well with the theoretical length scale expected for porosity wave instabilities controlled by viscous compaction (i.e., the compaction length) for permeability and viscosity values typical of Atg-serpentinite. Crystallization of granofels Chl-harzburgite was likely driven by compaction and near-equilibrium fluid drainage by porosity waves, while fluid pressure instabilities ––likely induced by short-lived hydrocracking–– allowed open-system arrival of external fluids that might explain the crystallization of spinifex Chl-harzburgite. The sequence of textural intervals of Chl-harzburgite probably records cyclic events of low and high fluxes that varied in extent by several orders of magnitude. This unique natural record provides the first field evidence supporting an episodic nature of the fluid release during high-P serpentinite dehydration in subducting slabs, as predicted by theoretical models of metamorphic devolatilization reactions. For the in depth study of the texture and microstructures of Almirez Chl-harzburgite, I have investigated the 3-D microstructure of centimeter-sized olivine crystals in these rocks using correlative X-ray micro-computed tomography (μ-CT) and electron backscattered diffraction (EBSD). The new innovative technique was applied to oriented samples across the Atg-serpentinite dehydration isograd to examine the textural evolution during serpentinite dehydration to prograde peridotite and investigate its relation to the paleo-stress orientation and the kinematics of the paleo-subducting Almirez slab. Above the Atg-out isograd, Atg-serpentinite textures record the long-term shear deformation in a foliation near the slab interface. Below the Atg-out isograd, Atg-serpentinite dehydrated to unfoliated, coarse-grained Chl-harzburgite. Crystallization of granofels and spinifex Chl-harzburgite records, respectively, a sequence of slow and fast fluid draining events during serpentinite dehydration under the same orientation of the principal stresses that resulted in the shear deformation of the Atg-serpentinite. The texture of the granofels Chlharzburgite formed by a topotactic dehydration reaction after Atg-serpentinite coupled to compaction leading to an olivine layering subparallel to the Atg-serpentinite foliation. The shape and crystal preferred orientations (SPOs and CPOs, respectively) of spinifex Chl-harzburgites are composed of clustered CPOs and SPOs that show a remarkable correlation with the orientation of the principal paleostresses, suggesting a cause-effect relationship. Oxide aggregate lineations in Chl-harzburgite and the platelet lineation of spinifex Chl-harzburgite might be correlated to along-strike fluid flow below the permeability barrier that constituted the Atg-out dehydration isograd. With this is shown that the kinematics of the slab, paleostresses, and fluid flow exert a dynamic control on the textures of Atg-serpentinite dehydrating to peridotite in subducting slabs. In the Almirez massif, the change in fluid flow expulsion mechanism during Atgserpentinite dehydration is exceptionally recorded in the morphological transition of olivine (i.e., in granofels vs. spinifex textures) in the prograde Chl-harzburgite. The study of rare samples of Chl-harzburgite preserving a varied-textured olivine morphology reveals the existence of composite olivine porphyroblasts made up of rounded cores mantled by coronas of tabular grains. The correlative X-ray μ-CT and EBSD study of these samples show that the morphological transition of olivine likely records the opensystem arrival of highly polymerized aqueous fluids during Atg-serpentinite dehydration. Besides reaction affinity and reaction rate, the results of this study show that surface-active molecules may play an essential role in shaping the morphology of growing crystals during metamorphic crystallization.