Mechanisms of serpentinite dehydration in subduction zones. Constraints from the almirez exhumed metamorphic terrane

Resumen

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. Despite their fundamental role in subduction dynamics, the time, flux and nature of deserpentinization fluid release are poorly constrained. A key devolatilization reaction is the discontinuous dehydration of antigorite serpentinite (Atg-serpentinite), liberating the largest amounts of slab fluids. The main aim of this PhD thesis is to constrain the dynamics and mechanisms of serpentinite dehydration fluid release from observations in an exhumed high-pressure metamorphic terrain, being the Almirez Ultramafic massif (SE Spain). This is a natural laboratory that uniquely preserves the dehydration front of the antigorite dehydration reaction into chlorite harzburgite (Chl-harzburgite) that used to be at sub-arc depth in a subducting slab. Until now, this is the only setting known worldwide to have this isograd with on both sides well-kept reactant and product. The key in studying the antigorite dehydration in this thesis lies within the differing textures observed within the dehydrated Chl-harzburgite. The combination of field data, textural, petrological, microstructural and geochemical analyses sheds more light on the episodic nature and physical mechanisms of antigorite dehydration, on its dynamics related to stress orientation and slab kinematics and on the role of its kinetics and fluid dynamics. In a first chapter, this work presents evidence of the episodic fluid release and crystallization of Chl-harzburgite during Atg-serpentinite dehydration as predicted by theoretical models. The Almirez massif records the alternation of granofels and spinifex textured Chl-harzburgite lenses, with similar lens thicknesses, volumes and calculated time-integrated water volumes. Their precursor Atg-serpentinite lenses have thicknesses (~ 15–35 m) that agree well with the theoretical length scale expected for porosity wave instabilities controlled by viscous compaction (i.e., the compaction length) for typical permeability and viscosity values of Atg-serpentinite. Crystallization of granofels Chl-harzburgite is linked to this compaction-driven, near-equilibrium fluid drainage by porosity waves. This would leave behind undrained metastable serpentinite domains, trapped by the rigid and impermeable granofels Chl-harzburgite surrounding them. Fluid pressure instabilities likely induced short-lived hydrocracking in the Chl-harzburgite that allowed open-system arrival of external fluids that might explain the textural crystallization of spinifex Chl-harzburgite. The minimum timescale of compaction-driven fluid expulsion for a 15–35 m thickness is estimated to be 10–23 yrs and can increase up to three orders of magnitude depending on fluid production rates and the order they are slower than viscous compaction rates. Fluid pressure instabilities linked to hydrocracking are estimated to last 12–70 days for linear kinetics or 14 h to 3 days for non-linear kinetics. The constrained size and shape of textural lenses provided an excellent opportunity to estimate time-integrated fluid fluxes at the Almirez massif. Unfocussed flow through porosity waves is estimated as 1 – 4 m^3 〖 m〗^(-2), while focused flow through narrow zones could vary between 101 m^3 〖 m〗^(-2) and 3·103 m^3 〖 m〗^(-2). The successive expulsion per textural interval herewith would have fluctuated between two orders of magnitude marking the cyclic events of low and high fluxes. This unique natural record of the Almirez thus provides the first evidence supporting the episodic nature of fluid release during high-P serpentinite dehydration by porosity waves. A more in depth study of textures and microstructures at Almirez is presented in a second and third chapter of the thesis, for which a new microstructure analysis technique has been developed. The 3-D microstructure is reconstructed of centimeter-sized olivine crystals in rocks from the Almirez massif using combined X-ray micro computed tomography (µ-CT) and electron backscatter diffraction (EBSD). The semi-destructive sample treatment involves geographically oriented drill pressing of rocks and preparation of oriented thin sections for EBSD from the µ-CT scanned cores. The µ-CT results show that the mean intercept length (MIL) analyses provide reliable information on the shape preferred orientation (SPO) of texturally different olivine groups. Statistical interpretation of crystal preferred orientation (CPO) and SPO of olivine becomes feasible because the highest densities of the distribution of main olivine crystal axes from EBSD are aligned with the three axes of the 3-D ellipsoid calculated from the MIL analyses from µ-CT. From EBSD data multiple CPO groups are distinguished and by locating the thin sections within the µ-CT volume, SPO is assigned to the corresponding olivine crystal aggregates, which confirm the results of statistical comparison. This work demonstrates that the limitations of both methods (i.e., no crystal orientation data in µ-CT and no spatial information in EBSD) can be overcome, and the 3-D orientation of the crystallographic axes of olivines from different orientation groups can be successfully correlated with the crystal shapes of representative olivine grains. Through this approach one can establish the link among geological structures, macrostructure, fabric and 3-D SPO-CPO relationship at the hand specimen scale even in complex, coarse-grained geomaterials. The second chapter of the thesis applies the novel combined µ-CT-EBSD technique in a detailed study of oriented samples across the Atg-serpentinite dehydration isograd to investigate the textural evolution during serpentinite dehydration to peridotite and its relation to stress orientations and the kinematics of subducting slabs. Above the Atg-out isograd, Atg-serpentinite shows a prograde mylonitic foliation and a weak Shape Preferred Orientation (SPO) of oxide aggregates defining a N–S stretching lineation. The antigorite Crystal Preferred Orientation (CPO) is characterized by [001]Atg perpendicular to the foliation, and the poles to (100)Atg and (010)Atg distributed in a girdle-like symmetry with [100]Atg nearly parallel to the stretching lineation. The antigorite microstructure and CPO are consistent with deformation by dislocation creep, twinning, and dissolution-precipitation creep. These textures record the long-term shear deformation near the slab interface where the main compressive stress, σ1, was at an acute angle to the foliation. Below the Atg-out isograd, Atg-serpentinite dehydrated to unfoliated, coarse-grained Chl-harzburgite with granofels or spinifex textures distributed in alternating decameter-sized lenses. 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. Both textural types exhibit coarse-grained textures with systematic mineral CPOs and SPOs and microstructures without evidence of major ductile deformation. The texture of the granofels Chl-harzburgite formed by a topotactic dehydration reaction after Atg-serpentinite coupled to compaction leading to an olivine layering subparallel to the Atg-serpentinite foliation. The olivines of granofels Chl-harzburgite are rounded and display a weak CPO that can be accounted for by the topotactic reaction <100>Atg||<100>Ol and (001)Atg||(010)Ol after Atg-serpentinite. Similarly, orthopyroxene shows a marked CPO consistent with the topotactic reaction (100)Opx||(001)Atg and [001]Opx||[100]Atg. Spinifex Chl-harzburgite displays systematic mineral SPO and CPO. Spinifex olivines are tabular on (100)Ol and elongated along [001]Ol (c>b>>a), and define a ESE–WNW platelet lineation. The average texture is characterized by [001]Ol,Opx subparallel to a strong ESE–WNW oxide aggregate lineation, and [100]Ol,Opx and [001]Ol,Opx within a plane of similar orientation to the Atg-serpentinite foliation. The SPOs and CPOs of spinifex Chl-harzburgites are composed of up to four orientation populations of tabular olivines, where one population is volumetrically dominant in all samples. Relative to the main orientation population, the CPO of olivine shows clustered distribution of [100]Ol and [010]Ol maxima rotated at systematic angles around [001]Ol. These clustered CPOs and SPOs show a remarkable correlation with the orientation of the principal paleostresses. The preponderant orientation population of tabular olivines lies on the plane of maximum compression (σ2–σ3 plane) and has [010]Ol and its (100)Ol tabular faces nearly perpendicular to the Atg-serpentinite foliation. This population can be accounted for by oriented growth of platy crystals perpendicular to σ1. The other populations lie in the plane perpendicular to the least compressive stress (σ3) and the planes of maximum shear. The driving force and causes for the oriented crystallization of tabular olivine in these planes is uncertain, but their correlation with paleostresses suggests a cause-effect relationship. Spinifex and granofels Chl-harzburgites show a marked ESE–WNW oxide aggregate lineation that differs in orientation from that of Atg-serpentinite and is approximately parallel to the intermediate compressive stress (σ2). These lineations and the platelet lineation of spinifex Chl-harzburgite may be due to along-strike fluid flow below the permeability barrier that constituted the Atg-out dehydration isograd. The study discussed in the second chapter therefore shows 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 third and last chapter, this thesis reports an exceptional record of the morphological transition of olivine formed during subduction metamorphism and high-pressure dehydration of Atg-serpentinite to prograde Chl-harzburgite. Uncommon samples of varied-textured Chl-harzburgite unveil the existence of olivine porphyroblasts made up of rounded cores mantled by coronas of tabular grains. Single olivine grains with a tabular morphology also occur in the matrix. The correlative μ-CT and EBSD study of two samples shows that tabular olivine in coronas is tabular on (100)Ol with c > b >> a, and grew in nearly the same crystallographic orientation as the rounded olivine cores. These tabular olivines strongly differ from previously described igneous and metamorphic tabular olivines, which are tabular on the (010)Ol with either a > c >> b, or a ≈ c >> b. The tabular olivines overgrown on rounded olivine are most likely due to a shift from isotropic to anisotropic olivine growth caused by inhibited growth on the (100) and, to a lesser extent, (010) olivine interfaces. Quantitative textural analysis and reaction mass balance point to a two-stage nucleation and growth process of olivine during the progress of the dehydration reaction of Atg-serpentinite to Chl-harzburgite. The first stage occurred under a low affinity and rate of the reaction and resulted in a low time-integrated nucleation rate and the isotropic growth of rounded olivine porphyroblasts. This stage was followed by a sudden increase in the affinity and reaction rate, which resulted in a relatively higher time-integrated nucleation rate of olivine and a shift from isotropic to anisotropic tabular olivine growth. The rounded and tabular olivines show differing trace element compositions that are likely due to the arrival of external fluids during the olivine tabular growth stage. Whatever the cause of the increase in the affinity and reaction rate, kinetic factors alone —due to increased nucleation rate and growth mechanisms— cannot account for the unusual morphology of olivine, which is tabular on the (100) face. Theoretical works in highly polymerized fluids predict inhibited growth on the (100) and (010) olivine interfaces due to the dissociative and molecular adsorption of water monolayers on these interfaces. It is likely that the morphological transition of olivine records the open-system arrival of highly polymerized aqueous fluids during Atg-serpentinite dehydration. Besides reaction affinity and reaction rate, surface-active molecules could play an important role in shaping the morphology of growing crystals during metamorphic crystallization.