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Examples of the three ultramafic rock-types recovered at the VLS: a. Porphyroclastic peridotite showing the mesh-textured serpentine + magnetite matrix (post-olivine) surrounding partially serpentinized and oxidized coarse-grained porphyroclastic pyroxenes (samples S1905-64); b. Dry mylonite showing a strongly oxidized reddish serpentine matrix embedding coarse-grained aligned porphyroclast (sample S1915-70); and c. Amphibole ultramylonite with layers of serpentine + magnetite alternating with oxidized reddish portion of the rock (sample S1907-23).
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Mantle peridotites from an exposed lithospheric section (Vema Lithospheric Section, VLS), generated during ~ 26 Ma at a ~ 80 km long Mid Atlantic Ridge segment (11° N), have been sampled and studied to understand the evolution of the serpentinization process. The VLS was uplifted due to a 10 Ma transtensional event along the Vema transform. Before...
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... serpentinization higher than d-mylonites, although a precise evaluation of the primary and secondary mineral assemblages is not easy. In addition, most of the samples show a reddish earthy appearance due to the deeply goethite-rich Fe-hydroxide late alteration, whereas mm-thick Fe-Mn crusts formed along the surface of the rocks, as well shown in Fig. 4c. However, Cipriani et al. (2009b) concluded that the mineral chemistry of the few mantle fine-grained relics points to a more fertile protolith with respect to porphyroclastic peridotites. Amphiboles were formed close to the ridge axis, most probably by the action of water rich fluids (not derived by seawater but of mantle ...
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The distribution of major, minor and trace elements in the Cenozoic alkali basalt from north-eastern Jordan indicates a homogeneous lava flow from a mantle source. These basaltic rocks contain abundant upper mantle xenoliths (spinal lherzolite, harzburgile and olivine- websterite). Theses xenoliths contain olivine, orthopyroxne, clinoporoxene and a...
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... Within the oceanic domain, over 70% of abyssal peridotites are residual harzburgites and lherzolites with up to 40% modal orthopyroxene and 15% clinopyroxene, while dunites (>90% olivine) represent only 6% of all ultramafic lithologies (Warren, 2016). Where detachment faults expose abyssal peridotites, serpentinite-hosted hydrothermal systems may develop over oceanic core-complexes (Kelley et al., 2001(Kelley et al., , 2005Boschi et al., 2006Boschi et al., , 2013. ...
Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs, as well as fluid and element transfer between the Earth’s mantle and crust. However, fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained. Here we present two constant temperature (230 °C) time-series of natural peridotite (77.5% olivine; 13.7% enstatite; 6.8% diopside; 2% spinel) serpentinization experiments: at 13.4 MPa; and 20.7 MPa. Al-enriched lizardite was the main secondary mineral in all runs after olivine (olv) and orthopyroxene (opx) serpentinization (without any detectable brucite, talc or magnetite), while primary spinel and diopside partially dissolved during the experiments. Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite, as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine, while the opposite is true for Mg and Fe, with homogenization of serpentines compositions after 40 days. The Ni/Cr ratios of serpentines, however, remain diagnostic of the respective primary mineral. Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day−1 (0.26 mmol·day−1) and 0.26 wt.%·day−1 (0.13 mmol·day−1) at 13.4 and 20.7 MPa, respectively. Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al, which accelerates olivine serpentinization while delays orthopyroxene dissolution. Additionally, time-dependent increase in solid products masses suggests rock volume expands linearly 0.37% ± 0.01% per serpentine wt.% independently of pressure. Mass balance constrains suggests olv:opx react at ∼5:2 and ∼3:2 M ratios, resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series (13.4 MPa) and high-pressure series (20.7 MPa), respectively. Elevated starting peridotite olv:opx ratio (7.94:1) therefore indicates orthopyroxene serpentinization is ∼3.3x and ∼5.4x faster than olivine at 13.4 MPa and 20.7 MPa, respectively. This contradicts previous assumptions that olivine should dissolve faster than orthopyroxene at experimental conditions. Finally, serpentinization-derived fluids develop pH > 10 and become enriched in H2, CH4, Ca2+ and Si within 6 weeks. Aqueous silica concentrations are highest after 5 days (265.75 and 194.79 µmol/kg) and progressively decrease, reaching 13.84 and 91.54 µmol/kg at 13.4 and 20.7 MPa after 40 days, respectively. These concentrations are very similar to the low-silica (M6) and high-silica (Beehive) endmembers of the Lost City Hydrothermal Field (LCHF). Beyond fluid characteristics, serpentinization products and conditions analogous to the LCHF suggest similar mechanisms between our experiments and natural processes. Our results demonstrate constant temperature serpentinization of a common protolith leads to distinct serpentine and fluid compositions at different pressures. Although additional data is necessary, recent studies and our experiments suggest peridotite serpentinization rates at 230 °C rapidly decrease with increasing pressures at least up to 35 MPa. Whether pressure directly influences olivine and orthopyroxene serpentinization kinetics or indirectly controls reaction rates due to spinel dissolution under hydrothermal conditions deserves further investigation.
... Furthermore, the near vertical incident rays and shortage of the ray crossing in the crust, result in unreliable teleseismic tomography results because of the high degree of smearing. While teleseismic methods can be a better choice to study deeper structures (e. g., the upper mantle, etc.), they may yield results with relatively poor resolution in the crust (see, e.g., Alinaghi et al., 2007;Rawlinson et al., 2010;Shad Manaman et al., 2011;Mahmoodabadi et al., 2020).The Ambient Noise Tomography (station-station system), ANT, which is based on the extraction of Green's functions from ambient noise data is an efficient method to study the upper earth structure (Boschi et al., 2013), but it is very dependent on the noise sources and density of the seismic stations that are again poor in the Zagros. Because of the high diffuse seismicity in the Zagros, using local earthquake surface waves can be a proper alternative to improve the resolution of the crustal structure in this region because the ray coverage is sufficient (Bonadio et al., 2021). ...
The Zagros fold-and-thrust belt is the principal response to the Arabia–Eurasia continental collision, which generates significant seismic activity in Iran. Surface wave tomography can be a proper tool for crustal structure imaging in this region. We used observed dispersion curves of Rayleigh wave derived from high-quality local-earthquake waveforms to obtain 2D tomographic maps in the period range 1-28 s beneath the Zagros collision zone. Subsequently, local pseudo-dispersion curves for individual grid points are inverted for shear-wave velocity structure, in order to obtain a 3D velocity model of the crust. Our results indicate shallow low-velocity structures near the suture zone. Moreover, a low-velocity flexure shape in the south of the Zagros Mountain Front Fault could be due to the thick sedimentary basin beneath the Dezful embayment. Relatively an uplifting of the deeper structures beneath Sanandaj-Sirjan and the high Zagros might could be led to high-velocity structures at mid-crustal depths. The variation of shear-wave velocity along the Zagros indicates the inhomogeneous structures along this folding and thrust belt.
... The rate of accretion of the oceanic crust varies dramatically, especially along ultraslow-slow spreading ridges, both in space and time, due to strongly fluctuating magma supply (Bown & White, 1994;Liu et al., 2022;Morgan & Chen, 1993). Studies from geophysical observations and numerical models indicated that the depth of detachment faults could reach 10-30 km, enhancing the hydrothermal circulation and alteration at spreading centers (e.g., Andreani et al., 2014;Bach et al., 2004;Beard et al., 2009;Bickert et al., 2020;Boschi et al., 2006Boschi et al., , 2013Klein et al., 2009;Maffione et al., 2014;Mével, 2003;Plümper et al., 2012Plümper et al., , 2014Schroeder et al., 2002). At the root of detachment faults, especially at amagmatic sections, strain and stress concentrations lead to dynamic recrystallization, forming grain size reduction zones (Bickert et al., 2021). ...
Although positive buoyancy of young lithosphere near spreading centers does not favor spontaneous subduction, subduction initiation occurs easily near ridges due to their intrinsic rheological weakness when plate motion reverses from extension to compression. It has also been repeatedly proposed that inherited detachment faults may directly control the nucleation of new subduction zones near ridges subjected to forced compression. However, recent 3D numerical experiments suggested that direct inversion of a single detachment fault does not occur. Here we further investigate this controversy numerically by focusing on the influence of brittle‐ductile damage on the dynamics of near‐ridge subduction initiation. We self‐consistently model the inversion of tectonic patterns formed during oceanic spreading using 3D high‐resolution thermomechanical numerical models with strain weakening of faults and grain size evolution. Numerical results show that forced compression predominantly reactivates and rotates inherited extensional faults, shortening and thickening the weakest near‐ridge region of the oceanic lithosphere, thereby producing ridge swellings. As a result, a new megathrust zone is developed, which accommodates further shortening and subduction initiation. Furthermore, brittle/plastic strain weakening has a key impact on the collapse of the thickened ridge and the onset of near‐ridge subduction initiation. In contrast, grain size evolution of the mantle only slightly enhances the localization of shear zones at the brittle‐ductile transition and thus plays a subordinate role. Compared to the geological record, our numerical results provide new helpful insights into possible physical controls and dynamics of natural near‐ridge subduction initiation processes recorded by the Mirdita ophiolite of Albania.
... To summarize, we propose a model that combines both the thermal structure and deformation history of an oceanic plate (Fig. 13) to explain the possible origin of deep lithospheric hydration necessary for the wide separation observed between the upper and lower planes of some Wadati-Benioff zones. Outside of slow to ultraslow spreading ridges with a spreading rate of <5 cm/yr and the immediate vicinity of fracture zones (e.g., Mével, 2003;Boschi et al., 2013;Rüpke and Hasenclever, 2017), serpentinization of the uppermost mantle is possible away from a spreading ridge but is likely limited (Mével, 2003). In the context of our model (Fig. 13A), the depth extent of serpentinization can be viewed as a result of the crust and uppermost mantle reaching a temperature below 600 °C, which thus triggers a switch from ductile to brittle behavior (Kohlstedt et al., 1995) in this portion of the lithosphere and then allows fluid access to the uppermost mantle. ...
The recycling of water into the Earth’s mantle via hydrated oceanic lithosphere is believed to have an important role in subduction zone seismicity at intermediate depths. Hydration of oceanic lithosphere has been shown to drive double planes of intermediate-depth, Wadati-Benioff zone seismicity at subduction zones. However, observations from trenches show that pervasive normal faulting causes hydration ~25 km into the lithosphere and can explain neither locations where separations of 25–40 km between Wadati-Benioff zone planes are observed nor the spatial variability of the lower plane in these locations, which suggests that an additional mechanism of hydration exists. We suggest that intraplate deformation of >50-m.y.-old lithosphere, an uncommon and localized process, drives deeper hydration. To test this, we relocated the 25 November 2018 6.0 MW Providencia, Colombia, earthquake mainshock and 575 associated fore- and aftershocks within the interior of the Caribbean oceanic plate and compared these with receiver functions (RF) that sampled the fault at its intersection with the Mohorovičić discontinuity. We examined possible effects of velocity model, initial locations of the earthquakes, and seismic-phase arrival uncertainty to identify robust features for comparison with the RF results. We found that the lithosphere ruptured from its surface to a depth of ~40 km along a vertical fault and an intersecting, reactivated normal fault. We also found RF evidence for hydration of the mantle affected by this fault. Deeply penetrating deformation of lithosphere like that we observe in the Providencia region provides fluid pathways necessary to hydrate oceanic lithosphere to depths consistent with the lower plane of Wadati-Benioff zones.
... For example, forearc-modified serpentine-dominated assemblages have been shown to have higher δ 11 B values (Benton et al., 2001;Savov, 2004;Savov et al., 2005Savov et al., , 2007Tonarini et al., 2007Tonarini et al., , 2011, whereas deeper parts of the mantle wedge beyond the forearcs tend to have much lower δ 11 B values (Cannaò et al., 2015;Martin et al., 2016). On the other hand, abyssal serpentinites, and by inference hydrated oceanic mantle, may also have high δ 11 B (Spivack and Edmond, 1987;Boschi et al., 2008Boschi et al., , 2013Vils et al., 2009;Harvey et al., 2014), but more work combining B isotopes with other isotopic and trace element tracers is needed to precisely constrain the contribution of the various serpentinite sources. In contrast, there is negligible boron isotope fractionation between the fluids and the secondary minerals formed as the slab descends (e.g., forsterite, diopside). ...
Serpentinites entering subduction zones show selective enrichment in trace elements and are the most important carriers of B in this setting. The evolving B isotopic composition (δ¹¹B) of serpentinite is used in studying water-mediated interactions between mantle and crust in subduction zones, but we lack a full understanding of the processes controlling B isotope fractionation during serpentinization that would allow this tracer to be better utilized. Boron isotope fractionation between lizardite/forsterite/diopside and aqueous fluids is investigated using quantum mechanics (density functional theory, DFT) and the first-principles molecular dynamics simulation (FPMD) here. The ¹¹B enrichment sequence in minerals follows the order of forsterite > clinopyroxene (diopside) > white mica > phlogopite > lizardite. On the basis of boron configurations in aqueous fluid acquired from FPMD simulations based on DFT, the reduced isotopic partition function ratio (β-factor) of aqueous B(OH)3 and B(OH)4⁻ are 1000lnβB(OH)3 = −20.1 + 37.07(1000/T) + 8.2(1000/T)² and 1000lnβB(OH)4- = −13.94 + 24.43(1000/T) + 9.12(1000/T)² at the P-T range of 0–500 MPa and 298–1000 K. This theoretical approach gives an equilibrium B isotope fractionation of Δ¹¹Blizardite-fluid = 15.89–22.88(1000/T) - 0.19(1000/T)² between lizardite and neutral serpentinization fluid in the temperature range from 298 to 673 K. By contrast, the boron isotope fractionation between minerals formed as the slab descends (e.g., forsterite, diopside) and fluid released from subducted crustal lithologies is limited, indicating that secondary olivine minerals may inherit the δ¹¹B signatures of the dehydrated serpentinites. This study provides new insights into how B isotopes can constrain boron sources and pathways in subduction zones, and trace phases recycled into the deep mantle and their later recycling in ocean island basalts.
... Serpentinization is a low-grade metamorphic process, which commonly involves large volumes of ultramafic rocks exhumed and hydrated during continental rifting. At present-day, these serpentinized bodies are mostly located in the offshore regions of passive margins, in correspondence of the oceanic ridge axes or along the oceanic transform faults (Boschi, et al., 2013), making their accessibility and evaluation for potential hydrogen generation difficult. ...
... The fluid-mobile elements (e.g. Rb, Ba, U, Pb), which are usually enriched in the altered oceanic crust or serpentinized abyssal peridotite to variously extents (Boschi et al., 2013), are well correlated with the fluid-immobile element Nb ( Supplementary Fig. 6). Note that our samples are distributed along the ridge segments as long as several hundreds of kilometers, including the segments controlled by both magmatic and tectonic activities (thick oceanic crust vs. mantle directly exposed, Fig. 1) respectively. ...
... The B concentrations and δ 11 B give further constraints to this issue. The positive and linear trends between B and F and Nb (Fig. 6) are hard to reconcile with the assimilation of serpentinized peridotite, which has rather high B content (usually >50 ppm) but low Nb (mostly <0.3 ppm) (Boschi et al., 2013) and the assimilation would lead to extensively elevation of B but no significant change of the latter. Although the uncertainty of B isotopic data in this study is large, assimilation of altered oceanic and/or box labeled as "mantle value" represents the ratios in mantle-derived basalts free of seawater assimilation. ...
... Although the uncertainty of B isotopic data in this study is large, assimilation of altered oceanic and/or box labeled as "mantle value" represents the ratios in mantle-derived basalts free of seawater assimilation. The data for the mantle, seawater and brines (with brine salinities higher than 50% salts) are from Kendrick et al. (2017) and the data for serpentinized peridotite from Boschi et al. (2013). In a, b and c, compositions of basalt glasses from the Arctic ridge (Mohns, Jan Mayer and Kolbein) and Mid-Atlantic ridge (33 • N∼40 • N, FAZAR cruise) are also plotted for comparison, the data are from the Dixon et al. (2017) and its supporting supplementary dataset. ...
Trace amounts of water in the sub-oceanic mantle play crucial role in the vigor of mantle convection and the production of oceanic crust, and other many geodynamic processes. Consequently, the cycling of H 2 O between the mantle and the exosphere in the mantle is one of the critical processes governing Earth's geodynamical and geochemical evolution. While the deep cycling of altered oceanic lithosphere was considered as the main way to replenish the water in oceanic upper mantle, the significance of the arc mantle wedge after the genesis of arc magmatism dragged down by the subducting slab concomitantly has been not well constrained. Here, we report that fresh depleted basaltic glasses from the ultraslow-spreading Southwestern Indian Ridge (SWIR), located far from any recent subduction zones, show unusually high H 2 O/Ce ratios (>600), water contents and heavy hydrogen isotopic compositions. These results could be best explained by recycling of water through melting of a residual hydrous mantle wedge after early melt extraction. Considering that such mantle wedges residue dragged down to the deeper mantle could occupy a volume one order of magnitude larger than that of the subducted lithosphere in the earth history, we suggest that the potential role of such shallow recycling should be considered in studies of global water recycling and the origin of water in the upper asthenosphere.
... On both Al 2 O 3 and TiO 2 vs. MgO diagrams (Fig. 11a, b), as well as on the Mg/Si vs. Al/Si diagram (Fig. 12), the studied serpentinites fall within the field of mantle wedge fresh and serpentinized peridotites (i.e., Izu-Bonin Mariana type). The talc-schist sample is however distinguished by lower MgO content and an anomalously low Mg/Si value, outside the field of mantle peridotites, a feature that may be explained by local SiO 2 enrichment during serpentinization (e.g., Snow and Dick, 1995;Niu, 2004;Bach et al., 2004;Paulick et al., 2006;Kodolányi et al., 2012;Boschi et al., 2013;Harvey et al., 2014;Malvoisin, 2015). ...
... Thus, the REE systematic observed in these samples is better explained by melt processes under lithospheric mantle conditions. Finally, we deduce that, apart from CaO and the FME elements Cs, (2004), Paulick et al. (2006), Godard et al. (2008), Boschi et al. (2013). Blue fields for Mantle wedge peridotites and serpentinites are from Parkinson and Pearce (1998), Savov et al. (2005Savov et al. ( , 2007, Kodolányi et al. (2012). ...
... Similar to Izu-Bonin Mariana mantle peridotites (Parkinson and Pearce, 1998;Savov et al., 2005Savov et al., , 2007Kodolányi et al., 2012), the North In-Tedeini samples show depleted concave-upward REE CN patterns and a higher tendency to clustered HREE compared with M-and LREE patterns. In contrast, the SIT samples show more enriched and flatter REE CN patterns, overlapping abyssal mantle peridotites (Niu, 2004;Paulick et al., 2006;Godard et al., 2008;Boschi et al., 2013) and the enriched mantle peridotites from Izu-Bonin Mariana system (Parkinson and Pearce, 1998;Savov et al., 2005Savov et al., , 2007Kodolányi et al., 2012). Such variations are generally ascribed to different degrees of partial melting in subduction zones. ...
The assembly of West Gondwana supercontinent involved several complex processes that led to the formation of the Hoggar shield during the Panafrican-Brasiliano event. We present the first petro-geochemical, geochrono- logical and field data of serpentinite lenses exposed along the In-Tedeini domain, within deformed talc-schist and magmatic-sedimentary formations. The serpentinites preserved the main characteristics of their parent perido- tites (Al/Si: 0.004–0.03; Mg/Si: 1.14–1.62; Al2O3: 0.15–1.37 wt%; Mg#: 85.3–94; Ti: 9.34–120 ppm; Nb: 0.007–0.46 ppm), attesting to a highly depleted mantle wedge protolith involved in a subduction zone. This is in agreement with the high Cr# (0.55–0.6), low to moderate Mg# (0.36–0.65) and low TiO2 contents (<0.1 wt%) of their constitutive Cr-spinels. Geochemical modelling suggest that both the North and South In-Tedeini serpen- tinite units have experienced intense and similar fluid-induced dynamic melting episodes. The evolution of these two units has diverged, with the Southern In-Tedeini unit being refertilized by a small volume of island-arc basaltic melts generated in the mantle wedge. Serpentinization of these rocks probably occurred under static conditions at high temperature corresponding essentially to amphibolite-facies conditions. Field relations suggest that the exhumation of the massive serpentinites occurred along major sinistral shear zones steeply dipping eastward, assisted by talc-schists that highly localized transpressive deformation. First U-Pb zircon ages obtained from a metasomatic chloritite in North In-Tedeini serpentinites; they may have recorded the age (770 ± 5 Ma) of the subduction related Panafrican island arcs and the emplacement (631 ± 10 Ma) of In-Tedeini serpentinites within the crust; or they may rather correspond to the serpentinization events endured by the rocks. All together, the reported results support the presence of a major suture zone, oriented NNW-SSE. This suture is outlined by mantle serpentinite lenses exhumed in a collisional accretionary wedge, which connects the western and the central Hoggar, following a Panafrican east-dipping subduction. This tectonic system would have contributed to the closure of the Goi ́as-Pharusian ocean.
... Ocean floor serpentinisation including at trench to shallow forearc settings produces characteristic element enrichments, either absolute ones to values exceeding PM concentrations or relative ones to elements of similar compatibility during mantle melting. Elements that can be absolutely enriched include B, As, Sb, U, W, Cl, Br, I, Sr, Li, and LILE, while relative enrichments are sometimes observed for Pb (compare Kodolányi et al., 2012;Vils et al., 2008;Savov et al., 2005Savov et al., , 2007Paulick et al., 2006;Kendrick et al., 2013;Jöns et al., 2010;Deschamps et al., 2011;Boschi et al., 2013;Debret et al., 2013;Andreani et al., 2014;Barnes et al., 2014;Kahl et al., 2015;Albers et al., 2020;Hattori and Guillot, 2003;Bonatti et al., 1984;Thompson and Melson, 1970). However, data for a series of elements have remained scarce to date for ocean floor serpentinites including W, In, Bi, Sn, Cd, and Tl, of which notably the chalcophile elements Tl and Sn were documented to be strongly enriched in altered MORB (Jochum and Verma, 1996). ...
Serpentinites are central to water (re)cycling in subduction zones and thus effect fluid-mediated element transfer between the hydrosphere and the Earth's mantle and back into continental crust via calcalkaline magmatism. Diverse and often controversial models exist on the relevance of various source contributions to the budget of fluid mobile elements of hydrous peridotites and how these evolve during the subduction cycle. This work offers novel constraints on ongoing debates. We present a comprehensive bulk rock and silicate mineral major to trace element study covering the antigorite dehydration reaction based on Cerro del Almirez antigorite-serpentinites and chlorite-harzburgites, including the systematics of As, Sb, B, W, Li, In, Tl, Bi, Cd, and Sn - so far unavailable for Almirez, and there exist only few such data for orogenic serpentinites in general. We integrate these with reviewed literature data and develop a general model for the geochemical systematics of subducting hydrous slab mantle covering magmatic peridotite conditioning, element enrichment upon oceanic hydration, compositional evolution with progressive subduction to peak temperature antigorite dehydration, and retrograde metasomatism upon exhumation.
Pre-hydration magmatic processes produce strong compositional variations on centimetre to metre to kilometre scales. Serpentinisation via seawater and sediment-equilibrated pore fluids produces highly variable fluid-mobile element (FME) bulk rock enrichments in B, As, Sb, W, Cs, ±Li, ±Bi, ±Pb, ±U exceeding primitive mantle concentrations. Hydration enrichment numbers represent a novel concept introduced in this work to refine the extent of hydration-mediated FME enrichment. They represent the measured ratio of fluid-mobile element over a fluid-immobile element of closely comparable magmatic compatibility normalised to its corresponding primitive mantle abundance ratio. Hydration enrichment numbers are highest for Sb and As (up to 650) and lowest for Ba and Rb (down to 0.06) for Almirez data, quantifying fractions of minimal enrichment (values >1) and minimal prograde subduction loss (values <1). FME enrichment occurred primarily in ocean floor to trench to shallow forearc settings where sediment-equilibrated pore fluids are relevant, while addition from deeper sediment metamorphic dehydration fluids with progressive subduction is subordinate at best. Prominent fractions of As, Sb, B, Rb, Sr, Cs, Ba, Pb, Zn, Cl, Br, Li, Na, K, and Ca are then lost to the fluid upon serpentinite dehydration including the antigorite-out reaction. We find no evidence in support of significant fluid-mediated element addition (e.g., Th, U, Ta, Sr, Pb, Cs, Rb, Li) upon antigorite dehydration as has been postulated in literature by simple comparison between Atg-serpentinite and Chl-harzburgite coexisting at Almirez. Magmatic preconditioning prior to serpentinisation can account for the differences in HFSE between Atg-serpentinite and Chl-harzburgite, while prominent FME addition upon retrogression as measured on retrograde serpentine and talc is demonstrated for Cs, Pb, Sr and Ba, thus adulterating bulk rock systematics for certain FME.
This work concludes that oceanic serpentinisation dominates the FME imprint of subducting slab serpentinites and that progressive subduction goes along with successive FME loss. We propose that serpentinites from fluid-dominated, highly hybridised and oxidised plate interface melange materials displaying spectacular FME enrichments with a sedimentary flavour are distinctly overrepresented in our sample record. Therefore, the combined data set of hydrous peridotites from Almirez and Erro Tobbio may offer a more representative compositional estimate of the bulk mass of subducted slab serpentinite, to be used in modelling of the geochemical impact of serpentinite-derived matter on fluid-mediated chemical cycling in subduction zones.
... Cependant, l'étude des systèmes hydrothermaux démontre que le processus de serpentinisation s'étend depuis des milliers d'années et s'observe également jusqu'à plusieurs kilomètres de la ride océanique. La serpentinisation et donc la perméabilité de la roche sont maintenues dans le temps (e.g., Boschi et al., 2013;. Le développement de fissures et fractures assure le maintien de la perméabilité en permettant l'écoulement des fluides Macdonald and Fyfe, 1985;. ...
La production abiotique de dihydrogène (H2) est observée et mesurée dans les environnements où l’altération hydrothermale des roches ultrabasiques (serpentinisation) a lieu. Cette réaction est associée à l’oxydation du Fe2+ contenu dans l’olivine et les pyroxènes, en Fe3+ hébergé par la magnétite et/ou serpentine. L’oxydation du fer est couplée à la réduction de l’eau conduisant à la formation d’H2. Les marges passives non volcaniques exposent du manteau serpentinisé sur une échelle km et présentent donc un grand intérêt pour la production d’H2. Notre étude est menée sur la paléo-marge Ibérique (NO Pyrénées), où des reliques de manteau affleurent.Cette thèse combine approches analytique et expérimentale. La première partie comprend une étude pétrostructurale (EBSD), minéralogique (XRD, Raman, XANES au seuil K du fer en roche totale et in-situ) et géochimique ((µ)-XRF, EPMA, (LA)-ICPMS) de 32 échantillons issus de 3 massifs de péridotites Pyrénéens : Urdach, Turon et Montaut. La deuxième partie présente les résultats d’une expérience hydrothermale longue réalisée sur des lherzolites du Turon.Les massifs étudiés sont principalement composés de lherzolites à spinelles issues d’un même manteau sous-continental, présentant des degrés de serpentinisation contrastés (teneurs en serpentine 14-100 pds%). Les trois massifs ont des teneurs variées en magnétite, indépendantes de leur degré d’altération et inversement proportionnelles à la teneur en Fe des serpentines. La précipitation de magnétite est attribuée à des températures de serpentinisation >250 °C.Les enrichissements sélectifs des éléments mobiles dans les fluides, en particulier Cs, Sb et Li, indiquent des interactions pour Turon et Montaut, avec des fluides dérivés des sédiments et de la croûte continentale. Ces résultats sont cohérents avec la position structurale de ces massifs, restés sous une unité de croûte continentale et de sédiments pré-rift au cours de l’événement extensif. Le massif d’Urdach, exposé sur le fond océanique, enregistre la formation d’ophicalcites et de serpentinites. Nos résultats montrent que les ophicalcites contiennent de la magnétite tandis que les serpentinites en sont exemptes. Ces contrastes sont interprétés comme représentatifs de chemins de réaction fluides/roches variés; avec des fluides riches en Ca-Sr pour les ophicalcites et des fluides riches en Si d’origines crustale pour les serpentinites. Ces conditions contrastées de serpentinisation résultent de contextes structuraux différents.A un même taux de serpentinisation, l’hydratation du manteau sous-continental produit autant d’H2 que le manteau océanique, mais affiche une contribution plus importante de la Fe3+-serpentine. La faible teneur en magnétite induite par des températures de serpentinisation plus basses et/ou une activité en silice du fluide plus élevée explique ce résultat. Compte tenu de nos résultats et des quantités de péridotites exposées aux marges passives, nous supposons que ces environnements ont une contribution significative dans le cycle global de l’H2.Le comportement du Fe a été étudié lors d’une expérience de serpentinisation pour mieux contraindre la distribution et la valence du Fe entre serpentine et magnétite. L’altération se produit dans des conditions hautement oxydantes avec la précipitation d’iddingsite et d’hématite, et un front de dissolution riche en Si se développe à la surface de l’échantillon. La nouvelle serpentine, située dans les veines du protolithe, est riche en Fe. En lien avec la fO2 élevée, nous suggérons que le fer est remobilisé de la magnétite instable vers la serpentine. Les résultats de la spectroscopie XANES au seuil K du fer montrent une valence du fer contrastée entre les surfaces exposées au fluide affichant des états d’oxydation élevés et la serpentine contenue dans la roche affichant des états d’oxydation faibles. Nous supposons que le comportement de l’H2 (piégeage ou libération) contrôle les conditions locales de fO2.
