Internal and External Fluid Sources for Eclogite-facies Veins in the Monviso Meta-ophiolite, Western Alps: Implications for Fluid Flow in Subduction Zones

Important amounts of fluids are released in subduction zones by successive dehydration reactions occurring both in the previously hydrated oceanic crust (and mantle) and overlying sedimentary cover. The release and circulation of such fluids in rocks have major consequences on both their mechanical and chemical behavior. Indeed, the presence of a free fluid phase strongly modifies the rock rheology, fracturing properties, and could be implicated in both intermediate-depth earthquake and slow slip events nucleation. Moreover, the scale of mass transfer, associated chemical changes in infiltrated rocks and element recycling in subduction zones are controlled by both the rock permeability and the amount and composition of such fluids. Thus, there is a crucial need to identify the major fluid sources, amounts and pathways to better constrain their impact on subduction dynamics.Metamorphic veins, as well as mineralized fractures and shear zones in exhumed fossil subduction zones are the best witnesses of fluid-rock interactions and fluid circulation pathways. However, their interpretation in terms of fluid sources, residence time, scale of circulation requires a good knowledge of the composition of potential fluid sources. In order to determine the composition of the fluid released by both oceanic crust and sediments at various depth along their subduction, we analyzed the composition of fluid inclusions contained in vein minerals formed at peak P-T conditions, in rock units buried at various depths in the Alpine subduction zone.The Schistes Lustr&#233;s complex is a slice-stack representing the deep, underplated part of the former Alpine accretionary wedge. These Alpine Tethys rocks are mainly composed of oceanic calcschists with fewer mafic and ultramafic rocks, buried to various depths before exhumation. From West to East, the juxtaposed Schistes Lustr&#233;s units show increasing peak P-T conditions from blueschist (300-350&#176;C - 1.2-1.3 GPa) to eclogite facies (580&#176;C - 2.8 GPa). This study focuses on the Schistes Lustr&#233;s - Monviso transect, which shows an almost continuous increase in metamorphic grade.In the Schistes Lustr&#233;s blueschist-facies sediments, fluid inclusions were analyzed in quartz from high-pressure veins, i.e. quartz that co-crystallized with prograde to peak metamorphic minerals such as lawsonite and Fe-Mg carpholite. In the metamorphosed mafic rocks, we analyzed fluid inclusions from the peak metamorphic assemblages, i.e. glaucophane +/- omphacite in blueschist facies rocks, omphacite in eclogite-facies slices. Raman spectroscopy data on these fluid inclusions suggest that fluids released during dehydration of calcschists in blueschist-facies conditions are aqueous fluids with low-salinity and small amounts of CO2 and CH4. In contrast, eclogitic fluids released from metagabbros are highly saline brines with low N2 content. These results, which will be associated with LA-ICP-MS analysis of fluid inclusions in metasedimentary quartz veins, will contribute to better constrain the evolution of composition of the fluids liberated by dehydration reactions with depth and protolith composition along the subduction interface.

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METAMORPHIC DEHYDRATION FROM OCEANIC CRUST PROVIDES FLUID SOURCES FOR DEEP SLOW SLIP AND TREMOR IN SUBDUCTION ZONES

10.1130/abs/2020am-357702 ◽

2020 ◽

Author(s):

Cailey Condit ◽

◽

Victor Guevara ◽

Jonathan R. Delph ◽

Melodie French

Keyword(s):

Oceanic Crust ◽

Subduction Zones ◽

Slow Slip ◽

Fluid Sources ◽

Metamorphic Dehydration

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Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Contributions to Mineralogy and Petrology ◽

10.1007/s00410-021-01806-4 ◽

2021 ◽

Vol 176 (7) ◽

Author(s):

Thomas Bovay ◽

Daniela Rubatto ◽

Pierre Lanari

Keyword(s):

Oxygen Isotope ◽

Western Alps ◽

Large Contribution ◽

Chemical Mapping ◽

Rock Interaction ◽

Rock Density ◽

Chemical Zoning ◽

Dehydration Reactions ◽

Subducted Oceanic Crust ◽

Eclogite Facies

AbstractDehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

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Chloritoid-bearing assemblages in mafic systems and eclogite-facies hydration of alpine Mg-Al metagabbros (Erro-Tobbio Unit, Ligurian Western Alps)

European Journal of Mineralogy ◽

10.1127/ejm/7/5/1149 ◽

1995 ◽

Vol 7 (5) ◽

pp. 1149-1168 ◽

Cited By ~ 41

Author(s):

Bruno Messiga ◽

Marco Scambelluri ◽

Giovanni B. Piccardo

Keyword(s):

Western Alps ◽

Eclogite Facies

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Post-magmatic fracturing, fluid flow, and vein mineralization in supra-subduction zones: a comparative study on vein calcites from the Troodos ophiolite and the Izu–Bonin forearc and rear arc

International Journal of Earth Sciences ◽

10.1007/s00531-020-01978-7 ◽

2021 ◽

Vol 110 (2) ◽

pp. 627-649

Author(s):

Dennis Quandt ◽

W. Kurz ◽

P. Micheuz

Keyword(s):

Fluid Flow ◽

Subduction Zones ◽

Growth Dynamics ◽

Pillow Lava ◽

Published Data ◽

Fracturing Fluid ◽

Deep Seawater ◽

Vein Formation ◽

Mineralization Processes ◽

Extensional Faulting

AbstractBased on the published data of pillow lava-hosted mineralized veins, this study compares post-magmatic fracturing, fluid flow, and secondary mineralization processes in the Troodos and Izu–Bonin supra-subduction zone (SSZ) and discusses the crucial factors for the development of distinct vein types. Thin section and cathodoluminescence petrography, Raman spectroscopy, fluid inclusion microthermometry, and trace element and isotope (87Sr/86Sr, δ18O, δ13C, Δ47) geochemistry indicate that most veins consist of calcite that precipitated from pristine to slightly modified seawater at temperatures < 50 °C. In response to the mode of fracturing, fluid supply, and mineral growth dynamics, calcites developed distinct blocky (precipitation into fluid-filled fractures), syntaxial (crack and sealing), and antitaxial (diffusion-fed displacive growth) vein microtextures with vein type-specific geochemical signatures. Blocky veins predominate in all study areas, whereas syntaxial veins represent subordinate structures. Antitaxial veins occur in all study areas but are particularly abundant in the Izu–Bonin rear arc where the local geological setting was conducive of antitaxial veining. The temporal framework of major calcite veining coincides with the onset of extensional faulting in the respective areas and points to a tectonic control on veining. Thus, major calcite veining in the Troodos SSZ began contemporaneously with volcanic activity and extensional faulting and completed within ~ 10–20 Myr. This enabled deep seawater downflow and hydrothermal fluid upflow. In the Izu–Bonin forearc, reliable ages of vein calcites point to vein formation > 15 Myr after subduction initiation. Therefore, high-T mineralization (calcite, quartz, analcime) up to 230 °C is restricted to the Troodos SSZ.

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Magnetite (U-Th-Sm)/He dating: analytical challenges and application

10.5194/egusphere-egu21-16379 ◽

2021 ◽

Author(s):

Marianna Corre ◽

Martine Lanson ◽

Arnaud Agranier ◽

Stephane Schwartz ◽

Fabrice Brunet ◽

...

Keyword(s):

Laser Ablation ◽

Subduction Zones ◽

Major Elements ◽

Western Alps ◽

Planetary Science ◽

Mantle Xenoliths ◽

Transform Faults ◽

Geological Processes ◽

History Of ◽

Geochronological Constraints

Magnetite (U-Th-Sm)/He dating method has a strong geodynamic significance, since it provides geochronological constraints on serpentinization episodes, which are associated to important geological processes such as ophiolite obductions, subduction zones, transform faults and fluid circulations. Although helium content that range from 0.1 pmol/g to 20 pmol/g can routinely be measured, the application of this dating technique however is still limited due to major analytical obstacles. The dissolution of a single magnetite crystal and the measurement of the U, Th and Sm present at the ppb level in the corresponding solution, remains highly challenging, especially because of the absence of magnetite standard. In order to overcome these analytical issues, two strategies have been followed, and tested on magnetite from high-pressure rocks from the Western Alps (Schwartz et al., 2020). Firstly, we purified U, Th and Sm (removing Fe and other major elements) using ion exchange columns in order to analyze samples, using smaller dilution. Secondly, we performed in-situ analyzes by laser-ablation-ICPMS. Since no solid magnetite certified standard is yet available, we synthetized our own by precipitating magnetite nanocrystals. The first quantitative results obtained by LA-ICP-MS using this synthetic material along with international glass standards, are promising. The laser-ablation technique overcomes the analytical difficulties related to sample dissolution and purification. It thus opens the path to the dating of magnetite (and also spinels) in various ultramafic rocks such as mantle xenoliths or serpentinized peridotites in ophiolites.Schwartz S., Gautheron C., Ketcham R.A., Brunet F., Corre M., Agranier A., Pinna-Jamme R., Haurine F., Monvoin G., Riel N., 2020, Unraveling the exhumation history of high-press ure ophiolites using magnetite (U-Th-Sm)/He thermochronometry. Earth and Planetary Science Letters 543 (2020) 116359.

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Hydrofractures and crustal-scale fluid flow

10.5194/egusphere-egu21-14503 ◽

2021 ◽

Author(s):

Paul D. Bons ◽

Tamara de Riese ◽

Enrique Gomez-Rivas ◽

Isaac Naaman ◽

Till Sachau

Keyword(s):

Fluid Flow ◽

Large Scale ◽

Low Flow ◽

Fault System ◽

Tectonic Activity ◽

Field Evidence ◽

Dehydration Reactions ◽

Matrix Flow ◽

Matrix Porosity ◽

Fluid Sources

Fluids can circulate in all levels of the crust, as veins, ore deposits and chemical alterations and isotopic shifts indicate. It is furthermore generally accepted that faults and fractures play a central role as preferred fluid conduits. Fluid flow is, however, not only passively reacting to the presence of faults and fractures, but actively play a role in their creation, (re-) activation and sealing by mineral precipitates. This means that the interaction between fluid flow and fracturing is a two-way process, which is further controlled by tectonic activity (stress field), fluid sources and fluxes, as well as the availability of alternative fluid conduits, such as matrix porosity. Here we explore the interaction between matrix permeability and dynamic fracturing on the spatial and temporal distribution of fluid flow for upward fluid fluxes. Envisaged fluid sources can be dehydration reactions, release of igneous fluids, or release of fluids due to decompression or heating.&#160;Our 2D numerical cellular automaton-type simulations span the whole range from steady matrix-flow to highly dynamical flow through hydrofractures. Hydrofractures are initiated when matrix flow is insufficient to maintain fluid pressures below the failure threshold. When required fluid fluxes are high and/or matrix porosity low, flow is dominated by hydrofractures and the system exhibits self-organised critical phenomena. The size of fractures achieves a power-law distribution, as failure events may sometimes trigger avalanche-like amalgamation of hydrofractures. By far most hydrofracture events only lead to local fluid flow pulses within the source area. Conductive fracture networks do not develop if hydrofractures seal relatively quickly, which can be expected in deeper crustal levels. Only the larger events span the whole system and actually drain fluid from the system. We present the 10 square km hydrothermal Hidden Valley Mega-Breccia on the Paralana Fault System in South Australia as a possible example of large-scale fluid expulsion events. Although field evidence suggests that the breccia formed over a period of at least 150 Myrs, actual cumulative fluid duration may rather have been in the order of days only. This example illustrates the extreme dynamics that crustal-scale fluid flow in hydrofractures can achieve.

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Crustal reworking and hydration: insights from element zoning and oxygen isotopes of garnet in high-pressure rocks (Sesia Zone, Western Alps)

Contributions to Mineralogy and Petrology ◽

10.1007/s00410-020-01745-6 ◽

2020 ◽

Vol 175 (11) ◽

Author(s):

Vho Alice ◽

Rubatto Daniela ◽

Lanari Pierre ◽

Giuntoli Francesco ◽

Regis Daniele ◽

...

Keyword(s):

Oxygen Isotopes ◽

Subduction Zones ◽

Sea Water ◽

Fluid Flows ◽

Western Alps ◽

Garnet Growth ◽

Trace Element Geochemistry ◽

Element Geochemistry ◽

Chemical Zoning ◽

Dehydration Reactions

Abstract Subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere. A better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial. In this study, major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the Sesia Zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin. Garnet shows a variety of textures requiring dissolution–precipitation processes in presence of fluids. In polycyclic metasediments, garnet preserves a partly resorbed core, related to pre-Alpine high-temperature/low-pressure metamorphism, and one or multiple rim generations, associated with Alpine subduction metamorphism. In eclogites, garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim. In metasediments, pre-Alpine garnet relics show δ18O values up to 5.3 ‰ higher than the Alpine rims, while no significant variation is observed among different Alpine garnet generations within each sample. This suggests that an extensive re-equilibration with an externally-derived fluid of distinct lower δ18O occurred before, or in correspondence to, the first Alpine garnet growth, while subsequent influxes of fluid had δ18O close to equilibrium. The observed shift in garnet δ18O is attributed to a possible combination of (1) interaction with sea-water derived fluids during pre-Alpine crustal extension and (2) fluids from dehydration reactions occurring during subduction of previously hydrated rocks, such as the serpentinised lithospheric mantle or hydrated portions of the basement.

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