Composition of fluids released along a subduction interface with increasing depth: insights from fluid inclusions analysis on the Schistes Lustrés - Monviso transect (Western Alps)

2020 ◽  
Author(s):  
Clément Herviou ◽  
Anne Verlaguet ◽  
Philippe Agard ◽  
Hugues Raimbourg ◽  
Michele Locatelli ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Fluid Inclusions ◽  
Subduction Zones ◽  
Shear Zones ◽  
Free Fluid ◽  
Accretionary Wedge ◽  
Continuous Increase ◽  
Dehydration Reactions ◽  
Eclogite Facies ◽  
Fluid Sources

<p>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.</p><p>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.</p><p>The Schistes Lustrés 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 Lustrés units show increasing peak P-T conditions from blueschist (300-350°C - 1.2-1.3 GPa) to eclogite facies (580°C - 2.8 GPa). This study focuses on the Schistes Lustrés - Monviso transect, which shows an almost continuous increase in metamorphic grade.</p><p>In the Schistes Lustrés 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 CO<sub>2</sub> and CH<sub>4</sub>. In contrast, eclogitic fluids released from metagabbros are highly saline brines with low N<sub>2 </sub>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.</p>

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

2020 ◽  
Author(s):  
Cailey Condit ◽  
◽  
Victor Guevara ◽  
Jonathan R. Delph ◽  
Melodie French
Keyword(s):  
Oceanic Crust ◽  
Subduction Zones ◽  
Slow Slip ◽  
Fluid Sources ◽  
Metamorphic Dehydration

scholarly journals Tracing fluid transfers in subduction zones: an integrated thermodynamic and <i>δ</i><sup>18</sup>O fractionation modelling approach

Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 307-328 ◽  
Author(s):  
Alice Vho ◽  
Pierre Lanari ◽  
Daniela Rubatto ◽  
Jörg Hermann
Keyword(s):  
Oxygen Isotope ◽  
Oceanic Crust ◽  
Subduction Zones ◽  
Integrated Modelling ◽  
Fluid Loss ◽  
Rock Composition ◽  
Oxygen Isotope Fractionation ◽  
Rock Interaction ◽  
Dehydration Reactions ◽  
Modelling Approach

Abstract. Oxygen isotope geochemistry is a powerful tool for investigating rocks that interacted with fluids, to assess fluid sources and quantify the conditions of fluid–rock interaction. We present an integrated modelling approach and the computer program PTLoop that combine thermodynamic and oxygen isotope fractionation modelling for multi-rock open systems. The strategy involves a robust petrological model performing on-the-fly Gibbs energy minimizations coupled to an oxygen fractionation model for a given chemical and isotopic bulk rock composition; both models are based on internally consistent databases. This approach is applied to subduction zone metamorphism to predict the possible range of δ18O values for stable phases and aqueous fluids at various pressure (P) and temperature (T) conditions in the subducting slab. The modelled system is composed of a mafic oceanic crust with a sedimentary cover of known initial chemical composition and bulk δ18O. The evolution of mineral assemblages and δ18O values of each phase is calculated along a defined P–T path for two typical compositions of basalts and sediments. In a closed system, the dehydration reactions, fluid loss and mineral fractionation produce minor to negligible variations (i.e. within 1 ‰) in the bulk δ18O values of the rocks, which are likely to remain representative of the protolith composition. In an open system, fluid–rock interaction may occur (1) in the metasediment, as a consequence of infiltration of the fluid liberated by dehydration reactions occurring in the metamorphosed mafic oceanic crust, and (2) in the metabasalt, as a consequence of infiltration of an external fluid originated by dehydration of underlying serpentinites. In each rock type, the interaction with external fluids may lead to shifts in δ18O up to 1 order of magnitude larger than those calculated for closed systems. Such variations can be detected by analysing in situ oxygen isotopes in key metamorphic minerals such as garnet, white mica and quartz. The simulations show that when the water released by the slab infiltrates the forearc mantle wedge, it can cause extensive serpentinization within fractions of 1 Myr and significant oxygen isotope variation at the interface. The approach presented here opens new perspectives for tracking fluid pathways in subduction zones, to distinguish porous from channelled fluid flows, and to determine the P–T conditions and the extent of fluid–rock interaction.

Fluid inclusions in granulites and eclogites from the Bergen Arcs, Caledonides of W. Norway

1990 ◽  
Vol 54 (375) ◽  
pp. 145-158 ◽  
Author(s):  
T. Andersen ◽  
H. Austrheim ◽  
E. A. J. Burke
Keyword(s):  
Fluid Inclusions ◽  
Shear Zones ◽  
Granulite Facies ◽  
Mole Percent ◽  
Molar Volumes ◽  
Granulite Facies Metamorphism ◽  
Laser Raman ◽  
Granulite Metamorphism ◽  
Facies Metamorphism ◽  
Eclogite Facies

AbstractThe Grenvillian granulite-facies complex on Holsnøy island, Bergen Arcs, W. Norway, has been metamorphosed at eclogite-facies conditions during the Caledonian orogeny (ca. 425 Ma). The granulite-eclogite facies transition takes place along shear zones and fluid pathways. Mineral thermobarometry indicates PT conditions of 800–900°C and 8–10 kbar for the Proterozoic granulite facies metamorphism and 700–800°C and 16–19 kbar for the eclogite-forming event. Quartz in the granulite facies complex contains CO2 fluid inclusions with less than 2.5 mole percent N2; the molar volumes are compatible with the PT conditions of the Proterozoic granulite metamorphism. Quartz in pegmatitic quartz + omphacite and quartz + phengite/paragonite veins coeval with shear-zone eclogites contain N2 ± CO2 fluid inclusions. Combined laser Raman microanalysis and microthermometry show that the least disturbed inclusions have XCO2 = 0.1–0.3, and molar volumes less than 40 cm3/mole, which may agree with the PT conditions during Caledonian high-pressure metamorphism. Younger, low-density N2 and N2-H2O fluid inclusions are the results of decrepitation and redistribution of early inclusions during the retrograde PT evolution of the eclogites.

scholarly journals Volatile Alteration and Metamorphic Facies: Release of H2O and CO2 in the Panjal Volcanics of the NW Himalaya

1995 ◽  
Vol 11 ◽  
Author(s):  
David A. Spencer
Keyword(s):  
Oceanic Crust ◽  
Theoretical Calculations ◽  
Coulometric Titration ◽  
Direct Determination ◽  
Metamorphic Grade ◽  
Loss On Ignition ◽  
Indirect Determination ◽  
Dehydration Reactions ◽  
Eclogite Facies

Whilst the bulk rock geochemistry of a basalt, subjected to progressively increasing metamorphic grade, may remain on the whole isochemical, it has been long known that volatiles are likely to be released. Three volatiles which should change through an increase in metamorphic grade are H2O, CO2 and O2. The various metamorphic grades of the Panjal Volcanics in the NW Himalayas (unmetamorphosed, greenschist, amphibolite and eclogite facies) provide an ideal opportunity to test these ideas and possibly act as chemical indicators of the prograde metamorphism. Direct and indirect determination of CO2 and H2O were made on numerous samples. Direct determination of CO2 content in a rock sample has been made by coulometric titration for five main groups: Zanskar Tethyan Shelf - Panjal Volcanic basalts (Ba); Lesser Himalaya- Panjal Volcanic basalts (PJ); Higher Himalaya Second Cover - Panjal Volcanic amphibolites (A); Higher Himalaya Second Cover - Panjal Volcanic dolerites (D) and Higher Himalaya Second Cover - Panjal Volcanic eclogites (E). The main observation on the occurrence of % CO2 in the Panjal Volcanics with increasing metamorphic grade is that there is a decrease in CO2 that appears to be fairly gradual. This therefore suggests that with increasing metamorphism, CO2 is being driven off by decarbonation reactions. Estimation of the volatile content in a sample can be made by the simple2weight loss on ignition, and used as a method of indirect determination of H2O. The loss on ignition (L.O.I.) and the H2O (+ FeO) in the Panjal Volcanics show that with an increase in metamorphic grade there is a decrease in the L.O.I. and H2O (+ FeO) content. This therefore suggests that with increasing metamorphism, volatiles are being removed by dehydration reactions. Comparison of the results of this work with the results of some published theoretical calculations are made for the wt. % H2O of a subducting, and therefore dehydrating, oceanic crust or tholeiitic basalt. Here, predictions were made by combining calculated pressure - temperature paths with a model of metabasalt phase equilibria, where a progression (for the upper parts of the oceanic crust) pass through greenschist to amphibolite to eclogite facies. The agreement between these two independently produced sets of data is clearly apparent.

Hydration- and dehydration-induced rheological heterogeneities on the deep subduction interface, and possible relationships to episodic tremor and slow slip

2020 ◽  
Author(s):  
Whitney Behr ◽  
Carolyn Tewksbury-Christle ◽  
Alissa Kotowski ◽  
Claudio Cannizzaro ◽  
Robert Blass ◽  
...  
Keyword(s):  
Subduction Zones ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Coarse Grained ◽  
Slow Slip ◽  
Brittle Deformation ◽  
Interface Shear ◽  
Dehydration Reactions ◽  
Viscous Shear ◽  
Episodic Tremor

&lt;p&gt;Episodic tremor and slow slip (ETS) is observed in several subduction zones down-dip of the locked megathrust, and may provide clues for preparatory processes before megathrust rupture. Exhumed rocks provide a unique opportunity to evaluate the sources of rheological heterogeneity on the subduction interface and their potential role in generating ETS-like behavior. We present data from two subduction interface shear zones representative of the down-dip extent of the megathrust: the Condrey Mountain Schist (CMS) in northern CA (greenschist to blueschist facies conditions) and the Cycladic Blueschist Unit (CBU) on Syros Island, Greece (blueschist to eclogite facies). Both complexes highlight the propensity for fluid-mediated metamorphic reactions to produce strong rheological heterogeneities:&lt;/p&gt;&lt;p&gt;In the CMW, hydration reactions led to progressive serpentinization of peridotite bodies that were entrained from the overriding plate and underplated along with oceanic-affinity sediments. The margins of each peridotite-serpentinite lens show extreme strain localization accommodated by dislocation glide and minor pressure solution in antigorite, whereas lens interiors show evidence for more distributed, alternating, frictional-viscous deformation, with abundant crack-seal veins occupied by antigorite, brucite and oxides that are in some places also ductilely sheared. Deformation in the surrounding metasedimentary matrix was purely viscous.&lt;/p&gt;&lt;p&gt;In the CBU on Syros Island, dehydration reactions in MORB-affinity basalts, subducted and underplated with oceanic and continental-affinity sediments, led to progressive development of strong eclogitic lenses within a weaker blueschist and metasedimentary matrix. The eclogite lenses are commonly coarse-grained and massive and show brittle deformation in the form of dilational and shear fractures/veins filled with quartz, white mica, glaucophane and/or chlorite. Brittle deformation in the eclogites is coeval with ductile deformation in the surrounding blueschist and metasedimentary matrix, indicating concurrent frictional-viscous flow.&lt;/p&gt;&lt;p&gt;Although we cannot easily distinguish transient deformation processes in exhumed rocks, we can use the following three approaches to assess whether these heterogeneities could have generated deformation behaviors similar to deep ETS: 1) We measure displacements within, and dimensions of the heterogeneities in outcrop/map-scale to estimate the maximum possible seismic moment that would be released when the frictional heterogeneities slip;&amp;#160; 2) We compare deformation mechanisms inferred from field and microstructural observations to their expected mechanical behavior from rock deformation experiments; and 3) We use seismo-thermo-mechanical modeling to examine expected slip velocities and moment-duration ratios for frictional-viscous shear zones that are scaled to observations from nature and the lab.&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;All three approaches suggest that frictional-viscous heterogeneities of the types and length-scales we observe in the exhumed rock record are compatible with ETS as documented in modern subduction zones.&lt;/p&gt;

Os isotopic composition of western Aleutian adakites: Implications for the Re/Os of oceanic crust processed through hot subduction zones

2021 ◽  
Vol 292 ◽  
pp. 452-467
Author(s):  
Rachel Bezard ◽  
Simon Turner ◽  
Bruce Schaefer ◽  
Gene Yogodzinski ◽  
Kaj Hoernle
Keyword(s):  
Isotopic Composition ◽  
Oceanic Crust ◽  
Subduction Zones

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

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.

The effect of temperature-dependent thermal parameters in thermal models of subduction zones

2021 ◽  
Author(s):  
Iris van Zelst ◽  
Timothy J. Craig ◽  
Cedric Thieulot
Keyword(s):  
Subduction Zones ◽  
Thermal Parameters ◽  
Dislocation Creep ◽  
Seismogenic Zone ◽  
Effect Of Temperature ◽  
Temperature Dependent ◽  
Thermal Models ◽  
Intermediate Depth ◽  
Dehydration Reactions ◽  
Model Setup

&lt;p&gt;The thermal structure of subduction zones plays an important role in the seismicity that occurs there with e.g., the downdip limit of the seismogenic zone associated with particular isotherms (350 &amp;#176;C - 450 &amp;#176;C) and intermediate-depth seismicity linked to dehydration reactions that occur at specific temperatures and pressures. Therefore, accurate thermal models of subduction zones that include the complexities found in laboratory studies are necessary. One of the often-ignored effects in models is the temperature-dependence of the thermal parameters such as the thermal conductivity, heat capacity, and density.&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;Here, we build upon the model setup presented by Van Keken et al., 2008 by including temperature-dependent thermal parameters to an otherwise clearly constrained, simple model setup of a subducting plate. We consider a fixed kinematic slab dipping at 45&amp;#176; and a stationary overriding plate with a dynamic mantle wedge. Such a simple setup allows us to isolate the effect of temperature-dependent thermal parameters. We add a more complex plate cooling model for the oceanic plate for consistency with the thermal parameters.&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;We test the effect of temperature-dependent thermal parameters on models with different rheologies, such as an isoviscous wedge, diffusion and dislocation creep. We find that slab temperatures can change by up to 65 &amp;#176;C which affects the location of isotherm depths. The downdip limit of the seismogenic zone defined by e.g., the 350 &amp;#176;C isotherm shifts by approximately 4 km, thereby increasing the maximum possible rupture area of the seismogenic zone. Similarly, the 600 &amp;#176;C isotherm is shifted approximately 30 km deeper, affecting the depth at which dehydration reactions and hence intermediate-depth seismicity occurs. Our results therefore show that temperature-dependent thermal parameters in thermal models of subduction zones cannot be ignored when studying subduction-related seismicity.&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;

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