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.

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

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.

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.

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

2019 ◽  
Author(s):  
Alice Vho ◽  
Pierre Lanari ◽  
Daniela Rubatto ◽  
Jörg Hermann
Keyword(s):  
Oxygen Isotope ◽  
Subduction Zones ◽  
Integrated Modelling ◽  
Fluid Loss ◽  
Oxygen Isotope Fractionation ◽  
Rock Interaction ◽  
Mafic Rocks ◽  
External Fluid ◽  
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 both 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-temperature (P-T) conditions in the subducting slab. The modelled system is composed by a sequence of oceanic crust (mafic) with sedimentary cover of known initial chemical composition and bulk δ18O. The evolution of mineral assemblage and δ18O values of each phase is calculated along a defined P-T path. Fluid-rock interactions may occur as consequence of (1) infiltration of an external fluid into the mafic rocks or (2) transfer of the fluid liberated by dehydration reactions occurring in the mafic rocks into the sedimentary rocks. The effects of interaction with externally-derived fluids on the mineral and bulk δ18O of each rock are quantified for two typical compositions of metabasalts and metasediments with external fluid influx from serpentinite. The dehydration reactions, fluid loss and mineral fractionation produce minor to negligible variations in bulk δ18O values, i.e. within 1 ‰. By contrast, the interaction with external fluids may lead to shifts in δ18O up to one order of magnitude larger. Such variations can be detected by analysing in-situ oxygen isotope 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 a Myr and significant oxygen isotope variation at the interface. This technique opens new perspectives to track fluid pathways in subduction zones, to distinguish porous from channelized fluid flows, and to determine the P-T conditions and the extent of fluid/rock interaction.

Petrogeochemical tools for simulating fluid-rock interaction processes in high-pressure metamorphic terrains

2021 ◽  
Author(s):  
Pierre Lanari ◽  
Thomas Bovay ◽  
Daniela Rubatto ◽  
Hugo Dominguez ◽  
Thorsten Markmann ◽  
...  
Keyword(s):  
High Pressure ◽  
Oxygen Isotope ◽  
Isotope Fractionation ◽  
Open Systems ◽  
Fluid Flows ◽  
Modeling Framework ◽  
Oxygen Isotope Fractionation ◽  
Rock Interaction ◽  
Interaction Processes ◽  
Dehydration Reactions

&lt;p&gt;Petrological models based on equilibrium thermodynamics have proven critical in assessing how mineral assemblages evolve with pressure (&lt;em&gt;P&lt;/em&gt;) and temperature (&lt;em&gt;T&lt;/em&gt;) conditions. Still, they remain limited for the investigation and simulation of fluid-rock interaction processes in open systems. The interaction between a reacting aqueous fluid and a (already water-saturated) rock at eclogite facies conditions, for example, can have no or very limited effects on the mineral assemblage&amp;#8212;beyond eventually triggering re-equilibration. Therefore, pervasive fluid flows that are not associated to intense metasomatism cannot be modeled using phase diagrams and often remain hardly noticeable even to experienced petrologists. Unlike major and minor elements used for thermodynamic modeling, stable isotopes (e.g. oxygen) are known to be more sensitive for recording interaction with a fluid in isotopic disequilibrium.&lt;/p&gt;&lt;p&gt;In order to extend the existing modeling capabilities, an integrated modeling framework was developed applicable to multi-rock open systems combining thermodynamic and oxygen isotope fractionation modeling based on internally consistent databases (Vho et al. 2019, 2020). The petrological model quantifies the effect of dehydration reactions on the bulk &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of a rock during prograde metamorphism and can simulate different degrees of fluid-rock interaction with the surrounding rocks. This approach, in combination with the measurement of isotopic composition in key minerals, can be used for characterizing the behavior of open vs closed systems in natural settings and quantify the degree of fluid-rock interaction. Estimation of integrated fluid fluxes across geologic units of the Western Alps then allows permeability changes to be quantified along with the metamorphic conditions under which these changes occurred. Such results open the door to the dynamic simulation of reactive fluid flows in high-pressure environmentthat are controlled by the compaction pressure of the rock matrix.&lt;/p&gt;&lt;p&gt;This project has received funding from the European Research Council (ERC) under the European Union&amp;#8217;s Horizon 2020 research and innovation programme (grant agreement No 850530).&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;ul&gt;&lt;li&gt;Vho, A., Lanari, P., Rubatto, D. (2019). An internally-consistent database for oxygen isotope fractionation between minerals. Journal of Petrology, 60, 2101&amp;#8211;2129&lt;/li&gt; &lt;li&gt;Vho, A., Lanari, P., Rubatto, D., Herman, J. (2020). Tracing fluid transfers in subduction zones: an integrated thermodynamic and &amp;#948;18O fractionation modelling. Solid Earth, 11, 307-328&lt;/li&gt; &lt;/ul&gt;

scholarly journals Extensive fluid‐rock interaction and pressure solution in a UHP fluid pathway recorded by garnetite Lago di Cignana, Western Alps

2020 ◽  
Author(s):  
Hugo W. van Schrojenstein Lantman ◽  
Marco Scambelluri ◽  
Mattia Gilio ◽  
David Wallis ◽  
Matteo Alvaro
Keyword(s):  
Western Alps ◽  
Pressure Solution ◽  
Rock Interaction ◽  
Fluid Pathway

Oxygen isotope fractionation between rutile and water and geothermometry of metamorphic eclogites

1979 ◽  
Vol 43 (327) ◽  
pp. 405-413 ◽  
Author(s):  
Alan Matthews ◽  
Robert D. Beckinsale ◽  
John J. Durham
Keyword(s):  
Oxygen Isotope ◽  
Isotope Fractionation ◽  
Western Alps ◽  
Titanium Metal ◽  
Isotopic Equilibrium ◽  
Oxygen Isotope Fractionation ◽  
Tauern Window ◽  
Theoretical Predictions ◽  
Aqueous Oxidation ◽  
Isotope Analyses

SummaryOxygen isotope fractionation between rutile and water has been studied from 300 °C to 700 °C, PH2O = 1 kb, using aqueous oxidation of titanium metal as the equilibration reaction. The mechanism of rutile formation (which is critical to the assessment of isotopic equilibrium) is an ‘armouring’ reaction in which rutile grows around grains of titanium metal by solution-precipitation processes. Mean fractionation factors expressed as 103 In αTiO2-H2O obtained in the present study are:−6.20±0.23‰ at 304±5 °C−6.64±0.27‰ at 405±6 °C−6.11±0.16%. at 508±6 °C−4.45±0.28%. at 608±6 °C−3.38±0.15%. at 698±6 °C.These data agree with those obtained at temperatures above 500 °C by Addy and Garlick (1974) but do not accord with theoretical predictions by Bottinga and Javoy (1973). A minimum in the calibration curve 103 ln α versus 106T−2 occurs between 300 °C and 500 °C but from 500 °C to 700 °C 18O fractionation between rutile and water may be expressed by the equation:103 ln α = −(4.72±0.40)106T−2+(1.62±0.53).Oxygen isotope analyses of rutile and quartz from metamorphic eclogites and schists from the Tauern Window, Austria, yield isotopic temperatures at about 550 °C in agreement with results obtained on similar rocks from the Sesia Zone (Western Alps, Italy) and elsewhere by other workers. Petrologic studies indicate that the latest metamorphism of the Tauern eclogites reached about 450 °C Thus the measured partitions of 18O between rutile and quartz indicating temperatures around 550 °C have been inherited from an earlier metamorphic event.

High-pressure metamorphic age and significance of eclogite-facies continental fragments associated with oceanic lithosphere in the Western Alps (Etirol-Levaz Slice, Valtournenche, Italy)

Lithos ◽  
2016 ◽  
Vol 252-253 ◽  
pp. 145-159 ◽  
Author(s):  
Kathrin Fassmer ◽  
Gerrit Obermüller ◽  
Thorsten J. Nagel ◽  
Frederik Kirst ◽  
Nikolaus Froitzheim ◽  
...  
Keyword(s):  
High Pressure ◽  
Oceanic Lithosphere ◽  
Western Alps ◽  
Eclogite Facies
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