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

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.

Textural evolution and fluid-rock interaction during upper crustal, seismic deformation: Insights from the carbonate-dominated fault rock suite of the Belluno Thrust, Italian Southern Alps

2020 ◽  
Author(s):  
Renato Diamanti ◽  
Costantino Zuccari ◽  
Selina Bonini ◽  
Gianluca Vignaroli ◽  
Giulio Viola
Keyword(s):  
Structural Characterization ◽  
Strain Localization ◽  
Hanging Wall ◽  
Damage Zone ◽  
Southern Alps ◽  
Deformation Mechanisms ◽  
Pressure Solution ◽  
Time Interval ◽  
Rock Interaction ◽  
Calcite Veins

<p>A multi-scalar, multi-methodological approach has been used to characterize the deformation mechanisms and fluid-rock interaction processes within the Belluno Thrust (BT), a regional-scale thrust cutting through Mesozoic carbonates of the eastern Southern Alps of Italy. We report the first results of a systematic analysis of the deformation mechanisms that steered strain localization within the BT fault zone during seismogenic faulting. The WSW-ENE-striking BT contributed to development of the south-verging thrust-and-fold belt of the Southern Alps during the Late Oligocene – present time interval.        We studied an outstanding exposure of the BT in the greater Feltre region, where the BT juxtaposes an Early Jurassic oolitic and micritic limestone (the Calcari Grigi Group) in the hanging wall against an Upper Jurassic-Early Cretaceous pelagic and cherty limestone (the Maiolica Fm.). The BT is defined by a 2 m-thick damage zone formed at the expense of both the hanging wall and footwall blocks. Atop the damage zone is a millimetric principal slip surface (PSS) that strikes WSW-ENE and dips 40° to the NNW. Kinematic analysis confirms the top-to-the SSE transport along the BT. Several structural facies have been identified by means of detailed structural mapping and sampled from the damage zone (from within both the hanging- and the footwall blocks) and the PSS. The outcrop structural characterization has revealed a number of physically juxtaposed, yet different, structural facies: i) cohesive, weakly foliated proto- to ultracataclasite; ii) uncohesive, clay-rich gouge; iii) foliated domains with SC-C’ structures. Relatively unstrained host rock lithons are wrapped by these variably strained domains. Petrographic and microstructural analyses show evidence of pervasive pressure solution, with abundant stylolites, slickolites and foliated domains indicating an overall ductile behaviour. Calcite veins are also common in all recognised structural facies showing mutual cross-cutting relationships with the pressure-solution seams. This structural characterization has provided the basis for detailed image analysis of selected cataclastic textures to calculate fractal parameters for the particle size distribution (Ds) and morphology (Dr) of the clasts aiming at better understanding the cataclastic flow active in the BT fault rocks. Results from a range of representative samples suggest corrosive wear to be the main cataclastic process (Ds 1,41 ÷ 2,00; Dr 1,51 ÷ 1,88). Cathodoluminescence imaging revealed multiple generations of cement and permitted discriminating the first-order chemical characteristics of parental fluids and constraining the relationships between calcite veining and cementation. Two syn-tectonic cements have been identified: i) a bright-orange cement, preferentially surrounding carbonate clasts with highly irregular margins, indicative of the involvement of carbonate-reactive fluids; ii) a dull, homogeneous brown/black cement coexisting with a siliceous matrix, mantled clasts and local sigmoidal structures. The latter is at times observed as thin injections and fluidized structures.        Our preliminary results suggest that overall deformation was accommodated by creep and low-T crystal-plastic deformation possibly during inter-seismic phases as indicated by the presence of pressure-solution seams and foliated fabrics. Transient spikes of coseismic rupturing possibly promoted by multiple batches of overpressured fluids were accompanied by significant cataclasis and brittle strain localization.</p>

High volumes of mineral dissolution by localized fluid pulses in UHPM metasediments of Lago di Cignana, Western Alps

2020 ◽  
Author(s):  
Hugo van Schrojenstein Lantman ◽  
David Wallis ◽  
Marco Scambelluri ◽  
Matteo Alvaro
Keyword(s):  
Host Rock ◽  
Boundary Migration ◽  
Oceanic Lithosphere ◽  
Western Alps ◽  
Mineral Dissolution ◽  
Partial Replacement ◽  
Pressure Solution ◽  
Spatial And Temporal Scales ◽  
Mass Removal ◽  
Geological Processes

<p>Fluids play key roles in many geological processes across wide ranges of spatial and temporal scales. A major challenge in establishing the impacts of fluids is that partial replacement of minerals by dissolution-precipitation produces gaps in the rock record. Finding the records of such processes can help in understanding and reconstructing the processes of fluid flow, mineral dissolution and related volume changes.</p><p>The ultra-high pressure metamorphic (UHPM) Lago di Cignana Unit (Zermatt-Saas Zone, Western Alps) has been intensively studied because it is a piece of exhumed coesite- and diamond-bearing oceanic lithosphere. In this unit, schistose quartzite hosts coesite-bearing garnet and contains lenses of garnetite, which previously have been attributed to local bulk-compositional differences. Almost the entire quartzite consists of a retrograde mineral assemblage, and therefore processes occurring during subduction are best recorded in garnet.</p><p>A combined microstructural and petrological study of the garnetite lenses and their host rock reveals evidence for compaction by dissolution during subduction, partially driven by intergranular pressure solution. As the host rock matrix is removed, garnet is preferentially preserved and concentrated into garnetite. Garnet-garnet contacts then result in pressure solution and grain boundary migration. Different garnet densities and microstructures in the garnetite, alongside dissolution-reprecipitation structures in host rock garnet, suggest a complex process driven by fluid pulses. Linking garnet composition and structures to P-T through barometry on inclusions reveals an evolving fluid pathway during prograde to peak metamorphism, resulting in significant mass removal by pressure solution in metasediments subducted to UHPM conditions.</p><p>The occurrence of pressure solution and mass removal at UHPM conditions in combination with the large amounts of fluids produced by slab dehydration suggests that dissolution may play a significant role in metasediments during subduction.</p><p> </p><p>Acknowledgements: This project has received funding from the European Research Council under the H2020 research and innovation program (N. 714936 TRUE DEPTHS to M. Alvaro)</p>

Multimineral chronology of a complex high pressure terrane: insights from the Theodul Gletscher Unit (Western Alps, Switzerland)

2020 ◽  
Author(s):  
Thomas Bovay ◽  
Matthijs A. Smit ◽  
Daniela Rubatto
Keyword(s):  
High Pressure ◽  
Isotope Analysis ◽  
Carbonaceous Material ◽  
Western Alps ◽  
Growth Stages ◽  
Rock Interaction ◽  
Mafic Rocks ◽  
Rock Types ◽  
Garnet Core ◽  
The Matrix

<p>Reconstructing the tectonic history of metamorphosed terranes is a key step towards establishing a comprehensive model for collisional orogens such as the Alps. Single chronometers tend to record one specific component of such history—be it inheritance, reactions or cooling—or record several of these, without a clear indication of what each age datum means. Resolving the complex evolution of such terranes requires chronometric data of different minerals, which on the basis of their chemistry, may be linked to distinct stages. Here we present a multi-mineral geochronology of the Theodul Gletscher Unit (TGU; Western Alps). The tectonic unit is a metamorphic sequence containing a variety of pelitic and mafic rocks that mainly record Alpine low-temperature, high-pressure metamorphism. In addition, however, the rocks are known to host age components related to events and processes in the Permian and Jurassic; these could be attributed to inherited components and pervasive fluid-rock interaction during oceanic alteration and subduction. To investigate this, we subjected pelitic schists and mafic rocks from the TGU to a multi-method analysis, involving thermometry, oxygen isotope analysis in garnet, and zircon U-Pb and garnet Lu-Hf dating.</p><p>Zircon crystals in all rock types are Permian in age and have no significant record of Alpine metamorphism; they are interpreted as dating the source of the felsic and mafic sediments. Complex garnet textures in the schists reveal multiple growth stages: whereas the garnet rim reflects the subduction stage, the relict nature of the garnet core allows for speculation of an older, perhaps Permian age (Bucher et al., 2019). A distinct and abrupt rim-ward drop in δ<sup>18</sup>O coherent with major-element zoning in garnet from the schists indicates open system fluid-rock interaction. Rutile included in the different garnet zones as well as in the matrix of the schists provided consistent Zr-in-rutile thermometry results of 520–560 °C (calculated at 2.5 GPa). Similarly, Raman spectroscopy of carbonaceous material in the same textural positions indicates 540–580 °C. These results indicate a single Alpine metamorphic cycle. To look back beyond that stage, Lu-Hf data will be presented for garnet with and without seemingly inherited cores, as well as for cores that were physically isolated from the sample material. The results, together, provide new insight into the petrological and tectonic processes that affected rocks in the TGU during and prior to their Alpine history.</p><p>REFERENCES:</p><p>Bucher, K., Weisenberger, T. B., Klemm, O., Weber, S. (2019). Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps. Journal of Metamorphic Petrology, 37, 1151-1169</p>

scholarly journals Fluid-controlled deformation in blueschist-facies conditions: plastic vs brittle behaviour in a brecciated mylonite (Voltri Massif, Western Alps, Italy)

2017 ◽  
Vol 155 (2) ◽  
pp. 335-355 ◽  
Author(s):  
C. MALATESTA ◽  
L. FEDERICO ◽  
L. CRISPINI ◽  
G. CAPPONI
Keyword(s):  
High Pressure ◽  
Fluid Pressure ◽  
Western Alps ◽  
Pore Fluid ◽  
Ductile Deformation ◽  
Pore Fluid Pressure ◽  
Plate Interface ◽  
Rock Interaction ◽  
Field Evidence ◽  
Slow Earthquakes

AbstractA blueschist-facies mylonite crops out between two high-pressure tectono-metamorphic oceanic units of the Ligurian Western Alps (NW Italy). This mylonitic metabasite is made up of alternating layers with different grain size and proportions of blueschist-facies minerals.The mylonitic foliation formed at metamorphic conditions of T = 220–310 °C and P = 6.5–10 kbar. The mylonite shows various superposed structures: (i) intrafoliar and similar folds; (ii) chocolate-tablet foliation boudinage; (iii) veins; (iv) breccia.The occurrence of comparable mineral assemblages along the foliation, in boudin necks, in veins and in breccia cement suggests that the transition from ductile deformation (folds) to brittle deformation (veining and breccia), passing through a brittle–ductile regime (foliation boudinage), occurred gradually, without a substantial change in mineral assemblage and therefore in the overall P–T metamorphic conditions (blueschist-facies).A strong fluid–rock interaction was associated with all the deformative events affecting the rock: the mylonite shows an enrichment in incompatible elements (i.e. As and Sb), suggesting an input of fluids, released by adjacent high-pressure metasedimentary rocks, during ductile deformation. The following fracturing was probably enhanced by brittle instabilities arising from strain and pore-fluid pressure partitioning between adjacent domains, without further external fluid input.Fluids were therefore fixed inside the rock during mylonitization and later released into a dense fracture mesh that allowed them to migrate through the mylonitic horizon close to the plate interface.We finally propose that the fracture mesh might represent the field evidence of past episodic tremors or ‘slow earthquakes’ triggered by high pore-fluid pressure.

scholarly journals Effects of fluid–rock interaction on 40Ar/39Ar geochronology in high-pressure rocks (Sesia-Lanzo Zone, Western Alps)

2014 ◽  
Vol 126 ◽  
pp. 475-494 ◽  
Author(s):  
Ralf Halama ◽  
Matthias Konrad-Schmolke ◽  
Masafumi Sudo ◽  
Horst R. Marschall ◽  
Michael Wiedenbeck
Keyword(s):  
High Pressure ◽  
Western Alps ◽  
Rock Interaction

scholarly journals Metamorphic gabbro and basalt in ophiolitic and continental nappes of the Zermatt region (Western Alps)

2021 ◽  
Vol 114 (1) ◽  
Author(s):  
Kurt Bucher ◽  
Ingrid Stober
Keyword(s):  
Volcanic Rocks ◽  
Western Alps ◽  
Rock Composition ◽  
Rock Interaction ◽  
Near Surface ◽  
Hp Metamorphism ◽  
Alteration Processes ◽  
Rock Alteration ◽  
Shallow Crust ◽  
Basic Rocks

AbstractThe composition of meta-gabbro and meta-basalt occurring abundant and widespread in all nappes of the nappe stack exposed in the Zermatt region of the Western Alp shows distinct patterns related to the geodynamic origin of metamorphic basic rocks. Eclogitic meta-basalts of the ophiolitic Zermatt-Saas Unit (ZSU) show enriched MORB signatures. The meta-basalts (eclogites) of the continental fragment of the Theodul Glacier Unit (TGU) derive from pre-Alpine metamorphic continental intraplate basalts. Meta-basalts (eclogites) from the continental basement of the Siviez-Mischabel nappe (SMN) derive from MORB thus a genetic relation to the TGU eclogites can be excluded. All basic igneous rocks experienced post-magmatic alteration by fluid-rock interaction ranging from processes at the seafloor, in the shallow crust, during subduction zone hydration, in the exhumation channel and late Alpine regional metamorphisms. The consequences of these alteration processes can be identified at various levels in the rock composition data. It was found that the REE data are little affected by fluid-rock alteration. Some trace elements, notably Cs, Rb, and Ba are typically massively altered relative to igneous compositions in all three groups of meta-basalts. Generally, meta-basalts from the TGU and the SMN preserved the features of the original composition whilst the ZSU meta-volcanic rocks experienced massive alteration. For the ZSU meta-volcanic rocks it is evident that Zr was gained and Y lost during high-pressure fluid-rock interaction indicating a mobile behavior of the two elements during HP-metamorphism in contrast to their behavior in hydrothermal near-surface fluid-rock interaction.

Scales of fluid-rock interaction and carbon mobility in the deeply underplated and HP-Metamorphosed Schistes Lustrés, Western Alps

Lithos ◽  
2020 ◽  
Vol 354-355 ◽  
pp. 105229 ◽  
Author(s):  
Gabe S. Epstein ◽  
Gray E. Bebout ◽  
Samuel Angiboust ◽  
Philippe Agard
Keyword(s):  
Western Alps ◽  
Rock Interaction
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