GMPV6.1

Serpentinization, the water-driven alteration of olivine-rich rocks, plays an integral role in solar system evolution. While much attention has been directed towards the role of serpentinization in the evolution of our own planet, it has also been proposed as a mechanism for warming and stabilizing liquid water on early Mars, controlling the fate of the Martian hydrosphere, and originating life early in the planet’s history. Because olivine is widespread on the Martian surface and highly reactive in the presence of water, many researchers have hypothesized that serpentinization would have been common during periods of Martian history when liquid water was present. Observations of serpentine, the most abundant by-product of serpentinization, in intimate association with olivine on the Martian surface lends fundamental support to this hypothesis.

H₂ and organic carbon production during typical serpentinization on Earth is fundamentally limited by the modest quantities of Fe in terrestrial mantle olivine, which is typically composed of just 10% of the Fe-endmember, fayalite. To explore how this limitation would differ during Martian serpentinization, we compiled analyses of olivines in Martian meteorites and those analyzed by Curiosity in Gale Crater, Mars. The results show that even the most magnesian Martian olivines contain around twice the Fe content of terrestrial mantle olivine, and most contain much more. Thus, to gain a better understanding of H₂ and organic carbon production during Martian serpentinization, we must study serpentinization of atypical, Fe-rich olivines on Earth. To this end, we have performed and compiled analyses of serpentinites of the Duluth Complex (USA), which solidified from tholeiitic magmas broadly similar to those that produced the Martian crust and contains ferroan olivines representative of those on Mars. The data show increases in Fe(III)/Fe(tot) with increasing extents of serpentinization (as measured by H₂O content) that mimic the trends observed during serpentinization of terrestrial mantle rocks.  However, because of the much higher primary fayalite content, the Duluth Complex serpentinites produced around 5 times the H₂ at any given extent of serpentinization than those in an equivalent compilation of terrestrial serpentinites. This observation implies that even weakly serpentinized (20%) rocks on Mars would have produced as much H₂ as fully serpentinized terrestrial mantle peridotite, and a formation as large and stratigraphically continuous as the Olivine Bearing Unit in Jezero Crater could have produced a very substantial amount of H₂, even if it were only partially serpentinized. Thus, although orbiter observations suggest serpentine may be uncommon on the Martian surface, this does not necessarily indicate that serpentinization, and the reduced gases that it produced, did not play a significant role in the planet’s biogeochemical evolution.

How to cite: Tutolo, B. and Tosca, N.: Terrestrial constraints on H2 generation during Martian serpentinization, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3157, https://doi.org/10.5194/egusphere-egu22-3157, 2022.

Peridotite alteration via serpentinization has been identified as a major potential sink of man-made carbon. Peridotite serpentinization provides a geochemical pathway to store CO2 as a solid, and releases natural hydrogen as a byproduct. Given the large quantities of peridotite available in oceanic environments, serpentinization could serve as a major component in mitigating man-made climate change. “Reaction driven cracking” has been proposed as an attractive mechanism to explain how peridotite is fully serpentinized.  In this process, when the peridotite alters and becomes a serpentinite, the volume of the rock grows producing stress on the surrounding rock. This process produces new fractures that allow water to enter new areas within the rock thus promoting new serpentinization. Cracking events due to this fracturing process being driven by peridotite alteration have never been detected in the environment (e.g., seismic data). As part of the Oman Drilling Project, a network of 12 hydrophones was deployed in two boreholes drilled in Oman over a period of nine months. This network was designed to detect the microseismic cracking events associated with reaction driving cracking. Surprisingly, it has served as an excellent detector of natural hydrogen degassing events. Hydrogen is a byproduct of the geochemical serpentinization process. These intermittent events come in short “spurts” where periods of quiescence alternate with short periods where many bubbles come out all at once. This poster presents evidence of hydrogen degassing events related to active serpentinization in Oman. We use the results to provide estimates of hydrogen released during the period of hydrophone deployment based on estimated bubble volumes.

How to cite: Aiken, J., Sohn, R. A., Kelemen, P. B., Renard, F., and Jamtveit, B.: Detecting H2 degassing events related to serpentinization in Oman, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-521, https://doi.org/10.5194/egusphere-egu22-521, 2022.

The reaction of serpentinized peridotites with CO₂-bearing fluids to listvenite (quartz-carbonate rocks) requires massive fluid flux and significant permeability despite increase in solid volume. Understanding the mechanic-hydraulic interplay and the conditions, mechanisms and structures that enhance or hamper progress of this reaction is key to estimate the scale of long-term carbon fluxes and reservoirs in mantle rocks and their potential for industrial CO₂ removal by mineral carbonation. Here we present a detailed microstructural analysis of listvenite and serpentinite samples from Hole BT1B of the Oman Drilling Project, which helps to understand the mechanisms and feedbacks during vein formation in this process [1]. The samples contain abundant magnesite veins in closely spaced, parallel sets and younger quartz-rich veins. These veins constitute large volumes of the listvenites, showing that fracturing and related advective fluid flow were integral to carbonation progress. Cross-cutting relationships suggest that antitaxial, zoned carbonate veins with elongated grains growing from a median zone towards the wall rock are among the earliest structures to form during carbonation of serpentinite. They show a bisymmetric chemical zoning of variable Ca and Fe contents with a systematic distribution of SiO₂ and Fe-oxide inclusions; this and cross-cutting relations with Fe-oxides and Cr-spinel indicate that they record progress of reaction fronts during replacement of serpentine by carbonate in addition to dilatant vein growth. Euhedral terminations and growth textures of carbonate vein fill together with local dolomite precipitation and voids along the vein – wall rock interface suggest that these antitaxial veins acted as preferred fluid pathways allowing infiltration of CO₂-rich fluids necessary for carbonation to progress. Fluid flow was probably further enabled by external tectonic stress, as indicated by the close spacing and subparallel alignment of these carbonate veins. As carbonation progressed, permeability was reduced during subsequent quartz veining and silica replacement of the matrix, but the scarcity of remnant serpentine in listvenite horizons indicates that penetration of CO₂-rich fluid through the vein and matrix permeability network was sufficient for carbonation to proceed to completion.

[1] Menzel, et al., Solid Earth Discussions [preprint], https://doi.org/10.5194/se-2021-152, in review, 2022.

M.D.M. and J.L.U. acknowledge funding of DFG grants UR 64/20-1, UR 64/17-1.

How to cite: Menzel, M. D., Urai, J. L., Ukar, E., Decrausaz, T., and Godard, M.: Progressive veining during peridotite carbonation: insights from listvenites in Hole BT1B, Samail ophiolite (Oman), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11056, https://doi.org/10.5194/egusphere-egu22-11056, 2022.

Fluid flow in subduction zones is related to geological processes such as seismic and volcanic activities. However, the timescale of fluid flow and its flux in the supra-subduction setting is unclear. In this study, we report the novel texture of the antigorite veins with a brucite-rich reaction zone in dunite in the crust-mantle transition zone of the Oman ophiolite, and investigated the timescale and time-integrated fluid flux during the vein formation.

The antigorite veins occur in the drilling cores of serpentinized dunite in the crust-mantle transition zone taken from the Oman Drilling Project CM1 site (Wadi Zeeb, Northern Sharqiyah). The dunite samples are completely serpentinized and consist mainly of lizardite, brucite, magnetite, and Cr-rich spinel and are cut by the antigorite vein networks and later chrysotile. These features indicate that the antigorite veins formed at the stage of obduction of the Oman ophiolite. The antigorite veins, which are distinct by ‘bright’ networks by X-ray CT due to magnetite, occur preferentially in dunite (at 160 – 313 m along the CM1A). The veins are filled with a mixture of randomly orientated antigorite crystals and fine chrysotile. Some antigorite vein contains fragments of the host rock. The brucite-rich reaction zone was developed at both sides of the antigorite veins with a thickness of 0.5 – 4 mm. The reaction zone is composed of brucite (39.4 area%), chrysotile (59.3 area%), and magnetite (1.3 area%). The microtexture of the reaction zone indicates the brucite silicification after the brucite reaction zone formation. Mass balance and thermodynamic calculation suggest that silica was leached from the host rock lizardite during antigorite vein crystallization, resulting in the formation of the brucite reaction zone. Given solution chemistry and the amount of leached SiO₂ during vein formation, the time-integrated fluid flux was estimated to be 10⁵ - 10⁶ m³_(fluid) m^-2_(rock). A diffusion-controlled model suggests that the reaction zone was formed in a short time, ~10^-1 - 10⁰ years. These results suggest that a large amount of high-temperature fluid passed through the fracture network over several hundred meters in a short time at the earlier stage of the obduction of the Oman ophiolite.

How to cite: Yoshida, K., Okamoto, A., Oyanagi, R., and Kimura, M.: Rapid fluid infiltration recorded in the brucite-rich reaction zone along the antigorite veins from the Oman ophiolite, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3483, https://doi.org/10.5194/egusphere-egu22-3483, 2022.

Serpentinization of ultramafic mantle rocks is one of the main reactions leading to a significant water incorporation into the oceanic lithosphere. The multiphase hydration is yet poorly constrained, in terms of sequence of events, their chemical and isotopic compositions, and the reaction conditions (temperature, fluid composition and the water/rock ratio). We present here the first study on Iberia and Newfoundland passive margins samples (oceanic drill core samples, ODP, from Site 1070 and Site 1277) that closely correlates petrographic observation, in situ oxygen isotopic data and major and trace elements mobility in serpentine phase.

The serpentine minerals lizardite and chrysotile are the main hydrous phases formed during the serpentinization reaction. These minerals crystallize in specific textures, depending on the primary minerals being replaced: (i) serpentine in mesh texture after the alteration of olivine, and (ii) serpentine as bastite pseudomorphing pyroxenes. As the textural control is the key to detect multistage fluid uptake during progressive hydration, we used Secondary Ion Mass Spectrometry (SIMS) to achieve a good spatial resolution of ∼20 µm for in situ oxygen isotope measurements. The trace elements analyses were analyzed with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and a spot size of ∼38 µm. Newfoundland samples show less variability in the oxygen isotope composition than Iberia samples and have values higher than the mantle composition, suggesting low temperature of serpentinization (110-200°C). One of the Iberia samples shows similar variability of the oxygen isotope composition but lower values than 𝛿¹⁸O_mantle (temperature around 140-200°C). The second sample has the highest variability: (i) homogeneous mesh rim texture with 𝛿¹⁸O = 4.4 ± 0.8 ‰ (2𝜎), (ii) wide compositional range for mesh centers with 𝛿¹⁸O = 4.0 - 7.7 ‰, and (iii) bastite with large isotopic variation from 5.5 to 13.5 ‰. These values suggest a large hydration temperature range from 60-200°C within a single sample.

Texturally controlled rare-earth element (REE) analyses of the serpentine minerals reveal inherited, typical melt depletion patterns of the protolith, for both localities. The trace element composition of serpentine in the different textural domains displays typical signatures related to the precursor minerals (olivine vs. pyroxene), particularly in terms of Ni/Cr ratio. Positive anomalies of fluid-mobile elements (e.g. B, Sr, As) confirm hydration of the mantle rocks during oceanic serpentinization.

How to cite: Vesin, C., Rubatto, D., Pettke, T., and Deloule, E.: Texturally controlled oxygen isotope analyses of serpentine phases record the multistage hydration history during abyssal serpentinization, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8711, https://doi.org/10.5194/egusphere-egu22-8711, 2022.

Olivine and enstatite, formed by dehydroxylation of serpentine, have been naturally and experimentally documented as evidence of paleo-earthquake rupture propagation within natural serpentinite-bearing slip zones. To investigate the rheological and textural evolution during dehydroxylation of serpentinite, we performed rotary-shear friction experiments on water-saturated serpentinite powders using drained and undrained conditions (where drained conditions allow for excess fluid pressure to escape the gouge holder). Using a purpose-built gouge sample holder containing a thermocouple 1.5 mm from the eventual principal slip zone (PSZ), the experiments were performed at a seismic slip rate (1 m/s) and 10 MPa normal stress. Mechanical results show that in undrained experiments, the apparent friction coefficient (μ) initially reaches a peak value of ~0.21-0.24, followed by dramatic weakening to a steady-state value of 0.12-0.09, associated with gouge compaction, while the temperature at the thermocouple steadily increased reaching a maximum of ~180°C. In drained experiments, a plateau-like friction coefficient with a value of ~0.42 was reached, associated with gouge compaction at a steady temperature of ~250°C at the thermocouple, followed by a drop to a steady-state value of ~0.19, associated with gouge dilation at a temperature of ~450°C. The friction coefficient then gradually increased, reaching a value of ~0.3 (i.e. restrengthening) with gouge compaction and a max. temperature at the thermocouple of ~635°C by the end of the experiment. The PSZ of the products were examined by scanning electron microscope, in-situ synchrotron X-ray diffraction, and focused ion beam transmission electron microscope. Microanalysis showed no mineral phase changes in undrained experiments, which we interpret to indicate that fluid vaporization and pressurization buffered the temperature to below that required for serpentinite dehydroxylation. However, the PSZ in drained experiments contains well-developed aggregates of nanometric, rounded to polygonal forsterite + enstatite, which provide evidence for serpentinite dehydroxylation at temperatures of >600°C within the PSZ. Our observations indicate that fluid drainage facilitates a significant temperature increase within the gouge layer at seismic slip rates. We conclude that dehydroxylation of natural serpentinite gouges may occur under relatively dry conditions or when co-seismic permeability increases (e.g. due to fracturing) allow for efficient fluid drainage and decrease the efficiency of thermal pressurization.

How to cite: Wu, W.-H., Kuo, L.-W., Smith, S. A. F., and Tarling, M. S.: Olivine and enstatite formation during seismic faulting in serpentinite: an experimental approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3267, https://doi.org/10.5194/egusphere-egu22-3267, 2022.

Fractures derived from earthquakes can create permeable conduits for fluids to flow, enhancing fluid-rock interactions, and potentially altering the strength and rheology of fault systems. In the dry lower crust, numerous field examples show mutually overprinting pseudotachylytes (solidified melts produced during seismic slip) and mylonitized pseudotachylytes (produced during the post- and interseismic viscous creep). The mylonites contain hydrous mineral assemblages, suggesting episodic pulses of fluids infiltration and rheological weakening triggered by the earthquake. Here, our aim is to understand the porosity generating mechanisms during the earthquake cycle and characterize the intermittent evolution of porosity.

The Nusfjord East shear zone network (Lofoten, Norway) is an exhumed lower crustal section composed largely of anhydrous anorthosites that contain mutually overprinting pseudotachylytes and mylonitized pseudotachylytes. We present a microstructural analysis focusing on the mechanisms generating, maintaining, and destroying porosity from an exhumed network of lower crustal coeval pseudotachylytes and mylonites using synchrotron X-ray microtomography (SμCT), focused ion beam scanning electron microscopy (FIB-SEM) nanotomography, electron backscatter diffraction (EBSD) analysis, and SEM imaging.

In the pristine pseudotachylyte, SμCT data show that porosity is concentrated within the pseudotachylyte vein (0.16 vol% porosity), especially around framboidal garnet clusters and single garnet grains. SEM observations reveal that garnets within the vein often contain an asymmetric rim of barium-enriched K-feldspar. The damage zone of the host anorthosite on the other hand is efficiently healed (0.03 vol% porosity) primarily with the growth of plagioclase neoblasts nucleated from pulverized fragments of the host anorthosite, and secondly with the precipitation of barium-enriched K-feldspar found lining intragranular microfractures. A FIB-SEM transect along one of these microfractures shows a myrmekite microstructure formed during fluid-rock interaction that completely sealed the porosity.

In the mylonitized pseudotachylyte, SμCT data show a porosity of 0.03 vol%, mainly concentrated within monomineralic domains of plagioclase, which are interpreted as recrystallized, sheared survivor clasts of wall-rock fragments. EBSD analyses indicate that deformation in these monomineralic domains was accommodated by diffusion creep and grain boundary sliding. Polymineralic domains along the mylonitic foliation, which primarily derive from the overprint of the original pseudotachylyte veins, also deformed by diffusion creep and grain boundary sliding. However, unlike in the monomineralic domains, they lack detectable porosity. We interpret these observations to reflect the efficient precipitation of hydrous phases into the pores during creep cavitation.

Dynamic fracturing during earthquakes is the primary mechanism for porosity generation in the lower crust. Our study shows that porosity is further reduced by up to 90% when a pristine pseudotachylyte is viscously re-worked under deformation conditions promoting grain-size sensitive creep and grain boundary sliding. We suggest that such porosity reduction eventually results in shear zone hardening, which may evolve in the development of new pseudotachylytes overprinting the mylonites, as frequently observed in Nusfjord. Thus, earthquake-induced rheological weakening of the lower crust is intermittent, and occurs only as long as the fluid can infiltrate in the shear zone, thereby facilitating diffusive mass transfer.

How to cite: Michalchuk, S., Menegon, L., Renard, F., Chogani, A., and Plümper, O.: Dynamic evolution of porosity in lower crustal faults during the earthquake cycle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2375, https://doi.org/10.5194/egusphere-egu22-2375, 2022.

Hydration of upper mantle rocks leads to serpentinization with drastic consequences for the geophysical and geochemical properties of the Earth’s lithosphere. Serpentinization takes place via a dissolution-precipitation process in which the fluid phase plays a key role both in the transport of dissolved constituents and in the supply of reactants. A limiting factor for serpentinization is the likelihood of pore-space clogging due to the large solid-volume increase and potential negative repercussions on reaction-induced fracturing [1]. However, small-angle neutron scattering [2] has shown that porosity remains abundant at the nanometer-scale ensuring that aqueous fluids can penetrate to the reaction front allowing serpentinization to progress. To further determine the nature of serpentinite nanoporosity and explore the consequences of fluids confined to nanometric dimensions, we couple multi-scale correlative electron microscopy to molecular dynamics simulations. In the analytical part, we combined electron backscatter diffraction (EBSD) with focused-ion beam scanning electron microscopy (FIB-SEM) nanotomography and transmission electron microscopy (TEM) to investigate two partially serpentinized peridotites from the mid-Atlantic ridge (ODP Leg 209) and the Røragen ultramafic complex, Norway. We determined the crystallographic orientation of the host olivine grains to constrain any potential orientational relationship between the host and the reaction-induced porosity within serpentine. No apparent correlation was found. Based on the EBSD maps, we excavated 23 FIB-SEM cross-sections across serpentine veins and the serpentine-olivine interface. At a pixel resolution of 3 nm, only three out of 23 cross-sections showed apparent pore space. Subsequent, FIB-SEM nanotomography of these three regions showed that vein porosity (average φ_FIB-SEM <50 nm) is concentrated at the olivine-serpentine interface and devoid in the vein middle. To further investigate pore space beyond the FIB-SEM resolution, we prepared eight electron-transparent foils for high-resolution TEM analysis. All foils show an apparent porosity between 1 to 4% with an average pore size of 5 nm. TEM-based energy-dispersive X-ray analysis reveals a 100-nm wide brucite-layer separating serpentine from olivine. Within the brucite-layer, total porosity ranges from 10 to 20% with pore size >10 nm. A higher porosity within brucite-bearing domains is also apparent at the SEM-scale, where we observe larger brucite-rich veins with a high density of nanopores. Hence, our microstructural investigations suggest that continued fluid transport to the reaction interface in the potential absence of reaction-induced fracturing could be sustained through a combination of a nanoporous serpentine network, porous brucite-rich veins, and a highly nanoporous brucite-layer at the olivine reaction interface. Overall, our observations that serpentinite porosity is constrained to the nanoscale have first-order implications for fluid transport and behaviour, because critical physicochemical properties, such as the dielectric constant ε, differ significantly in nanoscale-confined fluids when compared to their bulk counterparts. Initial molecular dynamics simulations of aqueous fluids confined in brucite nanochannels indicate that with the reduction of the width of the nanochannel, the perpendicular component of ε drops drastically which likely has a profound impact on minerals solubility hence overall reaction progress.

[1] Plümper et al. Geology (2012) 40(12): 1103-1106.

[2] Tutolo et al. Geology (2016) 44(2): 103-106.

How to cite: Chogani, A. and Plümper, O.: Nanoporosity in serpentinites and its consequences for fluid transport: a combined multi-scale electron microscopic imaging and molecular dynamics study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2291, https://doi.org/10.5194/egusphere-egu22-2291, 2022.

Dedolomitization i.e. replacement of dolomite by calcite, is an important fluid-mediated replacement process occurring during carbonate diagenesis in basins. Especially, dedolomitization impact local reservoir rock properties, affecting the reservoir quality and rheology. The process of dedolomitization have been the subject of several studies but still, the controlling mechanism is not fully understood.

We investigate samples from the Maestrat Basin in Spain, formed during the upper Jurassic- lower Cretaceous. Between the Eocene and the Oligocene, the bassin recorded a compressive event and a general surrection responsible of dedolomitization. The detailed investigation of the progressive replacement of the original dolostone to the newly formed rock composed by calcite provide a key example to understand the dedolomitization process. Across the replacement interface, crystallographic orientation (EBSD) of the parent dolomite crystal is preserved in the calcite and no chemical zonation for major elements (EPMA) are visible in both phases, supporting a dissolution-precipitation mechanism. In order to better constrain the chemical evolution of the reaction, quantitative trace elements mapping (fs-LA-ICP-MS) was carried out and coupled to mass balance equations to quantify the elements gained and lost during the reaction. Results show a net gain of mass (~5%) with a loss of heavy Rare Rarth Elements and a gain in light ones. The specific gain in Zn and Rb pinpoints that the infiltrated fluid flowed through MVT deposits already present in the area.

How to cite: Hoareau, G., Centrella, S., Beaudoin, N. E., and Callot, J.-P.: Reconstructing fluid pathways by studying dedolomitization process: example of the Benassal Formation, Maestrat Basin, Spain, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9789, https://doi.org/10.5194/egusphere-egu22-9789, 2022.

Fluids are a primary control on deformation processes, in particular in the upper, brittle portion of the crust. In the mechanical framework of poroelasticity or friction, used to describe brittle rock behavior, the influence of fluid is integrated through the fluid pressure. High fluid pressure reduce the deviatoric stress necessary for slip ; for example during seismic slip, the temperature rise due to frictional work in the fault core might result in a large drop in resistance to further slip and constitutes therefore a very efficient lubricating process. Another example of the influence of fluid pressure is observed in deep slow slip events in subduction zones, where the slipping portion of the plate interface and domains of high fluid pressure migrate conjointly.

While models and observations highlight the large mechanical role of fluid pressure, measurements of fluid pressure below a few kilometers of depths are very indirect and plagued by large uncertainties. Veins constitute one of the ubiquitous by-products of the fluid-rock interaction during deformation at depths. Vein-forming mineral, such as quartz and calcite, trap, as inclusions, the fluid that was present during crystal growth. Fluid inclusions constitute therefore one of the very few record of the physicochemical conditions of the deep fluid.

We examined in this work three examples of syn-deformation quartz veins, from a japanese accretionary complex. The crystals within veins show growth rims, bringing to light the time evolution of the rock-fluid system. Many fluid inclusions are trapped within the growth rims ; in particular methane-rich fluid inclusions, which minimize the problem of late-stage reequilibration and therefore unravel the fluid pressure at the time of trapping. In parallel, those growth rims can be divided into two types, with either low or large content in trace elements (in particular aluminum).

We correlated the median fluid pressure recorded in fluid inclusions with the average Al concentration in quartz : High/low fluid pressure correspond to low/high Al concentration, respectively. Based on literature data about crystal growth in hydrothermal and magmatic contexts, it appears that the higher incorporation of impurities can be accounted for by rapid, out-of-equilibrium growth of quartz. We propose therefore a model of vein evolution with repetitions of large fluid pressure drop, where crystal grew rapidly and incorporated a large concentration in Al, alternating with longer period of slower growth, at higher fluid pressure, with a reduced incorporation of Al. The highest fluid pressure variations are of the order of 70MPa, and the corresponding Al concentration variations of the order of 0.28wt%.

Quartz veins are abundant in most, if not all tectonic contexts. In addition, Al concentration in quartz is preserved throughout exhumation, unlike fluid inclusions signal, which is in many cases questionable because of reequilibration. In conclusion, quartz geochemistry can be considered as a promising sensor of fluid pressure variations, which can provide access to the conditions of the fluid attending deformation of the brittle crust.

How to cite: Raimbourg, H., Famin, V., and Canizarès, A.: The record of deep fluid pressure in veins : a new method based on quartz geochemistry, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4636, https://doi.org/10.5194/egusphere-egu22-4636, 2022.

The low permeability and excellent sealing properties of mudstones places such sedimentary rocks into focus of geo-engineering applications using clay formations as natural barriers for contaminant transport. Here we investigate microscale deformation structures in the Opalinus Clay in northern Switzerland, which is currently under investigation as a host rock for radioactive waste confinement. We aim to characterize paleo-faulting/fracturing as well as subsequent mineralization events. For this purpose, drill core samples were investigated macroscopically, as well as by low and high-resolution optical light microscopy and scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX). These data were combined with high-resolution trace element maps obtained by Synchrotron X-ray Fluorescence Microscopy (SXFM). By combining the observed microstructures with the micro-chemistry of the associated mineralization events we yield a grouping between different processes and, in combination with cross-cutting relationships, a relative timing of the different deformation events.

Commonly the Opalinus Clay is weakly deformed, with only few localized deformation structures. The latter include: calcite veins (mm thickness) as well as mineralized (calcite and celestite) thrusts, normal faults and strike-slip faults (all cm thickness). Micro-textural analysis shows that low-angle thrust faults with calcite slickensides on their dip-slip surfaces localize on pre-existing horizontal fibrous calcite veins. The horizontal veins imply an early deformation stage with temporarily high pore fluid pressures under sub-horizontal max. principle stresses within the highly anisotropic mudstone. The first order analysis of the major element chemistry between the calcite forming slickensides and the fibrous veins shows significant differences in Mg, Fe and Mn contents. From a fluid-mechanical perspective, this finding implies that generation of fibrous veins during a first fluid event provides the mechanical discontinuity, which is reused during later fluid assisted thrusting.

The overprinting relationships between fibrous veins and slickensides indicate that deformation/precipitation events occurred in a cyclic fashion. Such information is a key towards the understanding of fluid assisted deformation and mineralization processes in compacted and anisotropic clay formations. On a regional scale, variations in paleo-deformation-mineralization events in the Opalinus Clay imply regional differences likely related to a gradually varying intensity of compressional (thrusting) and extensional (normal faulting) tectonics throughout northern Switzerland.

How to cite: Akker, I. V., Herwegh, M., Aschwanden, L., Mazurek, M., and Madritsch, H.: Microscale deformation in a compacted and anisotropic mudstone: the Opalinus Clay, northern Switzerland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2313, https://doi.org/10.5194/egusphere-egu22-2313, 2022.

Detachment faults that bound continental metamorphic core complexes typically record slip magnitudes of tens of kilometers—sufficient to exhume crustal rocks in their footwall from below the brittle-ductile transition. However, initiation and slip on low-angle (dip <30°) normal faults are at odds with the predictions of Coulomb failure during horizontal extension. What allows low-angle normal faults to acquire large displacements? Key obstacle to addressing this question is the scarcity of presently exposed active detachment faults. With a strike length of >60 km and a dip of 16°–21°at the surface, the active Mai’iu low-angle normal fault in SE Papua New Guinea has self-exhumed a smooth and corrugated footwall fault surface of >29 km width in the extension direction. Progressive strain localization preserved relicts of older-formed fault rocks in structurally lower positions on the fault surface including, from bottom-to-top, non-mylonitic schists through mylonites to cataclasites and ultracataclasites. The rapidly exhumed metabasaltic footwall of the Mai'iu fault contains multiple generations of deformed calcite veins that crosscut the sequentially formed fault rock units. Microstructural and stable and clumped isotope data of the syntectonic calcite are combined to reconstruct a profile of crustal strength with depth.

Clumped isotope thermometry of calcite in non-mylonitic schists and mylonites (n=8) yielded temperatures of 150-200°C. These temperatures are well below peak metamorphic temperatures of the metabasalt and mostly below the temperature range estimated by calcite twin morphologies in the non-mylonitic schists and mylonites (250-400°). Thus, they do not document calcite crystallization, but represent blocking of isotope reordering in calcite during cooling, However, calcite veins in cataclasites (n=3) record T=130-160°C, mostly below calcite blocking temperatures, and thus may be interpreted as true calcite precipitation or recrystallization temperatures. At ~ 12-20 km depth (T=275-370°C), mylonites accommodated slip on the Mai’iu fault at low differential stresses (25-135 MPa) before being overprinted by localized brittle deformation at shallower depths. At ~6-12 km depth (T=135-270°C) differential stresses in the foliated cataclasites and ultracataclasites were high enough (>150 MPa) to drive slip on mid-crustal portion of the fault (dipping 30-40°).

Carbon isotope ratios of calcite veins from all fault rocks are within a narrow range: õ¹³C_Cc = +2.1 to -2.6‰, typical of marine carbonates. However, their õ¹⁸O_Cc values are more variable and span distinct ranges for individual rock types: non-mylonitic metamorphic rocks, 25 to 27.5‰ (n=5), mylonites, 21 to 24‰ (n=9) and cataclasites, 21.5 to 22.5‰ (n=2). The foliation-parallel calcite-rich seams from the non-mylonitic schist were derived from intercalations of pelagic limestones, metamorphosed together with their host metabasalt. Moving upwards into the fault-zone mylonites and cataclasites, both isotope ratiosdecrease sharply, suggesting that CO₂ derived by breakdown of organic matter was dissolved in groundwater introduced into the damage zone of the Mai’iu fault and mixed with the local metamorphic fluids.

How to cite: Katzir, Y., Mizera, M., Little, T., and Thiagarajan, N.: Evolution of an active continental detachment fault: clumped isotope thermometry of syntectonic calcite, Mai'iu fault, SE Papua New Guinea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3024, https://doi.org/10.5194/egusphere-egu22-3024, 2022.

During the Early Permian, the post-Variscan evolution of the present-day Alpine region was characterized by crustal extension combined with strong magmatic activity at different crustal levels (Schaltegger and Brack, 2007), which finally led to the development of intracontinental extensional basins filled with volcanoclastic sediments (e. g. the Orobic Basin). In the central Southern Alps (cSA) N Italy, the opening of these basins was controlled by low-angle normal faults (LANFs). We investigated several Early Permian faults of the Orobic Basin with emphasis on their original features, as they have exceptionally escaped most of the Alpine deformation (Blom and Passchier, 1997). The identified LANFs of the Orobic Basin are characterized by cataclastic bands sealed with cm to dm thick layers of dark, aphanitic tourmalinites (Zanchi et al., 2019). The tourmalinites formed in response to circulation of boron-rich fluids channelled along Early Permian fault systems related to opening of the Orobic Basin. The tourmalinized faults were first noted in various sites of the cSA during the 1990’s: several authors linked them to the uranium mineralization of the Novazza-Val Vedello district (Slack et al., 1996; Cadel et al., 1996; De Capitani et al., 1999), although their genesis has never been fully characterized and the connection with U-ore bodies has also not been deeply investigated so far.

In this work, we further characterize the occurrence and assess the cause of the regional hydrothermalism in the context of intracontinental extension during the Early Permian. Field-based structural analysis are combined with mineral and whole-rock geochemistry, geochronology, microstructural studies and boron- isotopic analysis of tourmalinites from different sectors of the study area, in order to evaluate the origin of hydrothermal fluids. Preliminary results demonstrate a temporal relationship between tourmalinites and Early Permian magmatism in the cSA. Geochemical data on major and trace elements together with B isotope ratios suggest a direct connection between tourmalinites and the U-mineralization at the basement-cover contact and along LANFs within the Orobic Basin.

Blom, J. C., and Passchier, C. W. (1997). Geologische Rundschau, 86, 627-636.

Cadel, G., et al. (1996). Memorie di Scienze Geologiche, 48, 1-53.

De Capitani, L., et al. (1999). Periodico di Mineralogia, 68, 185-212.

Schaltegger, U., and Brack, P. (2007). International Journal of Earth Sciences, 96(6), 1131-1151.

Slack, J., F., et al. (1996). Schweiz. Mineral. Petrogr. Mitt., 76, 193-207.

Zanchi A. et al. (2019). Italian Journal of Geosciences, 138, 184-201

How to cite: Locchi, S., Trumbull, R. B., Zanchetta, S., Moroni, M., and Zanchi, A.: Boron-rich hydrothermalism marking Early Permian extensional structures of the central Southern Alps, N Italy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5102, https://doi.org/10.5194/egusphere-egu22-5102, 2022.

Different microstructures and quartz recrystallization mechanisms can be observed in shear zones of granites that formed in similar greenschist-facies conditions. It is generally assumed that temperature plays a major role on quartz rheology and recrystallization. However, at low-grade conditions, fluid percolation also controls strain accommodation, by favouring the growth of weak phases as phyllosilicates. The relative importance of temperature over fluid-induced softening reactions on microstructures remains however poorly constrained mainly because comparative studies among low-grade shear zones are lacking.

The present work focusses on two granitic massifs of the central Pyrenees deformed at greenschist-facies conditions but showing different structural styles. While in Bielsa granitoid, shear zones are spaced of ~ 100-200 m, in Maladeta strain is localised in shear zones spaced of ~ 1.5 km. The Bielsa granitoid, is pervasively altered at late-Variscan time, as suggested by petrography and trace elements variations uncorrelated to strain gradients, and then at Alpine time. Alpine mylonites are made of white mica at ~ 50 % vol. Quartz poorly recrystallizes by bulging. Geochemical whole-rock analyses show systematic variations of alkali, fluid and volume with increasing strain. These results point to a pervasive fluid-rock interaction before and during deformation in Bielsa.  In contrast, in high strain rocks of Maladeta, the magmatic mineral assemblage is largely preserved. Quartz pervasively recrystallizes by sub-grain rotation and white mica is less abundant (20% vol). Consistently, geochemical whole-rock analyses show no or little major element transfer across Maladeta shear zone at constant volume. This point to a lower pre and syn-kinematic fluid-rock interaction in Maladeta than in Bielsa. Thermometry on metamorphic chlorite show similar temperature ranges for deformation in the two massifs (280-350°C).

Variations of pre-kinematic hydrothermal alteration therefore strongly affect quartz recrystallisation mechanisms and microstructures, by controlling the abundance of weak phases as white mica. This process is observed despite very similar temperature ranges. Such variations may also explain the difference of structural style in the two massifs (distributed vs localised deformation) up to an outcrop scale.

How to cite: Airaghi, L., Alaoui, K., Dubacq, B., Rosenberg, C., and Bellahsen, N.: Different microstructures in low grade shear zone formed at comparable temperatures: effect of pre and syn-kinematic fluid-rock interactions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7798, https://doi.org/10.5194/egusphere-egu22-7798, 2022.

In Holsnøy (Bergen Arcs, Norway), metastable granulite facies anorthosite rocks are partially eclogitised within hydrous shear zones, that have been interpreted as widening over time with fluid influx and strain. We here present a detailed petrological description of metre-scale shear zones from this area. The granulite protolith (originally plagioclase + garnet + two pyroxenes) is transformed into an albite + zoisite + garnet + clinopyroxene assemblage within a few tens of centimetres of the shear zones. The outer edge of the shear zones consists in a fine-grained heterogeneous assemblage of omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz. An eclogite composed of coarser omphacite + kyanite + garnet + zoisite + phengite quartz forms the core of the shear zones. As the shear zones widened over time, this lateral evolution from the edge to the core of the shear zones reflects the temporal evolution of the granulite from the beginning to the end of the eclogitisation reaction. The outer omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz assemblage therefore represents a transient eclogite facies assemblage. This transient assemblage appears to be mechanically weaker than both the starting granulite and the final eclogite, based on field and petrological findings. We investigate the impact of transient weakening during syn-tectonic metamorphism using a one-dimensional numerical model of a fluid-fluxed, reacting shear zone. Our numerical model shows that transient weakening is required to explain the field and petrological data. Furthermore, we show that, while fluid infiltration was predominantly responsible for the widening of the shear zones, strain hardening during the end of the eclogitisation reactions sequence had a noticeable widening effect on the shear zones.

How to cite: Bras, E., Baïsset, M., Yamato, P., and Labrousse, L.: Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3558, https://doi.org/10.5194/egusphere-egu22-3558, 2022.

The incipient development of diagnostic high-pressure assemblages –the `eclogitization'– of granitoids, such as plagioclase-breakdown and small-scale formation of garnet and phengite does not require exogenous hydration because unlike dry protoliths like basalt/gabbro or granulite, granitoids s.l. contain crystallographically-bound H₂O in biotite. During high-pressure overprint, partial biotite dehydration-breakdown causes a localized increase in the chemical potential of H₂O (µH₂O). Diffusion of H₂O into nearby plagioclase induces the formation of diagnostic eclogite-facies assemblages of jadeite–zoisite–K-feldspar–quartz ± kyanite ± phengite that pervasively replace former cm-sized plagioclase without requiring the participation of free H₂O. Depending on P–T evolution, similar textures may involve albite instead of jadeite. Plagioclase-breakdown may also occur due to simple burial because compression leads to an increase of µH₂O, without requiring additional influx of H₂O at the texture scale. However, diffusion of biotite-derived H₂O into plagioclase sites likely favors reaction due to its catalytic effect. In parallel, ~100 µm-thick complementary coronae involving garnet phengite–quartz develop at former biotite–plagioclase/K-feldspar interfaces due to the coupled diffusion of FeO–MgO–H₂O from biotite towards feldspars, and minor CaO in the opposite direction. The reaction textures likely create structural weaknesses and preferential fluid pathways, thereby promoting further hydration, deformation and equilibration along the prograde path. If exogenous H₂O is introduced, it is accommodated in phengite growing at the expense of igneous K-feldspar and possibly in epidote-group minerals. Upon decompression, such hydrated rocks would dehydrate, thereby favoring fluid-assisted retrogression and loss of diagnostic eclogite-facies assemblages at lower pressure. Whereas the prograde reaction textures are only preserved at closed-system conditions and in the absence of deformation, they are suggested to commonly form during orogenic metamorphism of granitoids and quartzofeldspathic gneisses that dominate the continental crust in high-pressure terranes such as the Western Italian Alps or the Western Gneiss Region (Norway).

How to cite: Schorn, S.: Self-induced incipient `eclogitization' of metagranitoids at closed-system conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3832, https://doi.org/10.5194/egusphere-egu22-3832, 2022.

Serpentinite-derived fluids are released at different P and T conditions through several quasi-discontinuous dehydration reactions, such as the breakdown of brucite and antigorite forming olivine at forearc depths, and the terminal breakdown of antigorite to olivine and orthopyroxene at subarc depths. In subduction-related metamorphic terranes, the record of the colder and shallower brucite-breakdown reaction (< 1.0 GPa and < 450 ºC) in serpentinite occurs as locally olivine veining, whereas the deeper and hotter high-pressure terminal antigorite dehydration (> 1.5 GPa and ca. 660 ºC) shows pervasive replacement patterns with varied textures. Previous works have provided important constraints on the contrasting fluid flow mechanisms associated with these dehydration reactions, but the potential role of the stress field in controlling the geometry of the structures and eventually dictating the fluid pathways remains poorly understood.

Here we report the results from field observations from Cerro del Almirez (Nevado-Filábride Complex, Betic Cordillera, S. Spain) that records the formation of olivine-rich veins in prograde serpentinite at temperatures lower than the terminal antigorite dehydration. We show the existence of two generations of abundant olivine-rich veins formed as open, mixed-mode and shear fractures during prograde metamorphism. Type I veins were likely synchronous with the development of the serpentinite main foliation, whereas Type II veins postdate the foliation indicating that Atg-serpentinites experienced punctuated brittle behavior events during subduction. Type I veins were formed by the fluid overpressure developed during the brucite breakdown reaction, whereas Type II were potentially formed by continuous compositional and structural changes in antigorite that released subordinate amounts of fluids. Type II olivine-rich veins were formed by brittle failure in a well-defined triaxial stress field and were not significantly deformed after their formation.

We interpret olivine-rich veins as due to the replacement of antigorite by olivine at the walls of the crack due to reactive fluid-flow dissolution and precipitation. The ultimate driving force for the dissolution and precipitation is the low and contrasting solubility of SiO₂ and MgO in the aqueous fluid in combination with fluctuations in the fluid pressure relative to the lithostatic pressure. Equilibria under lower fluid pressure in the crack caused the nucleation and growth of olivine at the expense of antigorite dissolution. Comparison of the principal stress orientation inferred from Type II veins with those formed at peak metamorphic conditions in the ultramafic rocks at Cerro del Almirez shows a relative switch in the orientation of the maximum and minimum principal stress. These relative changes can be attributed to the cyclic evolution of shear stress, fluid pressure and fault-fracture permeability allowing for stress reversal.

FUNDING: This work is part of the project DESTINE (PID2019-105192GB-I00) funded by MICIN/AEI/10.13039/501100011033  and the  FEDER  program  “Una manera de hacer Europa”. J.A.P.N. acknowledges a Ramón y Cajal contract (RYC2018-024363-I) funded by 452MICIN/AEI/10.13039/501100011033 and the FSE program “FSE invierte en tu futuro”.

How to cite: Padrón-Navarta, J. A., Jabaloy-Sánchez, A., López Sánchez-Vizcaíno, V., Gómez-Pugnaire, M. T., Hidas, K., and Garrido, C. J.: On the potential role of reactive flow precipitation due to fluid-pressure gradients for the genesis of olivine veins in subducted metaserpentinite, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11319, https://doi.org/10.5194/egusphere-egu22-11319, 2022.