Pervasive carbonation of serpentinized peridotite to carbonate-quartz rock (listvenite) due to infiltration of CO2-bearing fluid is a remarkable process because it can be geologically fast and it increases the rock’s carbon content from initially zero to > 30 wt% CO2. This pervasive conversion is related to an overall solid volume increase while at the same time requiring high time-integrated fluid rock ratios with permeability and diffusivity on all scales. Thus, porosity has to be created dynamically during reaction progress as otherwise fluid pathways become clogged by the reaction products carbonate and quartz, which is one of the major obstacles for artificial carbon storage by peridotite carbonation. Processes that can renew porosity and permeability during carbonation are fracturing and veining – in response to tectonic stress [1] or induced by reaction and crystal growth [2], or a combination of both –, reaction-enhanced ductile deformation [3,4], and spatial decoupling of dissolution and precipitation with solute transfer at different length scales.

Using SEM, EBSD, TEM and FIB nano-tomography, we investigated the microstructural record of local solute transfer and its role for porosity renewal in natural carbonate-bearing serpentinites, transitional serpentine-carbonate-quartz assemblages and listvenites from the Samail Ophiolite, Oman. The clearest indicators of pervasive replacement accommodated by local solute transfer are pseudomorphic replacement structures where carbonate and quartz occur in distinct microstructures corresponding to different inherited peridotite and serpentinite textures. A common pseudomorphic replacement structure in the Samail carbonated peridotite are quartz-fuchsite intergrowths replacing bastite (pseudomorphs after orthopyroxene) in listvenite with harzburgite protoliths. A local strong crystallographic alignment of quartz in each of the bastite/pyroxene pseudomorphs suggests that the anisotropic porosity structure of bastite serpentine favored oriented, epitaxial growth of quartz. Transitional serpentine-carbonate-quartz assemblages show that the first quartz and carbonate generations precipitate coeval but spatially separated, with distinct crystal habits. FIB nano-tomography, STEM analysis and high resolution SEM on ion-polished samples of a transitional serpentine-dolomite-quartz rock from the carbonation reaction front show nano-porous fluid channels in fibrous serpentine at high angle to a highly serrated carbonate-serpentine replacement contact. These nano-scale fluid channels facilitated bidirectional mass exchange of Ca, C, Mg and Si bearing solutes between sites of preferential replacement, such as the cores of serpentine mesh textures, and larger-scale permeability networks along veins and fractures. These observations imply that massive and pervasive solute transfer through the reacting serpentine matrix is possible on a small scale, without clogging of porosity by immediate co-precipitation of quartz or Mg-silicates. Spatial decoupling of dissolution and precipitation was likely caused by the dynamically evolving composition of the reacting fluid and/or due to the influence of differential stress and volumetric strain – a mechanism that can compensate on a local scale for the volume expansion expected of isochemical carbonation reactions.

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

[2] Kelemen & Hirth, EPSL, 2012; https://doi.org/10.1016/j.epsl.2012.06.018

[3] Menzel et al., Nature Communications, 2022; https://doi.org/10.1038/s41467-022-31049-1

[4] Kelemen et al., JGR, 2022; https://doi.org/10.1029/2021JB022352

Funding: Junta de Andalucía (Postdoc_21_00791); DFG grants UR 64/20-1, UR 64/17-1; and EU Horizon 2020 Transnational Access EXCITE _C1_2022_34.

How to cite: Menzel, M. D., Urai, J. L., Plümper, O., Ohl, M., and Schwedt, A.: Multiscale solute transfer and porosity evolution during pervasive replacement of serpentine by carbonate and quartz - insights from the Oman ophiolite, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11886, https://doi.org/10.5194/egusphere-egu23-11886, 2023.

Basaltic rocks are considered excellent candidates for CO2 storage by in situ mineral trapping, due to their large presence on Earth’ surface and their higher reactivity with CO2 to form calcium-rich minerals. Often carrying a high-volume fraction of vesicles, basaltic rocks can be an important reservoir horizon in petroleum systems. When the vesicle network has been filled by earlier mineralization the basalts can act as impermeable seals and traps. Characterizing the spatial and temporal evolution of the porosity and permeability is critical to understand the CO2 storage potential of basalts. We exploited X-ray computed tomography (XCT) to investigate the precipitation history of an amygdaloidal basalt containing a pore-connecting micro-fracture network now partially filled by calcite as an analogue for CO2 mineral trapping in a vesicular basalt. The fracture network likely represents a preferential pathway for CO2-rich fluids during mineralisation. We quantified the evolution of basalt porosity and permeability during pore-filling calcite precipitation by applying novel numerical erosion techniques to “back-strip” the calcite from the amygdales and fracture networks. We found that permeability evolution is dependent on the precipitation mechanism and rates, as well as on the presence of micro-fracture networks, and that once the precipitation is sufficient to close off all pores, permeability reaches values that are controlled by the micro-fracture network. These results prompt further studies to determine CO2 mineral trapping mechanisms in amygdaloidal basalts as analogues for CO2 injections in basalt formations.

How to cite: Macente, A., Dobson, K. J., MacDonald, J., Wadsworth, F. B., and Vasseur, J.: The Evolution of Paleo-Porosity in Basalts: Reversing Pore-Filling Mechanisms Using X-Ray Computed Tomography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-478, https://doi.org/10.5194/egusphere-egu23-478, 2023.

Fluid-rock interaction relies on the fluid’s ability to migrate through rocks, utilising permeable pore space. While we understand permeability in rocks that interact with fluids to evolve dynamically, e.g. in dehydration or carbonation reactions, we have very little quantitative information on these dynamics, as direct measurements of permeability in reacting rocks are inherently difficult.

Here, we present a series of permeability measurements that capture the evolving fluid transport properties of dehydrating gypsum samples. To derive these measurements, we used an X-ray transparent deformation rig to document gypsum dehydration in 4-dimensional µCT datasets and then modelled the permeability evolution for a segmented sub-volume numerically. In doing so, we were able to characterise the grain-scale porosity and permeability evolution of a dehydration reaction for the first time. We present analyses from two experimental time-series run at a fixed confining pressure, temperature and pore fluid pressure (Pc = 20 MPa; T = ~125 C; Pf = 5 MPa) but contrasting stress states: one with the largest principal stress (Δσ = 16.1 MPa) parallel to the sample cylinder axis and another the largest principal stress (Δσ = 11.3 MPa) being radial. In both cases, as pore space formed due to the negative change in the solid molar volume during the reaction, permeability evolved and increased congruently with porosity in time until ultimately reaching average values of 3.14E-13 m² and 4.55E-13 m², respectively. A clear spatial heterogeneity of fluid flow develops at the grain-scale along with the fabrics in the samples. Importantly,  the calculated permeability tensors are anisotropic from the onset, but  develop over different spatiotemporal trajectories and have different preferred orientations in the two experimental geometries: If the anisotropy is expressed as 1-(min_eigenvalue/max_eigenvalue) of the permeability tensor (where isotropy = 0), then the experiment with the largest principal stress applied radially has a final anisotropy of 0.45, with fluid flow efficiently focussed into a vertical lineation. In the case with an axial largest principal stress, the final anisotropy of permeability is 0.30 with fluid flow being channelled along a foliation that developed orthogonally to σ1.

Our results suggest that the spatial and temporal developments of permeability during a dehydration reaction are controlled by the orientation and relative magnitudes of the principal stresses of a tectonic environment, and that these two parameters exert a strong control on the efficiency of drainage and thus reaction progress. This has consequences for our understanding of fluid movements in thrust tectonics and subduction zones, but also in applications such as the in-situ carbonation of ultramafic rocks.

How to cite: Fusseis, F., Gilgannon, J., Jacob, A., Freitas, D., Rizzo, R., and Butler, I.: Emergent permeability in dehydrating rocks is controlled by the stress state and orientation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7041, https://doi.org/10.5194/egusphere-egu23-7041, 2023.

Retrogression and hydration commonly affect large portions of the crust, causing variable degrees of chloritization, sericitization and/or serpentinization depending on the protolith and the conditions of fluid ingression. Retrograde overprint involving hydration is a strongly exothermic process, and leads to a thermal perturbation around the pressure–temperature conditions of hydration, which in the case of chloritization of felsic rocks typically occurs at <500°C. These conditions of retrogression overlap with the closure temperatures of some isotopic systems commonly used for geochronology, for example ⁴⁰Ar/³⁹Ar in micas and feldspars. The exothermicity of hydration therefore disturbs the recorded apparent ages and cooling histories of reworked terranes. Using an average metapelite composition as case study, we estimate that hydration and retrogression of a high-grade amphibolite facies assemblage to a low-grade greenschist paragenesis involves approximately a twofold increase of the mineral-bound water content and releases about 50 kJ.kg^-1 latent heat. Using a simple 1-dimensional numerical model, we solve the heat equation for a steady-state continental geotherm that is advected towards the surface and track the cooling rates for markers that exhume from different depths. Assuming enthalpy production at 380°C to simulate exothermic hydration, the cooling rate is significantly reduced until the markers are exhumed to the temperature/depth of hydration and reaction. The calculated cooling paths feed into KADMOS (Moulas & Brandon, 2022), a set of MATLAB routines designed to calculate apparent ⁴⁰Ar/³⁹Ar ages as function of customized thermal histories. KADMOS solves the equation of ⁴⁰Ar production from ⁴⁰K decay and thermally-activated diffusive loss of ⁴⁰Ar for time (Fig. 1). Our results reveal that for intermediate exhumation rates, spherical muscovite grains with <100 µm in diameter are affected by a ~10% age error when latent heat is considered (Fig. 1b). Such muscovites in rocks exhuming with a velocity of, for example, 4 mm/year would record an apparent ⁴⁰Ar/³⁹Ar age of c. 10 Ma (Fig. 1a) and be affected by an absolute age error of ~1 Ma from thermal buffering by hydration, yielding an apparent age of 10 ± 1 Ma (Fig. 1b). Our calculations indicate that latent heat released from exothermic hydration may significantly disturb low-temperature isotopic systems, thereby complicating the cooling histories and obscuring the temporal constraints deduced from state-of-the-art geochronological systems.

Figure 1 – Exhumation velocity vs. grainsize contoured for apparent ⁴⁰Ar/³⁹Ar age in muscovite (a) and relative error when latent heat is considered (b)

REFERENCES

Evangelos Moulas, & Mark T Brandon. (2022). KADMOS: a Finite Element code for the calculation of apparent K-Ar ages in minerals (Version 1). Zenodo. https://doi.org/10.5281/zenodo.7358138

How to cite: Schorn, S., Moulas, E., and Stüwe, K.: Hot when wet: the consequences of exothermic hydration on geochronology, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5769, https://doi.org/10.5194/egusphere-egu23-5769, 2023.

Crustal rocks involved in orogenic processes frequently bear evidence for widespread fluid circulation. The hydration history of the granitic continental crust in inverted passive continental margins is of particular interest, as granitoids experience rheological weakening by fluid-rock interaction processes. Regrettably, it is often unclear if hydration occurs during rifting or during tectonic inversion. Hence, it is difficult to appreciate the interplay of pre- and syn-orogenic fluids inside continental crustal segments of rifted margins. The geochemical fingerprint of ancient hydration events is stored in hydrous minerals that crystallized directly from circulating paleo-fluids. Thus, such minerals can shed light on the nature of these ancient fluids, as well as provide temporal constraints if they can be dated. Hence, advances in geochronological methods applied to hydrous minerals may prove pivotal in untangling the history of fluid circulation in the granitic continental crust in orogens.

We applied U–Pb geochronology of epidote [i.e., Ca₂Al₂(Al,Fe³⁺)Si₃O₁₂(OH)] in hydrothermal veins hosted by a late Carboniferous/early Permian calc-alkaline granodiorite in the inverted Adriatic passive continental margin (hereafter “Err nappe”), both located in the eastern Swiss Alps. During Jurassic rifting leading to the break-up of Pangea, the continental crust in the Err nappe was hydrated, as seawater-derived fluids percolated along syn-rift faults. However, geochronological data of epidote reveal that the hydration of the granitic continental crust in the Err nappe occurred also later during inversion. Epidote U–Pb geochronology returned two age clusters: (1) 85.2 ± 9.7 Ma, related to Late Cretaceous compression; and (2) 59.9 ± 2.7 Ma, related to subsequent Paleocene extension. These age clusters unveil two distict events of fluid circulation, which are consistent with the timing of tectonic inversion and deformation proposed in the literature. As confirmed by Pb–Sr–O–H isotope geochemistry of epidote, Late Cretaceous fluid circulation was likely mediated by fluids released by underlying units undergoing metamorphism during Eo-Alpine compression. Notably, the Paleocene fluids circulating during extension were most likely surficial in origin (i.e., meteoric water and/or modified/connate seawater), and they percolated into the granitic continental crust by exploiting extensional faults.

In the context of existing data, our results show that the hydration of the granitic continental crust of the Adriatic passive continental margin was mediated by a repeated series of fluid circulation events. Our work advocates that the use of a multi-methodological approach, combining new geochemical and geochronological, tools provides unprecedented insight into complex processes of fluid circulation in the continental crust, and beyond.

How to cite: Peverelli, V., Berger, A., Wille, M., Mulch, A., Putlitz, B., Lanari, P., Pettke, T., and Herwegh, M.: Complementing the hydration history of an inverted passive continental margin using epidote U–Pb geochronology and isotope geochemistry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-732, https://doi.org/10.5194/egusphere-egu23-732, 2023.

The oxidation state of sulfur in slab fluids is controversial, with both dominantly oxidized and reduced species proposed. Here we use in situ X-ray absorption spectroscopy analysis of sulfur-in-apatite to monitor changes in the oxidation state of sulfur during high-P metasomatism by slab fluids in the subduction channel. Our samples include a 73 cm continuous transect of reaction zones between a metagabbroic eclogite block and serpentinite matrix from a mélange zone on the island of Syros, Greece. The block core consists of garnet, omphacite, phengite, paragonite, epidote-clinozoisite, and rutile. In this region, apatite is only observed as elongate inclusions in omphacite cores. From the core outwards micas are increasingly replaced by epidote-clinozoisite, garnets are smaller and more frequent, pyrite + bornite is observed as inclusions in recrystallized omphacite, and apatite is increasingly abundant in the matrix and inclusions in garnet. A major transition at 48 cm separates an assemblage of Ca-Na amphibole, omphacite, chlorite, pyrite, and apatite from the inner garnet-bearing eclogite assemblages. Omphacite disappears from the assemblage at ~56 cm and amphibole compositions sharply transition to tremolite at 59 cm. Finally, the assemblage tremolite + talc + pyrite is observed after ~70 cm.

Apatites in the eclogite assemblages exclusively display S⁶⁺ peaks in their absorption spectra. This includes apatite inclusions in omphacite in the least altered lithology, as well as matrix apatite and isolated apatite inclusions in garnet in the outermost metasomatized eclogite zone. In the intermediate pyrite-rich (~1–5 vol %) amphibole + omphacite + chlorite zone, apatite displays a strong S^1- absorption peak in most grains, with rare analyses showing mixed S^1- and S⁶⁺. Finally, apatite in the outermost tremolite-bearing assemblages only displays a S⁶⁺ peak. The pyrite-rich zone at 48 cm occurs at the initial interface between the serpentinite matrix and eclogite block, characterized by a dramatic decrease in Na content and Mg#. Our data suggest that reduction of S⁶⁺ in infiltrating fluids to S^1- in pyrite became focused as Fe diffused across the steep Mg# gradient, resulting in pyrite precipitation. In contrast, S reduction in the Mg-rich tremolite-dominant portions of the transect was limited by a lack of Fe, resulting in low modes of pyrite and fluid buffered S⁶⁺ in apatite. Finally, S⁶⁺-bearing apatite is also observed in reaction zone lithologies from elsewhere on Syros, suggesting our observations are not isolated.

Two important conclusions are drawn from these data and observations: (1) In the case of Syros, slab fluids at eclogite-facies conditions carried oxidized S⁶⁺, and (2) The interaction of these fluids with eclogites composed of ferrous-Fe silicates resulted in extensive sulfide precipitation.

How to cite: Walters, J., Marschall, H., Grützner-Handke, T., Klimm, K., Konecke, B., and Simon, A.: Apatite as a monitor for sulfur redox reactions during fluid-rock interaction in the subduction channel, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7876, https://doi.org/10.5194/egusphere-egu23-7876, 2023.

To evaluate fluid-rock interaction in subducted sediments buried down to seismogenic depths (250-330℃), we describe mineral reactions along the prograde path and their influence on the fluid budget, fluid-mobile element concentrations, and δ⁷Li of fluids. We focused in particular on metapelitic rocks from two paleo-accretionary complexes, the Kodiak accretionary complex, USA, and the Shimanto belt, Japan.

In metapelites from Kodiak and Shimanto, illite-to-chlorite transformation is the main mineral reaction in the temperature range from 250 to 330℃. Such reaction requires additional H₂O and the plausible explanation is the consumption of pore water, contributing to the increase in the salinity of the pore fluid from 250 to 330°C, trapped as inclusions in quartz veins. Textural evidences, mineral reactions, and mass-balance calculations suggest that the system behaved as closed in both studied sites in terms of major elements. However, trace elements provide a slightly different picture. Indeed, fluid-mobile elements (FME; Li, B, Rb, Sr, Cs, Ba) indicate opposite trends between the two sites: In Kodiak, FME whole-rock concentrations are preserved from 250 to 330℃, in agreement with FME concentrations in illite and chlorite that suggest redistribution between rock-forming minerals. In contrast, samples from Shimanto show significant loss of all studied FME from 250 to 330℃, reflecting a decrease in the FME content of individual mineral phases.

Further insight into the fluid-rock interactions were provided by the analysis of δ⁷Li both in quartz and its fluid inclusions (FI) by applying crush-leach technique. In Kodiak, the fluid is characterized by relatively higher δ⁷Li than Shimanto (+8.1 to +17.07‰ in comparison to +2.53 to +10.39‰, respectively). Such variations can be accounted for by mineral reactions and lithium concentrations in individual phases. Chlorite is the main host of lithium. In the Kodiak complex, lithium concentrations in chlorite remains statistically identical between 250℃ and 330℃ (⁓240 ppm), whereas in the Shimanto belt significant decrease of lithium is observed in chlorite (from ⁓320 ppm in chlorite at 250℃ down to ⁓120 ppm at 330℃). Hence, the higher δ⁷Li of fluids in Kodiak is explained by the chlorite crystallization as it preferentially consumes ⁶Li and the fluid remains enriched in ⁷Li. Conversely, fluids from Shimanto are isotopically lighter than from Kodiak, consistent with lithium loss in chlorite as temperature increases. Therefore, δ⁷Li of fluids in both Kodiak and Shimanto examples can be accounted for by local redistribution of lithium between reacting phyllosilicates and their isotopic fractionation.

Overall, major elements, FME, and δ⁷Li of fluids point to a local redistribution of elements in the Kodiak complex, suggesting that the system behave as closed, as the studied units are underplated as a part of thick turbiditic sequence far from any large-scale fault zones. In the Shimanto belt, the loss of FME suggest rather the open system opposite to major elements, as they are more sensitive indicators of transfers between rock and fluid. Such opposite trend between Kodiak and Shimanto is largely controlled by (i) the amount of internal strain within the different units and (ii) the proximity to large-scale fault zones.

How to cite: Rajič, K., Raimbourg, H., Richard, A., and Lerouge, C.: Fluid-rock interaction in sediments subducted to the seismogenic zone: Implication from mineral reactions, fluid-mobile elements and δ7Li isotopes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11946, https://doi.org/10.5194/egusphere-egu23-11946, 2023.

Clay-rich rocks occur in a wide range of tectonic settings. They are of great interest, for example, for the mechanical properties of shallow subduction zone interfaces, but also for natural barriers in nuclear waste deposits or as subsurface caprocks for CO₂ storage. In contact with a polar fluid (e.g., water), the interaction between clay minerals and fluid can lead to swelling or, under confined conditions, build-up of swelling stress. Many studies have focused on the closure of cracks in clay-rich sedimentary rocks by swelling (also referred to as ’self-sealing’). However, less is known about how water-clay interactions affect the stress state of clay-rich rocks and whether they may induce slip along pre-existing faults. We try to address this knowledge gap in the present study by conducting triaxial shear experiments.

The experiments are performed using oblique saw-cut cylindrical samples, where the top half consists of a clay-rich rock (Opalinus claystone) and the bottom half of a permeable sandstone (Berea sandstone). To estimate the frictional properties of the sandstone-claystone interface, dry experiments are performed at 4 to 25 MPa confining pressure and constant axial displacement of 0.1 mm/min. Fluid injection experiments, where fluids are injected through the permeable footwall sandstone, are performed at 10 and 25 MPa confining pressure, constant piston position (no axial displacement), and an initial differential stress of about 70 % of the expected yield stress. The effect of water-clay interactions on the stress state is estimated by comparing the fluid pressures required to initiate slip when a non-polar fluid is injected (no water-clay interactions are expected) and when a polar fluid is injected (water-clay interactions will occur). In some experiments, the sample assemblage is equipped with fiber optics strain sensors glued to the surface of the sample to distinguish between (poro)elastic deformation of the matrix, deformation due to water-clay interaction, and elastic relaxation due to slip along the saw-cut.

For fluid injection experiments with a non-polar fluid (decane), the mechanical data indicate that slip along the saw-cut occurs at fluid pressures close to what is expected based on the friction slip envelope determined for the dry state. For fluid injection experiments with a polar fluid (deionized water), a differential stress drop already occurs when the water initially reaches the sandstone-claystone interface at ambient fluid pressure (0.1 MPa), which is not expected based on the dry friction slip envelope. The fiber optics strain sensor data indicate that swelling of the claystone is followed by a microstructural collapse before slip along the saw-cut likely occurs. In summary, our data suggest that water-clay interactions may initiate slip due to (1) the alteration of the friction slip envelope, (2) build-up of swelling stress, and (3) collapse of the claystone microstructure. However, to what extent these three mechanisms contribute to the according differential stress drop requires further research.

How to cite: Rast, M., Madonna, C., Selvadurai, P. A., Wenning, Q., Ruh, J. B., and Salazar Vásquez, A.: Fault slip in clay-rich rocks due to water-clay interactions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11542, https://doi.org/10.5194/egusphere-egu23-11542, 2023.

Since the impoundment of the Shivajisagar Reservoir behind the Koyna Dam in 1962, numerous earthquakes have been felt in the Koyna-Warna Seismogenic Region of Western India. The mesoscopic and microscopic observations on the basement granitoid core samples, recovered under the Continental Deep Drilling Program of the Ministry of Earth sciences, reveal the precipitation of calcite and the formation of clay minerals (illite and chlorite) along the fractures and faults. The presence of these secondary minerals alongside the primary minerals like quartz and feldspar is further supported by X-ray Diffraction, which also points to the fracture scale chemical alteration as a result of fluid-rock interactions. It's interesting to note that the precipitation of these hydrophilic clay minerals along faults and fractures might promote slip by raising fluid pressure and lowering the shear strength of the faults. Thus, secondary mineralization due to fluid-rock interaction may have a contribution to the release of strain in form of seismic tremors. On the other hand, the neoformation of these hydrophilic clay minerals along fault/fracture surfaces may also cause rheological incongruity, which could lower the density as well as P and S wave velocities. Besides, hydrogen atoms in clay-bound water may influence neutron capture, leading to over-optimistic estimations of neutron porosity. Additionally, our study supports past geophysical anomalies found in the KFD1 borehole and infers that the geophysical anomalies correlating to the growing fracture density and fault system of the basement rocks are caused by chemical alteration due to fluid-rock interaction and subsequent secondary mineralization. So, this research offers important new understandings of geochemical activity in the context of geophysics and serves as a bridge between geochemistry and geophysics.

How to cite: Halder, P., Sharma, A., Shukla, M. K., and Kumar, K.: Is secondary mineralization playing a pivotal role in recurring seismicity at Koyna-Warna Seismogenic Region of India: a geochemical perspective?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-626, https://doi.org/10.5194/egusphere-egu23-626, 2023.

We consider a porous medium infiltrated by a reactive fluid which triggers coupled dissolution/precipitation reactions at pore surfaces. To study these processes, we model the porous medium as a system of interconnected pipes with the diameter of each segment increasing in proportion to the local reactant consumption. Moreover, the topology of the network is allowed to change dynamically during the simulation: as the diameters of the eroding pores become comparable with the interpore distances, the pores are joined together thus changing the interconnections within the network. With this model, we investigate different growth regimes in an evolving porous medium, allowing for both erosion and precipitation of the dissolved material.

The interplay of flow, transport and reaction in such a system can give rise to a variety of patterns: from spontaneous channeling to nearly homogeneous transformation of the entire rock matrix into the product phase. Interestingly, even if the product phase has a larger molar volume than the parent phase, clogging in such a system can be avoided, due to the interplay of dissolution and precipitation resulting in the continuous creation of new flow paths. These results can be relevant for the analysis of carbonation reactions, in which an important goal is to avoid clogging of the pore space that can lead to permeability reduction and the overall slowdown of the process.

How to cite: Szymczak, P. and Budek, A.: Channeling, clogging and permeability oscillations: different macroscopic regimes in mineral replacement   , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10432, https://doi.org/10.5194/egusphere-egu23-10432, 2023.

Strontium (Sr) is a common trace element in calcite which is incorporated during calcite precipitation through either inorganic or organic pathways. The ratio of Sr to calcium (Ca) in carbonate rocks and minerals has been widely used in studies of paleoceanography, marine sediment diagenesis, and hydrothermal alteration of oceanic crust. The partitioning coefficient of Sr (K_Sr) describes fundamentally the partitioning of Sr between calcite and aqueous solutions, but is a complicated function of environmental conditions and water composition. It has long been recognized that K_Sr is strongly dependent on the precipitation rate of calcite (R_p), which has been formulated in a surface kinetic model [1] and a subsequent ion-by-ion model [2]. We re-evaluate available experimental data of Sr partitioning in calcite and find apparent dependence of K_Sr on calcite oversaturation and solution pH [3]. An ion-by-ion model is developed that successfully reproduces the observed K_Sr values at given solution chemistry [4]. Our model also reproduces observed K_Sr-R_p relationships at various temperatures of 5, 25 and 40 ^oC. This model provides an opportunity to evaluate effects of past seawater composition on Sr partitioning and their possible roles in reconstructing seawater Sr/Ca ratio in the geological history [5], in using pore fluid Sr concentration to extract sediment-fluid exchange rates in deep sea carbonate sediments [5], and in understanding Sr partitioning in biogenic calcite such as foraminifera.

Reference

[1] DePaolo, D.J., Surface kinetic model for isotopic and trace element fractionation during precipitation of calcite from aqueous solutions. Geochimica et Cosmochimica Acta, 2011. 75(4): p. 1039-1056.

[2] Nielsen, L.C., J.J. De Yoreo, and D.J. DePaolo, General model for calcite growth kinetics in the presence of impurity ions. Geochimica et Cosmochimica Acta, 2013. 115: p. 100-114.

[3] Zhang, S. and D.J. DePaolo, Equilibrium calcite-fluid Sr/Ca partition coefficient from marine sediment and pore fluids. Geochimica Et Cosmochimica Acta, 2020. 289: p. 33-46.

[4] Jia, Q., et al., A model for pH dependent strontium partitioning during calcite precipitation from aqueous solutions. Chemical Geology, 2022. 608: p. 121042.

[5] Zhang, S., R.J. Zhou, and D.J. DePaolo, The seawater Sr/Ca ratio in the past 50 Myr from bulk carbonate sediments corrected for diagenesis. Earth and Planetary Science Letters, 2020. 530: p. 115949.

How to cite: Zhang, S., DePaolo, D., and Jia, Q.: Strontium partitioning in calcite and its controling factors, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10955, https://doi.org/10.5194/egusphere-egu23-10955, 2023.

The mobility of arsenic in aquatic environments is controlled by oxidation states of arsenic, stability of solid phases, and chemical composition of water (Meng et al., 2002). Binding of arsenic in the environment may occur through precipitation of low-solubility salts (Magalhães, 2002), like mimetite Pb₅(AsO₄)₃Cl. The aim of this study is to experimentally investigate reactions between cerussite (PbCO₃) and solutions containing AsO₄^3- at various conditions favouring mimetite formation. These observations may provide a new recognition for As immobilization, which might be relevant in remediation of contaminated natural waters.

The mechanism of cerussite reaction with arsenate solutions (50 mg As/L) was studied at pH 2 – 8 using synthetic cerussite powder and fragments of natural cerussite crystals (Mibladen, Morocco). The reaction was carried out by direct contact of 500 ml of As-containing solution with PbCO₃, in presence of Cl^- ions.Cerussite was reacted for up to 4 weeks at in situ and ex situ setups. X–Ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive Spectrometry (SEM-EDS) and Electron Microprobe Analysis (EMPA) were used for analysis of the solid products of the experiments. The solutions were tested for Pb with Atomic Absorption Spectroscopy (AAS) and for AsO₄^3-  using colorimetry.

Precipitation of mimetite on cerussite powder crystals is observed already after 1 day of the reaction with arsenate solutions, at the whole range of pH. Mimetite forms hexagonal rods or needles less than 1 µm in size precipitating in the form of incrustations on PbCO₃ crystals. Their size depends on the pH: a fine-grained precipitate forms at higher pH. Observations of natural crystals show replacement of cerussite by polycrystalline mimetite crust. The crust made of columnar and needle crystals is porous allowing for solution penetration and progress of the reaction. The replacement features indicate similarity to pseudomorphic reactions, and the mechanism elucidated as interface coupled dissolution - precipitation. Overall, cerussite replacement by mimetite reduces AsO₄^3- concentration from 50 ppm to below 1 ppm. It also depends on the pH.

This research was funded by AGH University of Science and Technology project No 16.16.140.315.

Magalhães, M. C. F. (2002). Arsenic. An environmental problem limited by solubility. Pure and Applied Chemistry, 74(10), 1843–1850.

Meng, X., Jing, C., & Korfiatis, G. P. (2002). A Review of Redox Transformation of Arsenic in Aquatic Environments. Biogeochemistry of Environmentally Important Trace Elements, 70–83.

How to cite: Stępień, E. and Manecki, M.: Experimental model of cerussite PbCO3 replacement by mimetite Pb5(AsO4)3Cl at pH 2 – 8, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1817, https://doi.org/10.5194/egusphere-egu23-1817, 2023.

Water-rock interaction determines how the geochemical cycles evolve from Earth surface to the deep interior, related to the fluxes, time, and reactivity between fluid phases and solids. Thus, quantifying mass balances from the global scale to the local one, for understanding planetary geodynamics as well as optimizing geothermal doublets, require understanding how the driving force is controlled between solids and dissolved phases [1, 2]. As a consequence, water-rock budget has a major role to drive the porosity accessible to flow. The standard approach is to consider the chemical potentials of the bulk phases assuming the interface to be infinitely thin and therefore thermodynamically negligible, except with highly divided materials and/or super-confined solutions. Our work is based on previous investigations evidencing the formation of an interphase layer/domain, up to one micron thick, having distinct thermodynamic features with respect to the bulk phase properties [3].  

Herein, diffraction limited FTIR micro-spectroscopy in transmission mode, based on confocal microscope coupled to broadband supercontinuum laser or synchrotron beam, was employed as an energetic probe to monitor the thermodynamic characteristics of liquid water as a function of beam location in a synthetic fluid inclusion (one pore micro-scale closed cavity). FTIR hyperspectral data was recorded to illustrate distance-dependent vibrational energy (absorption signatures) at room and homogenization temperatures. The vibrational energy was transformed to Gibbs free energy using a partition function [4].

The results showed that Gibbs free energy changes by 600 to 1000 J/mol up to 1μm far from the water-quartz interface. This variation indicates a significant change in the chemical reactivity of liquid water over a thick domain, rather defining an “interphase” instead of an “interface.” We observed that the thermodynamic property of this interphase domain has a thermal dependency, and by increasing the temperature the chemical potential has a higher value. The Gibbs free energy variation with T can be interpreted by either an enthalpic or an entropic contribution, or a combination of both. This surprising discovery calls for a shift in the paradigm of the bulk phases dominance in water-rock interaction.

References

1. Putnis, A., Fluid–Mineral Interactions: Controlling Coupled Mechanisms of Reaction, Mass Transfer and Deformation. Journal of Petrology, 2021. 62(12): p. egab092.

2. Putnis, A., J. Moore, and H. Austrheim, Fluid-rock reaction mechanisms and the inevitable consequences for mass transport and texture formation. 2022, Copernicus Meetings.

3. Bergonzi, I., et al., Oversolubility in the microvicinity of solid–solution interfaces. Physical Chemistry Chemical Physics, 2016. 18(22): p. 14874-14885.

4. Bergonzi, I., et al., Gibbs free energy of liquid water derived from infrared measurements. Physical Chemistry Chemical Physics, 2014. 16(45): p. 24830-24840.

How to cite: Mozhdehei, A., Mercury, L., and Slodczyk, A.: Experimental Measurements of Micron-thick Interphase Thermodynamics along a Water-Rock Interface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-463, https://doi.org/10.5194/egusphere-egu23-463, 2023.

Dendritic crystallisation is a key geological pattern-forming mechanism, typical of, and recording information about, rapid crystallization events. In this contribution we report on clinopyroxene (Cpx) dendrites in a basaltic rock fulgurite, which formed due to electrical discharge impacting a basaltic rock. Unusually, these dendrites exhibit a curved morphology. The curved, tapering main dendrite branches are up to 50 µm long, range from 3 µm to 100 nm thickness, and are surrounded by several higher orders of branches, which are also curved. The morphological curvature corresponds to lattice rotation, so branches have consistent elongation directions in crystal coordinates. Total rotation exceeds 180° for some branches, with the highest curvature found being 7° per µm. Such “bent” Cpx dendrites have been observed in experiments (e.g. Hammer et al. 2010), but the mechanism of bending was not previously understood.

By combining microstructural observations with crystallographic orientation maps from electron backscatter diffraction analyses of multiple Cpx dendrites, their three-dimensional morphological and crystallographic configuration was reconstructed. Dendrites feature a planar latticework of branches parallel to the Cpx (010) plane. Branches in this plane are elongated either parallel to {001}* (i.e. normal to the (001) plane) or <10-1>, and exhibit strong and weak lattice curvature, respectively. Sprouting out of this plane are branches parallel to {021}* (originating from {001}* branches) and <12-1> (originating from <10-1> branches), both types being weakly curved. Regardless of the crystallographic direction parallel to elongation, all branches exhibit a crystallographic rotation axis parallel to [010] of Cpx. Furthermore, the rotation sense is consistent regardless of elongation direction in crystal or sample coordinates.

The crystallographic control on the sense of bending and on the rotation axis indicates that bending is not caused by sample-scale compositional, thermal, or mechanical gradients. Instead, asymmetric compositional and thermal fields around branch tips are responsible for bending, supported by the fact that compositional gradients exist in the glass surrounding dendritic crystals. The specific cause of bending is inferred to be asymmetric distribution of melt supersaturation at branch tips, resulting from unequal growth rates of different facets. Branch-tip morphology alone poorly explains the constant sense of rotation of all branches, as the sense of morphological asymmetry is unlikely to be consistent for all branch types. The [010] rotation axis implies that lattice rotation is accomplished by incorporation of a single sign of [001](100) edge dislocations, with a maximum inferred density of 2*10¹⁴ m^-2.

This work provides new insights into fundamental processes occurring during rapid crystallization of Cpx and other minerals. Furthermore, microstructural observations suggest that higher degree of undercooling correlates with greater lattice curvature. Bent dendrites may thus encode information about spatial variations in the cooling rate and/or undercooling of samples. Finally, the consistent [010] rotation axis is expected to be preserved during recrystallization, offering a potential way to identify curved dendritic growth stages even after recrystallization.

References:

Hammer et al. (2010), Geology 38:367-370. https://doi.org/10.1130/G30601.1

Griffiths et al. (2022), J Petrol, egac125. https://doi.org/10.1093/petrology/egac125

This contribution was funded by the Austrian Science Fund (FWF): P 33227-N

How to cite: Griffiths, T., Habler, G., Ageeva, O., Sutter, C., Ferrière, L., and Abart, R.: What is the cause of lattice rotation in clinopyroxene dendrites?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9162, https://doi.org/10.5194/egusphere-egu23-9162, 2023.

Dissolution of carbonate rocks is a complex process in which the interplay of flow, transport, reaction, and geometric evolution plays an important role. The nonlinear couplings between these processes may lead to the formation of intricate patterns, including spontaneously formed channels (wormholes) [1].  It has been long established that the shapes of the dissolution patterns depend on fluid flow and mineral dissolution rates [2]. Recently, it also has become increasingly clear that pore-scale processes can impact large-scale morphologies [3,4]. However, the effects of pore-scale mixing on large-scale patterns remain unclear.

In this work, we investigate the effect of pore-scale mixing processes on the evolution of dissolution channels. Pore space is represented by a network of cylindrical tubes with the diameter of each segment increasing in proportion to the local reactant consumption. The inlet concentration of each pore is controlled by local mixing rules. Two different mixing protocols are considered: full mixing, in which the incoming reactant fluxes are assumed to be completely mixed at the intersection, and streamline routing, where the tracer follows the streamlines into the outgoing pores. We found that streamline routing enhances the flow focusing particularly strongly in moderate Damköhler number regimes where relatively wide dissolution channels appear spontaneously in the system. With the same initial conditions as the full mixing case, the winning channels obtained with streamline routing not only propagate faster but also could grow at a different location in the system. The enhanced flow focusing caused by streamline routing produces thinner wormholes and leads to shorter breakthrough times. Lastly, the evolution of velocity distribution is also found to be distinctive depending on the mixing rule.

[1] Hoefner, M. L. and Fogler,  H. S. AIChE J. 34: 45–54, 1988

[2] Golfier, F., et al.  J. Fluid. Mech. 457: 213-254, 2002

[3] Li, L., Peters, C. A., & Celia, M. A. Adv. Water Res., 29: 1351–1370, 2006

[4] R. Roded, P. Szymczak, R. Holtzman, Geophys. Res. Lett. 48:e2021GL093659, 2021

How to cite: Sharma, R. P., Kang, P. K., and Szymczak, P.: Effects of mixing at pore intersections on large-scale dissolution patterns, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-275, https://doi.org/10.5194/egusphere-egu23-275, 2023.