Fluids released from subducting hydrated rocks influence volcanism, tectonics, and geochemical cycling, but the mechanisms of fluid escape in subduction zones remain poorly understood. We address this issue by investigating the Erro-Tobbio meta-serpentinites (ET-MS), Italy, exhumed serpentinite rocks that preserve extensive dehydration vein networks formed by the porosity-generating breakdown of antigorite and brucite. We characterized the structure and morphology of these self-organized vein networks and evaluated their hydrodynamic properties using a novel approach. Specifically, we combined X-ray tomography and drone imagery with generative machine learning, electron microscopy, and equilibrium thermodynamics to model and analyze fluid pathways in the ET-MS. In both natural and simulated samples, these dehydration vein networks act as efficient drainage systems, enabling rapid fluid percolation even at porosities below 1%. The maximum network permeability is , several orders of magnitude higher than that of intact serpentinite. Fe-rich olivine and monticellite occur alongside relict brucite and magnetite in these veins. This assemblage indicates that the high permeability arises from porosity localized along brucite- and magnetite-rich veins, where infiltration of reducing fluids enhanced dehydration reactions. These findings demonstrate that serpentinite dehydration in subduction zones can produce flow-optimized vein structures that efficiently channel fluids at low porosity, potentially influencing fluid migration on local to regional scales before widespread dehydration occurs.

Fluid inflitration along brittle presursors is commonly with associated with hydration and deformation of the host rock. In many cases the relative timing of fracturing, fluid infiltration, reaction, and deformation is unclear, making it difficult to disentangle the relative importance of processes that facilitate advancement of the hydration front. Here we present the transition from an anhydrous and relatively undeformed precursor rock into a highly deformed and hydrated plagioclase-rich rock. The studied outcrop preserves both (1) the interface between the anhydrous granulite-facies parent lithology and a statically hydrated amphibolite-facies rock, and (2) a transition from statically hydrated amphibolite to the sheared amphibolite-facies lithologies. Detailed petrography, quantitative mineral chemistry and bulk rock analyses have been applied to investigate compositional variations and assemblage microstructure across both interfaces. Here, we produce hydro-chemical numerical models based on local equilibrium thermodynamics in an attempt to reproduce the characteristics of the hydration and deformation interfaces. Here, we present a comparison between the observed characteristics of the hydration front and those produced by modelling of the reaction front propagation.

The Cassini mission provided evidence for a global subsurface ocean and ongoing hydrothermal activity on Enceladus, based on results from Cassini’s mass spectrometers. Laboratory simulations of hydrothermal conditions on icy moons are needed to further constrain the composition of ejected ice grains containing hydrothermally altered organic material. Here, we present results from our newly established facility to simulate the processing of ocean material within the temperature range 80–150°C and the pressure range 80–130 bar, representing conditions suggested for the water–rock interface on Enceladus. With this new facility, we investigate the hydrothermal processing of triglycine (GGG) peptide and, for the first time, analyse the extracted samples using laser-induced liquid beam ion desorption (LILBID) mass spectrometry, a laboratory analogue for impact ionization mass spectrometry of ice grains in space. We outline an approach to elucidate hydrothermally processed GGG in ice grains ejected from icy moons based on characteristic differences between GGG anion and cation mass spectra. These differences are linked to hydrothermal processing and thus provide a fingerprint of hydrothermal activity on extraterrestrial bodies. These results will serve as important guidelines for biosignatures potentially obtained by a future Enceladus mission and the SUrface Dust Analyzer (SUDA) instrument onboard Europa Clipper. This article is part of the theme issue ‘Dust in the Solar System and beyond’.

Introduction: Because of early recurring impacts and later impact gardening, lunar crust surface turned into a layer of fragmented, variably shocked and occasionally melted impact breccias [Heiken et al., 199; White et al., 2020].One of the most severely affected group of rocks found on the lunar surface is the Mg-suite group. These are Mg-rich, primitive, plutonic to hypabyssal coarse-grained rocks which texture and bulk composition reflect magmatic accumulation of mineral phases [Heiken et al., 1991; White et al., 2020; Shearer et al., 2015; Černok et al., 2020].Whether the Mg-suite rocks formed by partial melting of the lunar mantle or they are impact-related is still debated [Taylor et al., 1993]. In the latter case, their origin could be related to the melting of early lunar crust and mantle caused by frequent hypervelocity impacts [Jolliff et al., 2006]. Some of their minerals, like Fe-Ni metals and sulfides, reflect mixing and melting of impactor(s) and target rocks and the proportion of impactor vs. target contributions [Tang et al., 2023; Day et al., 2020]. For example, a high Ni/Co ratio in Fe-Ni metal grains can be the consequence of the addition of either iron or chondritic impactors. Furthermore, those same minerals are extremely sensitive to processes that happened in Moon’s interior, such as fractional crystallization [Day et al., 2020]. For example, Ni/Co ratio decreases when mineral phases like pyroxene and plagioclase crystallize together because of the different compatibility of Ni and Co in both solid and melt.For these reasons, those minerals cannot only be considered important means to investigate lunar rocks origin, but also lunar evolution processes [Day et al., 2020]. In this study we concentrate on defining the mineral chemistry of Fe-Ni metal and sulfide grains in a set of variably shocked breccias from three different Apollo missions (15, 16 and 17). Figure 1. Fe-Ni metal grain in thin section 78235,38, BSE image Samples: Three different shocked breccia samples were selected from Apollo 15, 16 and 17 collections: shocked norite 78235 (thin sections 78235,38 and 78235,51), mainly consisting of glass veins and cumulus and partially fractured orthopyroxene and plagioclase, with much of the plagioclase converted into maskelynite [Meyer, 2010]; dimict breccia with shocked norite 15455 (thin sections 15455,27 and 15455,28), primarily made of fragmented orthopyroxene and plagioclase in a KREEP-rich, fine-grained igneous-textured groundmass containing plagioclase, olivine and pink spinel clasts [Meyer, 2010]; feldspathic polymict breccia 67915, showing two-main polymict lithologies, one white and one grey, containing heterogeneous lithic clasts cemented in shock-melted glass (thin sections 67915,76 and 67915,84) [Meyer, 2010].Methods: Non-destructive chemical analysis were performed at the Institute of Geological Sciences at the Freie Universität Berlin (Germany) using a JEOL JXA 8200 Superprobe on minerals selected with the help of QEMSCAN maps and BSE images. In both metals and sulfides we analyzed the concentration of siderophile (Fe, Ni, Co, Mn), chalcophile (S, Zn, Cu) and some lithophile elements (such as Ca, Mg, Si, Cr, P, used to evaluate possible interference by surrounding silicates). For all the elements detection limits lie between 100 and 200 ppm, while the beam size was usually 1 μm. Figure 2. Sulfide and Fe-Ni metal grains in thin section 78235,51, BSE image Initial results: In total, more than 150 grains were analyzed in four different thin sections, but most of them were very small (6700). Different is in 78235,38, where all metal grains plot within a small range of values (Ni/Co from ~0.7 to ~0.8, Fe/Ni from ~41.8 to ~48.7), while sulfide grains generally show a higher Ni/Co (from ~1.4 to ~3.1) and Fe/Ni (from ~1600 to >6800).In sample 15455 most of the selected grains were found in the impact melt, but are usually 4700 for pyroxenes, from ~9.3 to >4700 for plagioclase) compared to sulfides in 78235. In sample 67915 chemical analysis are yet to be performed.Preliminary conclusions: The calculated Ni/Co and Fe/Ni from the investigated Fe-Ni metals in differently shocked Apollo Mg-suite samples indicate that at least some are of possible impact-related origin. Fe-Ni metal grains within glass veins in sample 78235 and matrix in sample 15455 usually show Ni/Co ~20, compatible with possible iron meteorite contamination, as shown by Day et al (2020) and McCallum and Mathez (1975). FeNi metal grains belonging to other mineral assemblages have lower Ni/Co, usually ≤1, which is more compatible to an endogenous origin [Day et al., 2020]. This is also evident in sulfides, were a Ni/Co ratio that varies between ~0.3 and ~5 suggest a more endogenous origin. Fe/Ni values in Fe-Fe-Ni metals are mostly compatible with kamacite metal (>92% Fe,

The Cassini-Huygens space mission provided evidence for a global subsurface ocean [1] and ongoing hydrothermal activity within Enceladus, inferred primarily from the presence of silica (SiO2) nanoparticles and molecular hydrogen expelled from the depths of subsurface ocean [2,3]. The ocean material expelled from the south pole of Enceladus was sampled by Cassini’s mass spectrometers - ice grains were sampled by the Cosmic Dust Analyzer (CDA) whilst gases were sampled by the Ion and Neutral Mass Spectrometer (INMS). Enceladus hosts a variety of organic molecules, spanning a range of chemical properties. Postberg et al. [4] observed mass spectral signatures related to complex high-mass (> 200 u) refractory insoluble organic material in organic-containing ice grains. In a concurrent study, low-mass (< 100 u) volatile organics with oxygen-, nitrogen-, and aromatic moieties were also identified in the emitted ice grains [5]. Hydrothermal experiments are common in studies of the early and modern Earth, and exploration of terrestrial hydrothermal environments has yielded rich scientific return. Laboratory simulations of Enceladus’s subsurface chemistry [6,7] have thus far not considered how hydrothermally processed material would appear in impact ionization mass spectra, and therefore need to include measurements with analogue techniques for spaceborne instruments, such as laser-induced liquid beam ion desorption (LILBID). LILBID has been shown to accurately simulate the mass spectra produced by hypervelocity impacts of ice grains onto impact ionization mass spectrometers [8]. With a newly established hydrothermal facility at Freie Universität Berlin, we simulate hydrothermal conditions in the depths of Enceladus’s ocean and investigate the hydrothermal processing of triglycine (GGG; 9]. The hydrothermal setup is capable of operating at pressures of up to ~ 150 bar and temperatures of up to 150 °C, and supports sampling during the experiment. Here, for the first time, the extracted samples are measured using LILBID. The first investigation with this new experimental setup assesses the decomposition of triglycine peptide (GGG) at elevated temperature and pressure. It is a simple, well-studied peptide and scientifically valuable option for early experiments and calibration of the hydrothermal apparatus. 0.005 M solutions of GGG were processed at a pressure of 80 bar and a temperature of 80 °C, which lies within the range of conditions suggested for the alkaline (pH 8.5-10.5) hydrothermal systems of Enceladus. After 2 and 4 hours of hydrothermal processing, samples were extracted and measured using the LILBID facility. Here we present LILBID spectra of both hydrothermally processed and unprocessed material, showing their fragmentation pathways. These results will provide guidelines for biosignatures with a future Enceladus mission and also to the SUrface Dust Analyzer (SUDA), an impact ionization instrument onboard Europa Clipper. [1] Thomas, P.C. et al. Icarus (2016) [2] Hsu, H.-W., et al. Nature (2015) [3] Waite, J. H., et al. Science (2017) [4] Postberg, F., et al. Nature (2018) [5] Khawaja, N., et al. MNRAS (2019) [6] Sekine, Y., et al. Nature Comm. (2015) [7] Takahagi, W., et al. ACS Earth Space Chem. (2019) [8] Klenner, F., et al RCMS (2019) [9] Khawaja et al. (in revision), Phil. Trans. R. Soc. A.

The initiation of ductile shear zones commonly occurs spatially associated with fluid-rock reactions along brittle precursors. In many cases the relative timing of fracturing, fluid infiltration, reaction, and recrystallisation is unclear, making it difficult to disentangle mechanisms of shear zone initiation from subsequent deformation and recrystallisation. Here we present the study of the transition from a dry plagioclase-diopside-garnet-scapolite host granulite-facies lithology to (1) a low strain amphibolite-facies rock, and (2) a transition from low strain to high strain amphibolite-facies lithologies. Hydration of the granulite-facies precursor at amphibolite-facies conditions produces an assemblage comprised dominantly of plagioclase-amphibole-zoisite-clinozoisite-kyanite-scapolite-quartz. Detailed study of plagioclase chemistry and microstructures across these two transitions using Electron Backscatter Diffraction (EBSD) and Wavelength Dispersive Spectrometry (WDS) allows us to assess the degree of coupling between deformation and fluid-rock reaction across the outcrop. Plagioclase behaves dominantly in a brittle manner at the hydration interface and so the initial weakening of the rock is attributed to grain size reduction caused by fracture damage and fluid infiltration at amphibolite-facies conditions. Extensive fracturing-induced grain size reduction locally increases permeability and allows for continuing plagioclase and secondary mineral growth during shear. Based on plagioclase microstructures, such as, an inherited but dispersed crystallographic preferred orientation (CPO), truncation of chemical zoning, and the dominance of fine (5–150 µm), slightly elongate, polygonal grains we conclude that deformation is dominantly facilitated by dissolution–precipitation creep assisted by grain boundary sliding in the shear zone.

Abstract Copper isotopes (δ65Cu) in hydrothermal fluids have the potential to provide information on ore‐forming processes occurring below the seafloor, but Cu isotope data from high‐temperature fluids are scarce. Here, we examine the extent to which coexisting sulfide minerals in a hydrothermal chimney can preserve fluid Cu isotope ratios using a fluid‐solid pair of a black smoker (333°C) from the Roman Ruins vent area (PACMANUS) in the Manus Basin. Two ca. 3 cm long transects through the chalcopyrite‐rich chimney wall show an increase in δ65Cu from 0.48 to 2.28‰ from the interior to the exterior, coupled with limited variation in sulfide δ34S (1.52–4.72‰). The Cu isotopic composition of chalcopyrite from the innermost wall closely resembles the δ65Cu value of the paired hydrothermal fluid, indicating that chalcopyrite in the inner ∼5 mm of the chimney records the Cu isotope ratio of the venting fluid. Beyond this, an increase in sulfide δ65Cu toward the exterior correlates with an increase in the relative abundance of secondary Cu sulfides. The appearance of bornite coincides with the presence of small barite crystals, suggesting this represents a redox gradient between reduced hydrothermal fluids and oxidized seawater admixing inwards. Elevated δ65Cu in this zone can be explained by the precipitation of secondary Cu sulfides from 65Cu‐enriched fluids formed during oxidative chalcopyrite dissolution. Our findings indicate that interactions with oxidizing seawater shift chalcopyrite δ65Cu values over small spatial scales, and that caution must be applied if chimney sulfides are used to reconstruct δ65Cu values of high‐temperature hydrothermal fluids.

On Earth, subduction zones facilitate the cycling of volatiles between the Earth’s surface and interior. Volatile cycling has significant effects on the long-term state of the Earth’s climate and tectono-magmatic events, including volcanism and earthquakes. A key stage in the volatile cycle is the devolatilization of the subducting oceanic lithosphere, in which volatiles can escape the previously hydrated rocks. However, it is not well known how efficiently volatiles are transported at this stage. To better understand how volatiles escape at these conditions, we have analyzed the dehydration-related vein networks of the Erro-Tobbio meta-serpentinites (ET-MS), Italy. The ET-MS display well preserved networks of metamorphic olivine veins. These veins are the result of the dehydration reaction of antigorite and brucite to produce H2O and olivine. However, due to the low permeability of serpentinite at depth, the dehydration reaction requires the formation of self-organizing vein networks to allow the produced fluid to escape [1]. Thus, the metamorphic olivine veins in ET-MS may be used as a proxy for fluid flow pathways. We took a multiscale approach to analyzing the network architectures. For microscale (~16 µm voxel size) and mesoscale (~200 µm voxel size) resolutions, X-ray tomography methods are sufficient to visualize the three-dimensional structure of the networks. However, for large scale observations these methods are inapplicable. To solve this, we apply a novel workflow to analyze outcrop scale (~10 m) network systems in three dimensions using only two-dimensional data. By training a generative adversarial network (GAN) with two-dimensional data conditioned by spatial orientation, we can generate statistically representative three-dimensional networks that mimic those of the ET-MS. These representations also display similar characteristics in their respective pore-network-models. With this method, it is possible to produce reasonable three-dimensional approximations of the ET-MS vein networks using only photogrammetry data of the outcrops. In turn, this allows us to extract metrics, such as permeability, that describe the volatile transport efficiency of the ET-MS, and further, how these characteristics change at a broad range of scales. [1] Plümper et al. (2017) Nature Geoscience 10(2), 150-156.

We address in situ serpentinization and mineral carbonation processes in oceanic lithosphere using integrated field magnetic measurements, rock magnetic analyses, superconducting quantum interference device (SQUID) microscopy, microtextural observations, and energy dispersive spectroscopy phase mapping. A representative suite of ultramafic rock samples were collected, within the Atlin ophiolite, along a 100‐m long transect across a continuous outcrop of mantle harzburgite with several alteration fronts: serpentinite, soapstone (magnesite + talc), and listvenite (magnesite + quartz). Strong correlations between changes in magnetic signal strengths and amount of alteration are shown with distinctive contrasts between serpentinite, transitional soapstone, and listvenite that are linked to the formation and breakdown of magnetite. While previous observations of the Linnajavri ultramafic complex indicated that the breakdown of magnetite occurred during listvenite formation from the precursor soapstone (Tominaga et al., 2017, https://doi.org/10.1038/s41467-017-01610-4), results from our study suggest that magnetite destabilization already occurred during the replacement of serpentinite by soapstone (i.e., at lower fluid CO2 concentrations). This difference is attributed to fracture‐controlled flow of sulfur‐bearing alteration fluid at Atlin, causing reductive magnetite dissolution in thin soapstone zones separating serpentinite from sulfide‐mineralized listvenite. We argue that magnetite growth or breakdown in soapstone provides insight into the mode of fluid flow and the composition, which control the scale and extent of carbonation. This conclusion enables us to use magnetometry as a viable tool for monitoring the reaction progress from serpentinite to carbonate‐bearing assemblages in space and time with a caution that the three‐dimensionality of magnetic sources impacts the scalability of measurements.

Functionalizing biomaterials with conditioned media (CM) from mesenchymal stromal cells (MSC) is a promising strategy for enhancing the outcomes of guided bone regeneration (GBR). This study aimed to evaluate the bone regenerative potential of collagen membranes (MEM) functionalized with CM from human bone marrow MSC (MEM-CM) in critical size rat calvarial defects. MEM-CM prepared via soaking (CM-SOAK) or soaking followed by lyophilization (CM-LYO) were applied to critical size rat calvarial defects. Control treatments included native MEM, MEM with rat MSC (CEL) and no treatment. New bone formation was analyzed via micro-CT (2 and 4 weeks) and histology (4 weeks). Greater radiographic new bone formation occurred at 2 weeks in the CM-LYO group vs. all other groups. After 4 weeks, only the CM-LYO group was superior to the untreated control group, whereas the CM-SOAK, CEL and native MEM groups were similar. Histologically, the regenerated tissues showed a combination of regular new bone and hybrid new bone, which formed within the membrane compartment and was characterized by the incorporation of mineralized MEM fibers. Areas of new bone formation and MEM mineralization were greatest in the CM-LYO group. Proteomic analysis of lyophilized CM revealed the enrichment of several proteins and biological processes related to bone formation. In summary, lyophilized MEM-CM enhanced new bone formation in rat calvarial defects, thus representing a novel 'off-the-shelf' strategy for GBR.

The Mohns Ridge is a very slow-spreading ridge that, together with the Knipovich Ridge, marks the boundary between the North American and Eurasian plates in the Norwegian-Greenland Sea. In this study, we report the major and trace element composition of spatially associated basalts and peridotites from a gabbro-peridotite complex similar to 20 km west of the Mohns Ridge rift flank. Formation of the similar to 4-5 Myr crustal section involved accretion of normal mid-ocean ridge basalts with Na-content suggesting derivation from a depleted mantle source. This is consistent with the degree of partial melting estimated for clinopyroxene poor harzburgites using the Cr-number of spinel (14%-18%) and rare earth element modeling of orthopyroxene (16%-24%) and reconstructed whole-rock composition (14%-20%). If all the melting took place beneath the paleo-Mohns Ridge, a crustal thickness of similar to 7-8 km is expected, which is nearly double the observed thickness. Orthopyroxene trace elements are not consistent with typical fractional melting expected for mid-ocean ridges but rather resemble that seen in supra-subduction zone peridotites. The geochemistry of both the basalts and the peridotites suggests that a water-rich slab flux in the past has influenced the mantle source. In turn, this caused hydrous melting which increased the depletion of the pyroxene components, leading to a highly depleted mantle that is now underlying much of the Arctic Mid-Ocean Ridges and represents the source for the spreading related magmatism.

Subduction zones are principal pathways for the cycling of volatiles such as hydrogen and carbonfrom the Earth’s surface to the mantle and back to the atmosphere. This cycling has significant long-term effects on Earth’s climate. However, the processes that lead to volatile release during subduction and total volatile fluxes are poorly understood. In our study, we will quantify and characterize the network architecture of dehydration pathways exhibited as mineralized olivine-bearing metamorphic veins in the exhumed meta-serpentinites from the Erro-Tobbio unit, Italy [1]. Applying network analytical methods and graph theory both macroscopically and microscopically can provide the mode of propagation and describe the controlling factors affecting the evolution of these dehydration networks. Furthermore, multiscale observations can confirm the scalability of the vein network and if quantitative results such as permeability or volatile flux can be extrapolated to larger scales. Along with 2-D network analysis, these vein networks will be analyzed in 3-dimensions using X-ray tomography and sophisticated machine-learning methods, such as generative adversarial networks. The results of both will be compared, which can then assure whether current machine-learning methods can effectively create statistically equivalent copies of these networks. Lastly, the synthesis of 2-D and 3-D multiscale results should provide meaningful parameters for accurate calculations of volatile flux during the dehydration of subducting slabs. [1] Plümper et al. (2017) Nature Geoscience 10(2), 150-156.

Chalcopyrite (CuFeS2) is the most common copper-bearing mineral and represents more than half of global copper mineral reserves. The decrease in copper reserves and increase in demand for copper has led to an increased interest in hydrometallurgical approaches to copper mining. Copper extraction from copper sulfides such as chalcopyrite, requires oxidant addition for dissolution. Although numerous lixiviants have been evaluated, limited information is available regarding fluid-rock interactions during the leaching of coarse chalcopyrite samples at high temperature, including their stabilities and secondary product formation. This study compares the thermal stability of eleven oxidant/lixiviant systems (sulfuric acid, methanesulfonic acid, ammonium hydroxide, hydrochloric acid and glycine, and oxidants, including oxygen, iron(III), copper(II), dichromate, sodium nitrate, peroxydisulfate and hydrogen peroxide) and their ability to leach copper from chalcopyrite at 110 degrees C (below the sulfur melting point) and 170 degrees C (above the sulfur melting point) under similar conditions. Tests were conducted for up to 576 h using solid cuboids (4 mm side lengths) to simulate in-situ recovery conditions. Sealed batch and Parr reactors were used to simulate an anoxic environment that may be encountered in a subterranean application. This paper presents the findings from the study and compares the leaching systems at 110 degrees C and 170 degrees C based on copper release into solution, reaction products, passivation phenomena and textural analysis. The application potential of the systems for in-situ recovery from deep copper ore bodies is discussed based on the performance of the chosen systems.

Interactions of hydrous fluid and melt with dry mantle rocks are known to result in metasomatic alteration of the lithospheric mantle. Here we investigate such interactions that occurred beneath the Hangay Dome, Mongolia, in 22 mantle xenoliths, which were recovered from Cenozoic basalts at the Tsagan, Zala, Horgo, and Shavaryn-Tsaram localities near the village of Tariat. The xenoliths are medium- to coarse-grained spinel lherzolites that exhibit variable degrees of reaction with silicate melt and fluid (indicated by their Fe- and LREE-enrichment and the presence of secondary clinopyroxene, amphibole, phlogopite, apatite, and sulfide). According to their normalized REE patterns and microstructures, the spinel lherzolites were divided into three groups. Group 1 lherzolites contain LREE-depleted clinopyroxene and whole-rock compositions, exhibit a greater number of preserved deformation textures, and are the least affected by metasomatism. These lherzolites are interpreted to represent the sub-continental lithosphere before the rejuvenation processes that occurred during the Cenozoic. Group 3 lherzolites are characterized by partial annealing of pre-existing textures, and LREE-enrichment in clinopyroxenes and whole-rock compositions compared to Group 1 lherzolites. Group 3 lherzolites are interpreted to be the result of the interaction of depleted lithospheric mantle with a basaltic melt during the Cenozoic. The lack of correlation between the intensity of metasomatism, the degree of annealing, and the calculated temperatures of the lherzolites suggests that the Cenozoic basaltic melt percolation postdates the static recrystallization. Group 2 lherzolites exhibit characteristics from both Groups 1 and 3. Numerical modelling of the interaction between depleted lherzolites and basaltic melts evidences that: (i) a single initial liquid may fractionate to produce a range of element patterns: the observed spectrum of REE compositions of the Tariat lherzolites cannot have resulted from simple mixing of basaltic melt with a depleted mantle rocks; instead, it can be explained by chromatographic fractionation during reactive porous melt flow; (ii) highly fractionated element patterns are derived from conventional initial melt compositions and do not imply the existence of exotic melts; and (iii) strong element fractionation can be produced along short distances even at the thin section and mineral scale; this opposes the view that long percolation distances are required to produce significant chromatographic effects: the clinopyroxene core-rim disequilibrium demonstrates that REE variations in clinopyroxene rims were acquired in response to interactions with a more evolved REE-rich melt.

The alkaline playas near Atlin, British Columbia, Canada are likely one of the few surface environments on Earth where contemporaneous formation of hydromagnesite and magnesite occurs at temperatures that do not exceed 15 degrees C. This environment offers a unique opportunity to examine the impact of different formation mechanisms on Mg isotope compositions of Mg-carbonate minerals at low temperature. In this study, we report the Mg isotope composition of ultramafic bedrock, Mg-carbonate sediments, and both surface and ground waters in this geological setting. The composition of hydromagnesite suggests a Rayleigh-type distillation effect on the fluid Mg isotope ratios in unsaturated sediment above the water table. Through this mechanism of formation, hydromagnesite is progressively depleted in Mg-24 obtaining delta Mg-26 values as high as +1.14 degrees% near the sediment surface. In contrast, magnesite formation is characterized by enrichment of the solid phase in Mg-24. The apparent Mg isotope fractionation factor during magnesite formation at similar to 10 degrees C ranges between -0.7 +/- 0.1%o and -1.8 +/- 0.1%o. The distinct Mg isotope composition of hydromagnesite in comparison to magnesite supports magnesite formation occurring by precipitation from the fluid, or dissolution-reprecipitation, rather than solid-phase transformation from a hydrous Mg-carbonate precursor. Overall, the results provide insights on low temperature Mg-carbonate mineral formation that has implications for long-term storage of CO2.

The overall rates of multi-component reaction networks are known to be controlled by feedback mechanisms. Feedback mechanisms represent loop systems where the output of the system is conveyed back as input and the system is either accelerated or regulated (positive and negative feedback respectively). In other words, feedback mechanisms control the rate of a reaction network without external influences. Feedback mechanisms are well-studied in a variety of reaction networks (e.g. bio-chemical, atmospheric); however, in fluid-rock interaction systems they are not researched as such. Still, indirect evidence, theoretical considerations and direct observations attest to their existence [e.g. 1, 2, 3]. It remains unknown how mass and energy transport between distinct reaction sites affect the overall reaction rate and outcome through feedback mechanisms. We propose that feedback mechanisms are a missing critical ingredient to understand reaction progress and timescales of fluid-rock interactions. We apply the serpentinization of ultramafic silicates as a relatively simple reaction network to investigate feedback mechanisms during fluid-rock interactions. Recent studies show that theoretical timescale-predictions appear inconsistent with natural observations [e.g. 4, 5]. The ultramafic silicate system is ideal for investigating feedback mechanisms as it is relevant to natural processes, is reactive on timescales that can be explored in the laboratory, and natural peridotite typically consists of less than four phases. Our preliminary observations indicate a feedback between pyroxene dissolution and olivine serpentinization. Olivine serpentinization appears to proceed faster in the presence of pyroxene. Furthermore, the bulk system reaction rate increases with increasing fluid salinity, which is opposite to the salinity effect on the monomineralic olivine system. Dunite (>90% olivine) is rare, which is why it is crucial to explore the more common pyroxene-bearing systems. The salinity effect is important to investigate due to the inevitable increase in fluid salinity from the boiling-induced phase separation and OH-uptake in the formation of serpentine. Here we present preliminary textural and chemical observations, which will subsequently be used for kinetic modelling of feedback.[1] Ortoleva P., Merino, E., Moore, C. & Chadam, J. (1987). American Journal of Science 287, 997-1007.[2] Centrella, S., Austrheim, H., & Putnis, A. (2015). Lithos 236–237, 245–255.[3] Nakatani, T. & Nakamura, M. (2016). Geochemistry, Geophysics, Geosystems 17, 3393-3419.[4] Ingebritsen, S. E. & Manning, C. E. (2010). Geofluids 10, 193-205.[5] Beinlich, A., John, T., Vrijmoed, J.C., Tominaga, M., Magna, T. & Podladchikov, Y.Y. (2020). Nature Geoscience 13, 307–311.