Dypingite has recently been found to have promising applications in material and environmental sciences. However, its exploitation is limited due to its ambiguous identification via powder X-ray diffraction (PXRD), which is caused by the occurrence of several phases with similar structures and PXRD patterns to that of dypingite. Multiple dypingite-like phases with partially overlapping X-ray diffraction traces appear to exist. To better understand dypingite, we studied samples from Dypingdal and Feragen Ultramafic Body, Norway. Dypingite sensu stricto (s.s.), several dypingite-like phases, and hydromagnesite were identified by PXRD. Three regions of 2θ (CuKα) were recognized where diffraction peaks are readily observed to transition between dypingite s.s., dypingite-like phases, and hydromagnesite. The position of the peak at around 8.5° 2θ varies with the H2O contents measured by the thermogravimetric and Fourier transform infrared analyses, suggesting that structural and compositional characteristics of dypingite-like phases form a continuum between dypingite s.s. and hydromagnesite. Humidity incubation experiments at room temperature indicated a reversible change of H2O content between dypingite s.s. and dypingite-like phases. High humidity conditions increased H2O contents of dypingite-like phases to dypingite s.s., whereas low humidity conditions reversed the direction towards hydromagnesite, but without complete reaction. Such transition behavior suggests that the structures of dypingite s.s. and the dypingite-like phases are very similar, probably identical in building unit topology but with different degrees of hydration, whereas the structure of hydromagnesite is less closely related. This study draws attention to the sample processing on dypingite hydration series, contributes to the applications of this series in CO2 sequestration, and gives suggestions to the refinement of structures of dypingite and dypingite-like phases.

In the middle to lower continental crust, mineral reactions can play a key role in controlling rheological behaviour by generating fine-grained, mechanically weak domains. In granulite-facies rocks from Kross & oslash;y (Bergen Arcs, Norway), we document how garnet breakdown produced interconnected networks of orthopyroxene + plagioclase +/- spinel that formed along grain boundaries and reaction fronts. We interpret the reaction as being initiated during early Caledonian decompression and cooling, which changed from Grenvillian high-pressure granulite- to lower pressure granulite-facies. The networks are characterized by fine-grained Ca-enriched plagioclase (relative to the surrounding coarse-grained plagioclase) and orthopyroxene symplectite (10-50 mu m) surrounded by larger feldspar grains (1-2 mm). Phase-equilibrium modelling indicates that the formation of fine-grained Ca-enriched plagioclase networks occurred under lower pressure granulite-facies conditions and predates the main amphibolite-facies overprint. These reaction products form mm- to cm-scale bands that align under stress and are reactivated during amphibolite-facies conditions. Our observations show that reaction-driven grain size reduction, even when limited to 5%-10% of the rock volume, produces mechanically weak plagioclase networks that are potential sites for later strain localization. This suggests that the early formation of fine-grained reaction products can locally weaken the crust, forming distinct rheological domains and controlling where deformation and later metamorphic evolution are concentrated.

[This corrects the article DOI: 10.1107/S1600576725007915.].

The detailed mineralogical study of serpentinized peridotites collected at 13 localities at mid-ocean ridges, in ophiolites and ultramafic bodies shows the formation of two alteration zones around olivine grains. At the olivine contact, a fine-grained mixture of serpentine, Fe-brucite ($\frac{Mg}{Mg + Fe}$ molar ratio comprised between 0.66 and 0.82) and awaruite occurs (Reaction Zone 1). X-ray mapping indicates limited mass transfer during Reaction Zone 1 formation, suggesting isochemical serpentinization except for the addition of water. The measured distribution of Fe and Mg between serpentine and brucite in Reaction Zone 1 is well reproduced with thermodynamic modelling incorporating the latest data for the Fe$\left ( OH ight )_{2}$ endmember. Thermodynamic modelling also reveals that, at low water to rock ratio, awaruite formation limits H$_{2}$ production to values more than one order of magnitude lower than previous estimates. The predicted H$_{2,aq}$ concentrations are comprised between 10$<^>{-3}$ and 10$<^>{-2}$ mol/kg, that is in the same range than the maximum values measured in fluids expelled at ultramafic-hosted hydrothermal sites. At a water to rock ratio of 1, the updated thermodynamic model only predicts magnetite formation after olivine at temperatures above 300 $<^>{\circ }$C, that is above the temperature estimates for serpentinization based on published oxygen isotope data. Nevertheless, a second assemblage composed of Ni-bearing magnetite, serpentine and Mg-brucite ($\frac{Mg}{Mg + Fe}$ molar ratio comprised between 0.83 and 0.98) can be found at the mesh rim (Reaction Zone 2). Reaction Zone 2 can display a symplectite microtexture. Transmission electron microscopy reveals the presence of porosity and Fe-brucite relics in the symplectite, suggesting Reaction Zone 2 formation after Reaction Zone 1 by dissolution-precipitation. Significant mass transfer is observed in Reaction Zone 2 at the mesh rim, probably associated with the solid volume decrease of 7$\%$ during Reaction Zone 1 reaction to form Reaction Zone 2. The mineralogy and the composition of Reaction Zone 2 are reproduced with thermodynamic modelling of olivine serpentinization not by increasing the water to rock ratio but rather by removing H$_{2}$ from the system. This indicates that H$_{2}$ diffusion is the main driver for magnetite formation during serpentinization. The H$_{2,aq}$ concentrations at the equilibrium with Reaction Zone 2 fall in the 10$<^>{-7}$-10$<^>{-3}$ mol/kg range. The relative proportion of Reaction Zone 1 and Reaction Zone 2 in serpentinized peridotites has first-order impacts on H$_{2}$ production.

Dypingite, a hydrated magnesium carbonate hydroxide mineral [Mg5(CO3)4(OH)2·XH2O, X = 5-6], exhibits promising catalytic and purification properties. Although it was discovered 55 years ago, the crystal structure of this compound has remained unknown due to its aggregated morphology and structural disorder. This work investigates the origin of this phenomenon through a systematic analysis of synthetic and natural mineral samples, using synchrotron powder X-ray diffraction, thermogravimetric analysis and transmission electron microscopy. The findings reveal that ambient humidity significantly influences dypingite's structural properties at room temperature. High humidity (80% relative humidity at 22 °C) causes inhomogeneous expansion of the unit cell along the crystallographic c axis, leading to long-range structural disorder. Conversely, at 20% relative humidity at 22 °C, the mineral structure exhibits a shorter c lattice constant and reduced structural disorder. Chemical analysis reveals that samples kept at 80% and 20% relative humidity for 10 days differ by one molecule of water of hydration, yielding Mg5(CO3)4.5(2)(OH)0.96(3)·6.0(2)H2O and Mg5(CO3)4.5(2)(OH)1.02(4)·5.0(2)H2O, respectively. The results obtained demonstrate that the crystal structure of dehydrated dypingite [Mg5(CO3)4.5(2)(OH)1.02(4)·5.0(2)H2O] derives from hydro-magnesite's unit cell tripled along the a axis. The analysis of the mineral crystal structure provides insight into the role of humidity on the structural properties of dypingite, including unit-cell dimensions and long-range disorder.

Fluid migration within the Earth's crust significantly influences the development of shear zones. The Bergen Arcs have been a focal point for investigating the localization of rheologically weaker zones that facilitate shear zone formation, with various hypotheses proposed (Jamtveit et al. 2019; Incel et al. 2022). These zones may originate from seismic events, inducing brittle fracturing and creating pathways for mineral re-equilibration and ductile deformation. This research concentrates on the island of Krossøy, located in the northernmost part of the Bergen Arcs, Western Norway, offering a unique perspective on deformation, textural evolution, and metamorphism compared to the extensively studied southern regions of Holsnøy and Radøy (Austrheim 1987; Mukai et al. 2014; Moore et al. 2020). Krossøy exposes anorthosites from the old granulitic basement, intruded by a series of subparallel mafic granulitic dykes forming a distinctive "dyke swarm," not documented elsewhere in the Bergen Arcs. We present our results on microstructural analysis through Electron Backscattered Diffraction (EBSD) and, mineral and chemical evolution using Electron Microprobe analysis (EMPA). Given the plagioclase-rich nature of anorthosites, our results delineate the chemical and textural evolution of feldspars, tracing their journey from early-stage granulitic anorthosite formation approximately 930 Ma ago to the development of Caledonian mylonite (440-420 Ma) during shear zone activity under amphibolite facies conditions. By examining the fluid pathways, we seek to determine their relationship with the formation of rheologically weaker areas in the crust and how the dynamic interplay between fluid infiltration and deformation mechanisms may play an important role by changing the metamorphic conditions, textures and mineral assemblages in the rocks. Austrheim, H. (1987). Eclogitization of lower crustal granulites by fluid migration through shear zones. Earth and Planetary Science Letters, 81(2–3), 221-232. Incel, S., Labrousse, L., Hilairet, N. et al. (2022). Reaction-induced embrittlement of the lower continental crust. Geology, 47(3). Jamtveit, B., Petley‐Ragan, A. et al. (2019). The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust. Journal of Geophysical Research: Solid Earth, 124(8), 7725-7755. Moore J., Beinlich A., Piazolo S., Austrheim H. and Putnis A. Metamorphic differentiation via enhanced dissolution along high permeability zones. Journal of Petrology 61, 10, egaa096 (2020) Mukai, H., Austrheim, H., Putnis, C. V., and Putnis, A. (2014). Textural Evolution of Plagioclase Feldspar across a Shear Zone: Implications for Deformation Mechanism and Rock Strength Journal of Petrology, 55(8), 1457-1477.

Holsn & oslash;y, Norway, offers a world-class natural laboratory for studying the impact of fluid on subducting lower crust. Holsn & oslash;y is composed of dry, metastable lower crustal granulite that was infiltrated by fluids along shear zones and seismic fractures during subduction. The infiltration facilitated the localized growth of eclogite facies mineral assemblages along the fluid flow pathways. The duration of the eclogite facies metamorphism, however, remains uncertain. Previous garnet diffusion chronometry studies have estimated timescales ranging from hundreds of years to millions of years based on diffusional relaxation between metastable granulite facies garnet cores and eclogite facies garnet rims and fractures. The shorter timescales are inferred from extremely sharp Ca gradients across chemical contacts present in some garnets whereas the longer timescales are from wider Mg and Fe profiles present in all garnets. The different timescale estimates have led to divergent models for the region's tectonometamorphic evolution. Here we show that the sharp Ca contacts can be explained by diffusion-induced compositional stress. As Ca is significantly larger than Mg and Fe, its movement strains the crystal lattice and generates stress that limits the relaxation of sharp chemical contacts. When compositional stress is accounted for, the sharp contacts yield timescales that are consistent with the wider Mg and Fe diffusion profiles. We determine that eclogite facies conditions (670-700 degrees C, 1.5-2.2 GPa) lasted a maximum of c. 300 kyr. The relatively short duration of eclogite facies conditions requires that multiple transient heating events were superimposed on a longer (>10(6) yr) overall timescale of metamorphism. Granulite facies garnet cores are surrounded by multiple generations of eclogite facies rims formed by interface-coupled dissolution-reprecipitation (ICDR) reactions. The garnet rims indicate two rapid, regional-scale fluid pulses and additional smaller, more localized pulses. The fluid pulses may be linked to episodes of seismic moment release as well as transient heating via exothermic hydration reactions and/or shear deformation. Our model results predict up to 400 MPa of differential stress at the garnet core-rim contacts, consistent with observed eclogite facies microfractures that extend into relic granulite facies garnet cores. The microfractures indicate that ICDR was aided by compositional stress: diffusion ahead of the reaction front generated stress and fracturing that created porosity for further ICDR. Thus, compositional stress can markedly impact both diffusion systematics and intracrystalline deformation. Together, these results show that despite their brevity, transient thermal, fluid flux, and/or baric episodes may exert the primary controls on the mineralogical and rheological development of subducted lithologies, in contrast to the long, slow burial and exhumation typically envisioned for regional metamorphism.

Fluid-rock interactions play a key role in the formation, evolution and recycling of the Earth's crust. For fluids to infiltrate rocks and enable and sustain fluid-mediated mineral transformations, fluid pathways are required. In this study, we examined the potential mechanisms of formation of such pathways via detailed mineralogical, petrophysical and thermodynamic analysis of a dry, essentially 'non-porous' gabbro that was hydrated and transformed into an amphibolite under amphibolite-facies conditions. During a previous regional HP eclogite-facies metamorphism, the gabbro did not equilibrate and preserved almost entirely its igneous textures and magmatic minerals. Rock transformation during amphibolitization was triggered by fluid infiltration through a newly opened N-S striking fracture network. An equally spaced fracture network formed by mode I opening related to the formation of an E-W striking shear zone at the northern and southern borders of the gabbro body. The amphibolitization process allowed the fluid to pervasively infiltrate the rock from the fracture into the pristine gabbro. The essentially fully amphibolitized sample exhibits some unaffected gabbroic mineral relicts. Even though the amphibolitization process led to the formation of similar to 70 vol.% hydrous phases, it was accompanied by densification and related porosity formation. The modes and compositions of minerals within partly amphibolitized rocks indicate that besides the uptake of H2O, no significant mass exchanges were necessary for this transformation, at least on the thin section scale. Thermodynamic modelling and petrological data show that the transition from gabbro to amphibolite favours porosity formation. In the model, the reaction front proceeded as soon as the gabbro at the reactive interfaces of the affected minerals was sufficiently transformed. At this point, fluid was not consumed further but remained as a free fluid phase, which progressed through the newly formed pore space and advanced amphibolitization. Once the gabbro was almost entirely amphibolitized, its mineral content and mineral chemistry no longer changed, so the progress of amphibolitization progress was controlled by fluid availability. This case study shows that fluid-rock interaction leading to hydration of a rock can be efficiently maintained in almost non-permeable, dry and mafic crust and, therefore, strongly affects the petrophysical properties of the Earth's crust.

The island of Kross & oslash;y in the Bergen Arcs, southwestern Norway, provides an ideal geological setting to study the mechanisms of grain size reduction in plagioclase during shear zone development. The transition from a granulite facies anorthosite to mylonitic textures under amphibolite facies conditions is investigated through field observations, petrographic analysis, electron microprobe analysis, and electron backscattered diffraction (EBSD) techniques. Initial ductile deformation, evidenced by complex twinning and intragrain misorientation, leads to strain hardening and subsequent brittle fracturing. This results in grain size reduction primarily through fragmentation, forming a bimodal grain size distribution. Syntectonic recrystallization produces compositionally similar neoblasts. Subsequent interactions with fluids induce compositional recrystallization, leading to further grain size reduction and the formation of zoned plagioclase grains. These processes highlight the critical role of fluid-mediated recrystallization in shear zone development and strain localization. The spatial variation in deformation and hydration stages across Kross & oslash;y is attributed to both mechanical anisotropy and chemical reequilibration driven by fluid pathways. These findings enhance our understanding of the interplay between mechanical anisotropy, brittle failure and fluid infiltration in the lower crust, providing insights into the conditions that facilitate the formation and evolution of shear zones in the Bergen Arcs.

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.

Occurrences of natural magnesium alumina silicate hydrate (M-(A)-S-H) cement are present in Feragen and Leka, in eastern and western Trøndelag Norway, respectively. Both occurrences are in the subarctic climate zone and form in glacial till and moraine material deposited on ultramafic rock during the Weichselian glaciation. Weathering of serpentinized peridotite dissolves brucite and results in an alkaline fluid with a relatively high pH which subsequently reacts with the felsic minerals of the till (quartz, plagioclase, K-feldspar) to form a cement consisting of an amorphous material or a mixture of nanocrystalline Mg-rich phyllosilicates, including illite. The presence of plagioclase in the till results in the enrichment of alumina in the cement, i.e., forms M-A-S-H instead of the M-S-H cement. Dissolution of quartz results in numerous etch pits and negative quartz crystals filled with M-A-S-H cement. Where the quartz dissolution is faster than the cement precipitation, a honeycomb-like texture is formed. Compositionally, the cemented till (tillite) contains more MgO and has a higher loss of ignition than the till, suggesting that the cement is formed by a MgO fluid that previously reacted with the peridotite. The M-(A)-S-H cemented till represents a new type of duricrust, coined magsilcrete. The study of natural Mg cement provides information on peridotites as a Mg source for Mg cement and as a feedstock for CO2 sequestration.

Serpentinization and carbonation have affected ultramafic rocks on Noachian Mars in several places. Among the most prominent systems revealing mineral assemblages characteristic of serpentinization/carbonation is the Nili Fossae region [1]. Jezero crater – the target of the Mars 2020 rover –hosted a paleolake which constitutes a sink for sediments from Nili Fossae [1]. Thanks to the near infrared spectrometer onboard Mars2020 [2], the mission has the potential to offer ground truth measurement for other putative serpentinization/carbonation system documented on Mars. Several important aspects that may be addressed are: Do carbonates result from primary alteration of olivine-rich lithologies or are they derived by reprocessing of previous alteration minerals [3]? What is the composition? and nature of the protolith, which appear to be constituted of considerable amounts of olivine [4]? To reveal critical information regarding the conditions of serpentinization/carbonation, accessory minerals need detailed studies [1; 5]. In case of Jezero Crater, and serpentinization on Mars in general, the main alteration minerals are identified, but little is known about the accessory minerals. The Nili Fossae-Jezero system has potential analogues in terrestrial serpentinized and carbonated rocks, such as the Leka Ophiolite Complex, Norway (PTAL collection, https://www.ptal.eu). Here, distinct mineral assemblages record different stages of hydration and carbonation of ultramafic rocks [6]. We perform petrological and mineralogical analyses on thin sections to characterize the major and trace minerals and combine with Near Infrared (NIR) spectroscopy measurements. We study the significance of the mineralogical assemblages including solid solution composition and nature of accessory minerals. Effect of the presence of accessory minerals on the NIR signal is investigated and their potential incidence on the amount of H2/CH4 production in mafic or ultramafic system is discussed [5; 8]. This could improve our understanding of serpentinization and carbonation processes on Mars, which can guide future in-situ operations and also help for a better interpretation of the remote sensing data acquired on other possible serpentinization/carbonation systems. References: 1. Brown, A. J., et al. EPSL297.1-2 (2010): 174-182. 2. Wiens, R.C., et al. Space Sci Rev217, 4 (2021). 3. Horgan, B., et al. Second International Mars Sample Return. Vol. 2071. 2018. 4. Ody, A., et al. JGR: Planets118.2 (2013): 234-262. 5. Klein, F., et al. Lithos178 (2013): 55-69. 6. Bjerga, A., et al. Lithos227 (2015): 21-36. 7. Bultel, B. (Doctoral dissertation, Lyon). (2016).

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.

Alteration of serpentinized peridotites of the Highland Border Complex in Scotland took place in two steps. Listvenite-like dolomite–quartz rocks formed by addition of CaO, Sr and CO 2 at constant MgO and SiO 2 involving a mass increase of c. 140%. Stage two involved the dissolution of dolomite, evinced by the abundant pores and rhombohedral grains of quartz, to form Cr- and Ni-rich jasper and quartzites. Formation of the jasper–quartzites involves a mass reduction of c. 80%. The listvenite-like and jasper–quartzite rocks show enrichment in the fluid-mobile elements Ba, Sr, Cs, As and Sb. The As is present in the Aluminium–Phosphate–Sulfate group of minerals formed during alteration of Cr-spinel. Cr-spinel also alters to porous hematite and ferrihydrite with patches containing up to 5.5 wt% As 2 O 3 . Enrichment of As, related to alteration of chromite, is previously unknown from natural rocks, but strongly resembles efficient methods used for remediation of this toxic element. Formation of quartzite and jasper from peridotite and their common presence as pebbles in the Devonian Old Red conglomerates, the Highland Border Complex and Devonian basins in the Scandinavian Caledonides highlight their importance and potential for provenance and tectonostratigraphic correlations. Supplementary material: Supplementary data tables are available at https://doi.org/10.6084/m9.figshare.c.6764598 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

The Bergen Arcs, in Norway, consist of several arcuate nappes formed during the Caledonian orogeny 440-420 Ma ago (Bingen et al., 2001; Glodny et al., 2008) when the western margin of Baltica was subducted below Laurentia. This Caledonian orogeny overprinted many of the anorthosites that formed the 930 Ma old (Bingen et al., 2001) granulitic basement. This overprint resulted in both amphibolites and eclogites and have been observed in shear zones within the rocks of the well-studied island of Holsnøy, located on the western margin of the Lindås Nappe. On the adjacent island of Radøy, the Caledonian overprint is associated with amphibolite facies shear zones (Mukai et al., 2014; Moore et al., 2020).In the northern margin of the Bergen Arcs, near the Bergen Arcs Shear Zone, the much less-studied island of Krossøy also exposes the anorthosites from the old granulitic basement and here the Caledonian overprint also resulted only in amphibolite facies metamorphism. The anorthosites in Krossøy are intruded by a series of subparallel mafic granulitic dykes forming the Krossøy dyke swarm, that has never previously been described elsewhere in the Bergen Arcs. The style of deformation in the granulites and the textural evolution in the amphibolite facies overprint are also markedly different from the rocks on Holsnøy and Radøy. The development of ductile Caledonian shear zones may have been facilitated by initial brittle failure of the basement accompanied by fluid infiltration (Jamtveit et al., 2018). Here we investigate the influence of this deformation and fluid infiltration on different features observed on these rocks such as: the occurrence of plagioclase coronas around the garnets on the dykes; the presence of different types of symplectites; the variability of size, deformation and composition observed on the anorthositic feldspars; or the local changes of fluid composition along cm- long fractures. We will show our first analytical results on some of these key features and discuss their relevance in the context of the previous studies of the Bergen Arcs. Bingen, B., David, W. J., & Austrheim, H. (2001). Zircon U-Pb geochronology in the Bergen Arc eclogites and their Protereyoic protoliths, and implications for the pre-Scandian evolution of the Caledonides in western Norway. In GSA Bulletin (Issue 5). https://doi.org/10.1130/0016-7606(2001)113<0640:ZUPGIT>2.0.CO;2Glodny, J., Kühn, A., & Austrheim, H. (2008). Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction-collision system, Bergen Arcs, Norway. Contributions to Mineralogy and Petrology, 156(1), 27–48. https://doi.org/10.1007/s00410-007-0272-yJamtveit, B., Moulas, E., Andersen, T. B., Austrheim, H., Corfu, F., Petley-Ragan, A., & Schmalholz, S. M. (2018). High Pressure Metamorphism Caused by Fluid Induced Weakening of Deep Continental Crust. Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-35200-1Moore J., Beinlich A., Piazolo S., Austrheim H. & Putnis A. (2020). Metamorphic differentiation via enhanced dissolution along high permeability zones. Journal of Petrology 61, 10. https://doi.org/10.1093/petrology/egaa096Mukai H., Austrheim H., Putnis CV. & Putnis A. (2014). Textural evolution of plagioclase feldspar across a shear zone: implications for deformation mechanism and rock strength. Journal of Petrology. 55, 1457-1477. https://doi.org/10.1093/petrology/egu03

Proterozoic foliated and nodular sillimanite gneisses from the Bamble lithotectonic domain, South Norway, are analysed to unravel their microfabric evolution with mineral reactions during metasomatism and associated deformation. The nodules form cm-scaled spherical to ellipsoidal sillimanite-quartz aggregates that locally grade into foliated sillimanite gneisses. Independent on their fabric, they record incomplete breakdown reactions of biotite and K-feldspar recorded by muscovite lamellae and associated Fe-oxide needles in biotite and by muscovite-quartz aggregates after K-feldspar. Muscovite is partly replaced by sillimanite. Based on immobile Al, the nodular gneiss forming reactions give excess K, Mg and H2O that may leave the nodular gneiss to form a metasomatic agent and caused regional metasomatism (scapolitisation) in the surrounding rocks. Quartz in the foliated gneisses shows a pronounced shape but no marked crystallographic preferred orientation. There is no indication of major strain accumulation by quartz dislocation creep. Muscovite shows lobate phase boundaries to quartz, which is interpreted as reaction fabric, from the breakdown reactions of K-feldspar and biotite. The nodular and sillimanite gneisses formed during metasomatic mineral reactions, where major elements K, Mg and H2O leave the rock and an Al-rich metasomatic restite remains. We suggest that the metasomatism involved a molar volume loss, where reactions forming muscovite, quartz and sillimanite occurred by incongruent dissolution-precipitation creep at low stresses forming the nodular and foliated gneisses. Our study demonstrates that metasomatism with chemical rock changes and mass transfer associated with incongruent dissolution-precipitation contributed to the observed reaction and deformation microfabric.