Alissa Jeanne Kotowski

Basal lherzolite tectonites of supra-subduction zone ophiolites are thin (~10–100s m) fertile layers commonly interpreted to be inherited from oceanic spreading. Yet, their structural and chemical similarities between ophiolites may suggest that they record processes generic to subduction infancy. Here, we document a systematic compositional, thermal, and strain gradient through the basal mantle of the Mont Albert ophiolite (~450 Ma, Québec). Peridotites up to ~600 m above the contact with underthrust lithologies exhibit major, trace and rare earth element (REE) enrichments. Fertility increases toward the contact and coincides with cooling from >900°C to ~750°C, syn-kinematic decrease in spinel Cr# and mylonitization. We interpret the coupled changes to reflect addition of slab-derived hydrous melts to a cooling, deforming mantle wedge during subduction infancy. We propose that basal lherzolites are fossil reaction fronts that capture the earliest chemo-mechanical development of subduction interfaces; refertilization thus postdates original lithosphere formation but predates forearc spreading to form ophiolite crust. Reports of basal lherzolites show that they thin with time over the Phanerozoic; this may reflect a waning role of slab-melts in nascent subduction processes.

Tectonic plate motions reflect a balance of driving and resisting forces and typical velocities of 1–4 . Here, we show that an ancient island arc now exposed in Hokkaido (Japan), together with its associated subduction system, migrated across the mantle beneath the NW Pacific region at ~25 cm/yr between 85 and 50 Ma, based on new paleomagnetic data. Such exceptional rates require an unusually weak upper mantle. There is no evidence for past or present thermal anomalies that, elsewhere, have been proposed to explain anomalous velocities ^2,3 . Instead, seismological data show surprisingly high upper‑mantle seismic attenuation in the NW Pacific, signaling reduced grain size and/or elevated water content that likely reflect water and sediment input into the mantle from past intra‑oceanic subduction, which both could cause major mantle weakening. Seismic tomography shows that intra-oceanic subduction also occurred in the NW Pacific before 85 Ma ^5–7 , possibly preconditioning the NW Pacific mantle for anomalously high plate rates. Together with emerging evidence of geochemically heterogeneous mantle provinces ⁸ that preserve relics of ancient subduction or plume activity ^9–13 , our findings demonstrate that the distribution, composition, and rheology of mantle heterogeneities record that stem from past geodynamic processes may exert first‑order control on past and present plate motions.

Shear zones act as preferential fluid pathways during prograde and retrograde stages of metamorphism. Nonetheless, we still have limited knowledge of the drainage and permeability of natural settings. The preserved signature of fluid in exhumed rocks provides insights into fluid flow during burial and exhumation. This study investigates fluid flow processes recorded by garnet in quartz‐schists from the As Sheik shear zone (Saih Hatat window, NE Oman). We observed garnet as equant, oblate and honeycomb (i.e., skeletal) shapes that document distinct fluid‐related growth stages from peak‐pressure to early exhumation, both occurring at eclogite facies conditions. Garnet nucleated at 2.0–2.2 GPa and 500°C–550°C after the chloritoid‐out dehydration reaction. Subsequent decompression and heating (1.3–1.5 GPa, 600°C–650°C) promoted further fluid release and honeycomb garnet growth. We infer that dissolution, transport and precipitation rates primarily influenced whether garnets grew as oblate grains (i.e., as pseudomorphs on peak‐pressure chloritoid grains) or newly nucleated equant grains. Honeycomb garnet was interpreted to show the permeability network that records migration and drainage of fluid. This internally produced fluid escaped from the shear zone using pre‐existing grain boundaries and reaction‐formed pathways. Assuming 1 km of shear zone thickness, we computed a time‐integrated fluid flux of ~34 m ³ m ⁻² for the entire duration of garnet growth. This study highlights garnet morphology as a tracer of transient fluid pathways during a burial–exhumation cycle of an eclogitic shear zone. The close connection between garnet morphology and fluids calls for a re‐evaluation of similar microstructures in different tectonic settings.

Reconstructing palaeogeography from accretionary records is challenging owing to the difficulty of integrating data sources transcending a range of fields and scales. Here, we present the ‘orogenic architecture diagram’ method to systematically compile geological data in temporal and spatial context at the scale of the ‘building blocks’ of orogens and to use their interpreted geological histories as a base for tectonic and palaeogeographical reconstruction. We identify lower plate-derived Continental or Ocean Plate Stratigraphy, and upper plate-derived ophiolites and magmatic units. We apply this framework to the Newfoundland Appalachians, Canada, which record accretion of oceanic and Gondwana- and Laurentia-derived continental units to Laurentia during the Cambro-Ordovician closure of the Iapetus Ocean. Our diagrams allow for straightforward connection of modern geological records to opening and closure of marginal oceanic basins and illustrate that the Iapetus Ocean itself left little accretionary record. Our approach highlights the source of contrasting interpretations in Newfoundland reconstructions and may motivate targeted field campaigns here and elsewhere within the orogen to interrogate proposed models. Our application to Newfoundland also demonstrates that our protocol, based on modern-style plate tectonics and orogenesis, still applies to early Paleozoic orogens and may provide a tool to reconstruct the emergence of plate tectonics.

Nanoindentation is a mechanical testing technique used to quantify material properties (e.g., hardness) and deformation behaviour (e.g., plasticity). By controlling the indenter tip with great precision in all dimensions, the range of available methods can be expanded to include rapid property mapping, constant-stiffness stress-strain curves, topography mapping, and scratch and frictional testing. We have set up a complete, affordable, and fast workflow centred around nanoindentation with a Bruker Hysitron TriboIndenter 990 and complemented by electron microscopy to tackle a variety of research questions concerning the behaviour of earth materials.This contribution showcases this workflow as applied to several common rock-forming high-pressure metamorphic minerals. We begin with first-order sample characterisation of thin sections using optical microscopy and electron backscatter diffraction to quantify crystal orientations to determine which crystal axes are being indented. Transitioning to the mechanical testing phase, we use spherical tips to obtain stress-strain curves to analyse the transition from elastic deformation to low-temperature plasticity, and to quantify the yield hardness. Stress-strain curves can be calculated from regular constant loading rate indentation experiments, only valid within the elastic domain, or with constant stiffness measurements using tip oscillations to provide a full stress-strain curve including plastic behaviour. We image the residual indent sites with surface probe mapping, which measures surface topography with a vertical resolution down to 0.1 nm and thus produces 3D maps with which we can quantify the dimensions and geometries of indent pits.The results of our case studies on glaucophane, omphacite, and garnet show that plastic yielding is controlled by the availability of nucleation points for dislocations, provided by pre-existing defects. The degree of this effect varies per mineral, and further depends on crystal orientation. Overall, we demonstrate an efficient workflow for mechanical and microstructural characterization of low-temperature plasticity with nanoindentation applicable to most silicates and other minerals. This workflow can also be adjusted to analyse and quantify many other aspects of the properties and behaviour of earth materials.

The number of geophysics students is rapidly declining across Europe, despite the great demand for geophysicists. Many students are interested in quantitative sciences with Earth science applications, but they are often not aware of geophysics as a career option as it is not typically taught in school (Jenkins et al., 2024). Furthermore, the spread of misinformation, conspiracy theories, and anti-scientific movements across Europe have highlighted the need for scientific literacy (Siarova et al., 2019 European Parliament). Now more than ever, there is a need to introduce the general public to the basics of the scientific method and what it means to do research, particularly in geosciences (EGU Barcelona Manifesto for the Teaching of Geosciences, 2023). We are therefore developing materials for Science Storytellers: scientists who go into their community to engage with students and the general public. Our curiosity-driven teaching materials are based around well-known tales and incorporate elements of narrative, performance, and play. Through this original framework, we provide scientists and teachers with the tools to effectively engage students. This year, we focus on the classic tale of “The House At Pooh Corner” by A. A. Milne (1928). In this novel, Eeyore is relieved that there has not been an earthquake lately. The book takes place in Sussex (UK), where earthquakes are not typical. Why then, is Eeyore talking about them? We present the science case behind this story in the General Seismology session (SM1.1) and here we show how we developed this research into teaching and outreach material. We developed a set of different questions around the research-project, which naturally arise from the Winnie-the-Pooh premise to encourage children to follow their own curiosity to discover why Eeyore is worried about earthquakes. With this abstract, we seek feedback from fellow science communicators and teachers and invite everyone along to the try-out of the corresponding "The Science Storyteller" theatre show, which will make its debut on the Friday afternoon of EGU in a dedicated splinter meeting. We hope to reach even more young people and curious minds through this interactive, musical journey through the science story behind Winnie-the-Pooh. References Jenkins, J., Gilligan, A., & Bie, L. (2024). Who wants to be a geophysicist?. Astronomy & Geophysics, 65(5). Siarova, H., Sternadel, D., & Szőnyi, E. (2019). Research for CULT committee–Science and scientific literacy as an educational challenge.

Shear zones are preferential fluid pathways during prograde and retrograde stages of subduction cycles, but the drainage and permeability of subduction interfaces are poorly quantified. Analyzing exhumed rocks for preserved signatures of fluid production and flow provides insights into fluid circulation during burial and exhumation.Here, we investigated fluid flow processes recorded by garnets in quartz-schists from the As Sheik shear zone (Saih Hatat window, NE Oman) that records evidence for burial during subduction and local overprinting during exhumation. Garnet occurs as equant, oblate, and honeycomb (i.e., skeletal) shapes, which each documents distinct fluid-related growth stages from peak-pressure to early exhumation associated with a thermal excursion, both occurring at broadly eclogite facies conditions. We show with thermodynamic models and microstructures that garnet first nucleated at 2.0–2.2 GPa and 500–550°C after the chloritoid-out dehydration reaction, which promoted dissolution–precipitation processes. We infer a pseudomorphic replacement of peak-pressure chloritoid by garnet, and based on the absence of internal lattice strain, we suggest that elongate garnet morphology reflects reaction-controlled growth rather than plastic deformation. Our microstructures and models suggest that subsequent decompression and heating (1.5–1.3 GPa, 600–650°C) promoted further fluid release and a renewed stage of honeycomb garnet growth.We present a conceptual model in which dissolution, transport, and precipitation rates primarily influenced whether garnets grew as oblate grains (i.e., as pseudomorphs on peak-pressure chloritoid grains), or as newly nucleated equant grains. In addition, we argue that honeycomb garnet represents a snapshot of the permeability network that allowed the fluids to escape from the shear zone using grain boundaries and through reaction-forming pathways.Using measured maximum mass fraction of fluid released from all the hydrous phases modelled by thermodynamic modelling on a representative rock-scale column of 1000 meters, we estimate the time-integrated fluid flux of the studied shear zone was ~34 m3 m-2 at eclogite facies conditions for the entire duration of garnet growth. This volume represents a limited time-range of the shear zone lifetime during garnet growth, i.e., from peak-pressure to incipient exhumation still at eclogite facies conditions. Therefore, the full lifetime of the shear zone during prograde and retrograde conditions would indeed provide a higher fluid flux.The different garnet morphologies analyzed all resulted from the chloritoid dehydration reaction, but reflect different rates of dissolution–precipitation and efficiency of dissolution. This study highlights garnet morphology as a tracer of transient fluid pathways during a burial-exhumation cycle of an eclogitic shear zone. The close connection between garnet morphology and fluids calls for a re-evaluation of similar microstructures in different tectonic settings.

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.

Subduction initiation often begins with slow, forced convergence, switches “on” catastrophically as the slab collapses into the mantle, and then evolves to steady‐state, self‐sustained sinking that drives global plate movements. Numerical models suggest that the collapse phase implies sudden weakening of the plate interface. However, geological records of subduction infancy preserved as metamorphosed oceanic crust accreted beneath ophiolites (i.e., metamorphic soles) present a paradox. During the pre‐collapse period that may last 2–15 million years, the nascent plate interface is hot, whereas during collapse, shear zone temperatures plummet, which typically strengthens rocks. So, how could cooling cause weakening? Here, we show microstructures of metamorphic sole rocks from Mont Albert (Québec, Canada) that demonstrate that upon cooling, metamorphic mineralogy became more heterogeneous, average grain size decreased, and deformation mechanisms shifted from dislocation‐accommodated to fluid‐assisted and grain size sensitive, which culminated in drastic rheological weakening. Quartz piezometry indicates that flow stress increased with cooling, but flow laws indicate that the colder rocks exhibited lower viscosity and therefore could localize strain. Interface viscosity initially rose with cooling, but upon reaching a threshold where major metamorphic minerals changed, dropped from >10 ¹⁸ to 17 Pa‐s. Cooling‐induced mineral‐mechanical changes thus drove rheological weakening, and provide a general mechanism explaining slab collapse and the transition to self‐sustaining subduction. This implies that strain localization is inherent to modern metamorphosed oceanic lithosphere and does not require a “stress drop.” The next step to understanding subduction initiation is identifying causes of high temperatures and incipient cooling during the pre‐collapse phase.

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 ≈ 10-15 m2, 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.