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Abstract: Recently, carbonate microbialites were discovered in Lake Alchichica, Mexico, forming below the oxycline (down to 40 m depth), under seasonally anoxic conditions, while conspicuous microbialites also grow at shallower depths under continuously oxic conditions. Here, we investigated sulfur (S) speciation in these microbialites at submicrometer resolution using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy, complemented by laboratory X-ray fluorescence (XRF) mapping, scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDXS). Our findings revealed that S is pervasive in both shallow and deep microbialites. However, S speciation varied with depth: while carbonate-associated sulfates (CAS) and organic S compounds were present at all depths, more reduced S species were enriched in the aragonitic layers of deep microbialites, particularly in association with remnants of biogenic structures. These variations in S speciation suggest that deep Alchichica microbialites continue to accrete during seasonal anoxia. Moreover, our results indicate that S speciation in modern microbialites is influenced by ambient redox conditions. By documenting the processes affecting S speciation in a modern microbialitic system, this study provides a framework for future investigations into ancient microbialites that formed under sulfidic conditions.

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Abstract: Tracking carbon dioxide (CO2) flow and its consequent effects in subsurface rocks has received considerable attention in geological carbon sequestration. However, existing research on CO2 flow in reservoir-caprock systems with well-developed pores and microfractures is limited, and our understanding of its mechanisms remains incomplete. Here, we present the first observation of gaseous and supercritical CO2 (scCO2) behavior within a reservoir-caprock couplet using high-resolution synchrotron X-ray imaging. We found that high-speed gaseous CO2 flow reshaped part of the fracture framework but did not induce additional secondary fractures. High-speed scCO2 caused significant fracturing, connecting natural fractures in shale with newly developed secondary branches and creating larger pore spaces in sandstone. Consequently, the simulated permeability increased by approximately 2.6-fold in shale and 8.6-fold in sandstone. The concentrated strains around the main fracture in shale and the web-like strain patterns along granular mineral boundaries in sandstone highlight the distinct modes of scCO2 action during its passage through the reservoir-caprock system. This work provides new insights into the complex reservoir-caprock system and offers practical guidelines for fluid injection and production activities.

Open Access

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Abstract: Arc magmas have long been considered significantly more oxidised than their ocean island and mid-ocean ridge counterparts, a characteristic widely attributed to infusion of the mantle wedge by fluids from subducted lithologies. However, here we show that at comparable degree of differentiation and sulfur content, arc magmas have comparable oxidation state to ocean island magmas. Our study is based on measurements of Fe3+/∑Fe along with major and volatile elements in olivine and plagioclase hosted melt inclusions and matrix glasses from eleven volcanic systems located in arc settings worldwide. Accounting for fractional crystallisation (to MgO = 6 wt. %) we find that all systems lie on a reducing trend accompanying sulfur degassing, from QFM +0.9 (±0.2, 1σ) when S > 2000 ppm to QFM −0.2 (±0.6, 1σ) when S < 100 ppm (where QFM stands for the Quartz-Fayalite-Magnetite buffer). These findings reconcile the observed discrepancy between the oxidation states of xenoliths in arc magmas and gas emissions from arc volcanoes. We further show that fractional crystallisation influences the redox evolution of arc magmas to a comparable extent as, and sometimes counteracting, sulfur degassing.

Open Access

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Abstract: Seismic velocity at the near surface drops during ground motions due to remote earthquakes and can recover afterwards over decades. In the laboratory, seismic velocity of rock samples decreases after dynamic deformations (e.g., shaking) and gradually recovers towards the original level. These observations at different scales are referred to as slow dynamics in granular materials (e.g., rocks and concrete), but the underlying mechanisms remain debated. We explore the physics behind seismic velocity transients during and after dynamic deformations using Stór Mjölnir — a triaxial pressure loading apparatus featuring two piezoelectric transducers mounted in the top and bottom pistons and an X-ray transparent aluminium pressure vessel that houses a cylindrical core sample of Clashach sandstone (10 mm in diameter and 25 mm in length). We present the mechanical, acoustic, and X-ray microtomography results of two triaxial loading experiments, conducted at room temperature with a confining pressure of 20 MPa and a pore fluid pressure of 5 MPa. Both experiments involve first increasing the ram pressure at a constant strain rate of 1x10-5 s-1 until the onset of sample yielding, indicated by a deviation from the linear stress–strain curve. In the first experiment, we further hold the ram pressure constant and then abruptly reduce the pressure by 30 MPa before rapidly returning the pressure to the previous hold level; this perturbation is repeated for 32 cycles until catastrophic failure of the rock sample. In the second experiment, we apply the same cyclic loading protocol after sample yielding, except for the abrupt pressure drop of 150 MPa; the sample survives only two loading cycles before catastrophic failure. These cyclic loading protocols are designed to induce transient seismic velocity responses, which are monitored by active acoustic surveys acquired every 8 s and in-situ 3D X-ray tomography synthesised every 6 min at the beamline I12-JEEP, Diamond Light Source (Oxfordshire, UK). We observe nearly linear relationships between the small stress perturbations (30 MPa) and corresponding seismic velocity changes, indicating minimal slow dynamics in the rock sample. In contrast, large stress perturbations (150 MPa) cause nonlinear velocity changes, although the recovery time scale is limited by the small size of the experimental sample. The time-resolved 3D X-ray volumes from both experiments show no resolvable transient structural changes in the rock samples, despite ongoing microfracture accumulation and pore enlargement driven by background creep until catastrophic failure. These results demonstrate that active seismic waves can detect nonlinear velocity transients in triaxial loading experiments, which likely originate from microstructures (e.g., grain contacts) below the X-ray imaging resolution (voxel edge length ~ 7.9 µm). These experiments also motivate further study on seismic velocity transients using our next-generation experimental apparatus that accommodates larger samples (18 mm in diameter and 45 mm in length) and six acoustic transducers. Ultimately, we aim to assess seismic velocity transients as a proxy for rocks’ susceptibility to small stress perturbations, which could provide a method to map the proximity to catastrophic failure and hence help mitigate induced seismicity associated with hydraulic fracturing.

Open Access

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Abstract: Nanoscale crystals, or ‘nanolites’, are becoming increasingly recognised in both experimental products and natural samples of volcanic eruptions, across a range of magma compositions and explosivity. Nanolites can increase magma viscosity and influence eruptive style, due to the rheological impact of the nanoparticle suspension, by inducing chemical and structural changes in the residual melt and by facilitating heterogeneous bubble nucleation. Due to their large surface area, nanolites are also prone to aggregation. However, their morphology, spatial distribution and interaction in 3D has not yet been investigated. Here we present a 3D, nanometre-scale visualisation and quantification of nanolites within scoriae of highly explosive basaltic volcanic eruptions, obtained using X-ray ptychography, a nanoscale microscopy technique. We find that titanomagnetite nanolites aggregate, forming elongate, irregular structures in 3D. Compositional heterogeneities are also observed within the matrix glass, as extraction of Fe and Ti from the melt during nanolite crystallisation forms differentiated, Si-rich boundary layers surrounding nanolites with higher viscosity. We support our 3D nanoscale observations with images acquired using SEM and STEM, utilising multi-scale imaging methods to visualise nanolite crystallisation in basaltic magmas. We find that syn-eruptive nanolite crystallisation can increase magma viscosity through their aggregation and impact on the composition of the residual melt, increasing the potential of magma fragmentation during ascent. Our results provide insight into the nanoscale structure of volcanic products and also the driving mechanisms of highly explosive basaltic volcanic eruptions.