One of the challenges of storing hydrogen in saline aquifers is the possible geochemical reactions between host rock, formation solution, and hydrogen. This study investigated the effect of hydrogen treatment on the mechanical properties of sandstone samples from Hulett member of Sundance Formation, a potential underground hydrogen storage host in Wyoming, USA. Cylindrical specimens were treated with brine, brine+50%H-2, and brine+100%H-2 for two weeks at a pressure of 15 MPa and a temperature of 83 C-degrees. X-ray diffraction (XRD), scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET), uniaxial compression, and triaxial compression tests were conducted on specimens. Results showed that specimens treated with brine+50%H-2 and brine+100%H-2, on average, exhibited 24 and 41% lower peak strength, respectively, compared to that of specimens treated with the brine. XRD results showed a decrease in dolomite and clay content of specimens after exposure to hydrogen, causing degradation of the mechanical and elastic properties of the specimens.

Diverse assemblages of metal alloys occur within ultramafic rocks from a wide range of tectonic settings and geological environments. Alloys are typically small and can be difficult to find and identify, but they can host Re and Os, which are significant for geochronology, as well as other elements of economic interest, and can act as catalysts that help to form organic compounds. The alloys typically form from elements that show chalcophile and siderophile characteristics, indicating that alloys generally form under reducing, S-poor conditions. These conditions can be generated by a range of mechanisms that differ amongst alloy-forming environments, but include desulfidation during melting, exsolution from magmatic S-bearing phases during melt crystallisation, auto-hydrothermal, and sub-solidus processes involving a reduction in fS2, and electron transfer processes related to serpentinisation. Alloy formation during serpentinisation is typically associated with the formation of a companion phase that hosts Fe3+, such as Fe3+-bearing serpentine, magnetite, ferrit-chromite, or andradite, recording disproportionation of Fe2+ released by olivine alteration to form Fe0 and Fe3+. An additional source of electrons for alloy formation is provided by the reduction of H+ in water to H2, and the reduction of water activity couples with a reduction in fS2, driving desulfidation reactions and facilitating alloy formation. Other electron donors, such as methane, may also contribute to alloy formation. Alloy-forming serpentinites are strongly internally-buffered, but can follow a range of trajectories through compositional space. Serpentinite-hosted alloys offer opportunities to better understand a range of geological processes with applications within earth and material sciences. However, these advances require new thermodynamic data and activity composition models for all but the most common alloys, and better characterisation of alloy compositions.

The tectonic window at Pito Deep, in the southern Pacific Ocean, permits study of the formative processes of uppermost East Pacific Rise (EPR) gabbroic ocean crust. Here we present a detailed microstructural and crystallographic study of 17 gabbroic samples fromthe uppermost similar to 800m of plutonic crust exposed in the Pito Deep Rift. We integrate two-and three-dimensional measurements of crystal size, shape, spatial distribution and orientation, with petrographic observations and geochemical data to constrain the formation of fast spread gabbroic ocean crust. The shallowest samples, collected < 55 metres below the sheeted dikes (mbsd), have evolved bulk-rock compositions, elongate plagioclase crystals, a clear plagioclase shape- and crystallographic-preferred orientation, and preserve only minor amounts of intracrystalline strain. The characteristics of these rocks and their proximity to the sheeted dike complex, suggests they formed by crystallization at the lateral tip of an axial melt lens that solidified as it moved off axis. Underlying samples from 96-724 mbsd, record more primitive bulk-rock compositions, less elongate plagioclase crystals and exhibit increasing strength of both plagioclase shape- and crystallographic-preferred orientation with depth below the sheeted dikes. These samples host plagioclase crystals that show increasing intracrystalline strain with depth, suggesting magmatic to hypersolidus submagmatic flow within the mush zone beneath the axial melt lens. These observations, together with inclined-to-steeply dipping mineral layering preserved below similar to 180 mbsd, are interpreted to record the downward transport of crystal-rich magma originating at the bottom of the melt lens through the uppermost kilometre of the mush zone at the EPR. The location of initial crystallization along the floor of the axial melt lens determines the magmatic processes that affect the crystal-rich magma en route to solidification as lower ocean crust.