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Lithium and boron are trace components of magmas, released during exsolution of a gas phase during volcanic activity.
In this study, we determine the diffusivity and isotopic fractionation of Li and B in hydrous silicate melts.
Two glasses were synthesized with the same rhyolitic composition (4.2 wt% water), having different Li and B contents; these were studied in diffusion-couple experiments that were performed using an internally heated pressure vessel, operated at 300 MPa in the temperature range 700-1250 degrees C for durations from 0 s to 24 h. From this we determined activation energies for Li and B diffusion of 57 +/- 4 kJ/mol and 152 +/- 15 kJ/mol with pre-exponential factors of 1.53 x 10(-7) m(2)/s and 3.80 x 10(-8) m(2)/s, respectively.
Lithium isotopic fractionation during diffusion gave beta values between 0.15 and 0.20, whereas B showed no clear isotopic fractionation.
Our Li diffusivities and isotopic fractionation results differ somewhat from earlier published values, but overall confirm that Li diffusivity increases with water content. Our results on B diffusion show that similarly to Li, B mobility increases in the presence of water.
By applying the Eyring relation, we confirm that B diffusivity is limited by viscous flow in silicate melts.
Our results on Li and B diffusion present a new tool for understanding degassing-related processes, offering a potential geospeedometer to measure volcanic ascent rates.
Geochemical homogeneity in shale is often assumed when tracing subsurface fluids and characterizing sedimentary basins. This study presents measurements of the bulk gas composition, stable isotopes, and noble gas volume fraction and isotopes for shale gas samples collected from gas wells in the Wufeng-Longmaxi Shale, the southern Sichuan Basin, China. The dryness [C-1 /(C-2 + C-3)] ranging from 166.3 to 251.2, combined with delta C-13(1) and delta DC1 that vary from -28.8 to -27.3 parts per thousand and - 153 to -145 parts per thousand, respectively, point to a late mature thermogenic origin of hydrocarbon gas. He-3/He-4 ratios of gas samples are around 0.01 times the air value suggesting dominantly crust-derived He. Ne-21/Ne-22 and Ar-40/Ar-36 ratios of many gas samples are higher than the corresponding air values indicating the mixing of crustal and atmospheric noble gases. Multiple dichotomous patterns are observed in noble gas signatures of forelimb and backlimb samples, and depression and crest samples. Ne-20/Ne-22 ratios of some crest samples are higher than that of depression samples in the backlimb, pointing to the presence of diffusion-driven fractionation that is likely caused by the long-distance migration from depression to crest. Elemental ratios of air-derived noble gas isotopes - Ne-22/Ar-36, Kr-84/Ar-36, and Xe-132/Ar-36 are compared to the recharge water values, suggesting the interactions of oil, gas, and water phases in the shale over geologic time. Forelimb samples generally display older ages than backlimb samples, indicating a larger flux of external radiogenic He-4 due to the higher density of deep faults in the forelimb area caused by the basementinvolved deformation. The basement-involved deformation also causes pore collapse especially in the forelimb leading to a lower porosity that results in a more pristine noble gas signature in the forelimb due to the reduced impact of younger recharge water.
Transport properties of potential host rocks for nuclear waste disposal are typically determined in laboratory or in-situ experiments under geochemically controlled and constant conditions. Such a homogeneous assumption is no longer applicable on the host rock scale as can be seen from the pore water profiles of the potential host rock Opalinus Clay at Mont Terri (Switzerland). The embedding aquifers are the hydro-geological boundaries, that established gradients in the 210 m thick low permeable section through diffusive exchange over millions of years. Present-day pore water profiles were confirmed by a data-driven as well as by a conceptual scenario. Based on the modelled profiles, the influence of the geochemical gradient on uranium migration was quantified by comparing the distances after one million years with results of common homogeneous models. Considering the heterogeneous system, uranium migrated up to 24 m farther through the formation depending on the source term position within the gradient and on the partial pressure of carbon dioxide pCO2 of the system. Migration lengths were almost equal for single- and multicomponent diffusion. Differences can predominantly be attributed to changes in the sorption capacity, whereby pCO2 governs how strong uranium migration is affected by the geochemical gradient. Thus, the governing parameters for uranium migration in the Opalinus Clay can be ordered in descending priority: pCO2, geochemical gradients, mineralogical heterogeneity.</p>