Isotopic Gradients Across Fluid–Mineral Boundaries

Abstract

Isotopic Gradients Across Fluid-Mineral Boundaries 357 composition is likely governed by the partitioning associated with a reaction (Lemarchand et al. 2004; Fantle and DePaolo 2007; DePaolo 2011). The accumulation of newly formed solids is also anticipated to vary with the local transport regime, which in turn exerts an infl uence on the isotopic composition of the fl uid-solid interface. This relationship between the dominant mechanisms of transport and fractionation suggests that over a representative volume of porous media a variety of fractionation factors may be observed. For example a fractionation factor characteristic of diffusive transport between the solid-fl uid surface and the well-mixed fl uid, or a fractionation factor associated with the difference in isotope composition between the fl uid and solid at each reactive surface. The continuum-scale or observable fractionation factor over the timescale of the reaction (e.g., seconds to years) is then subject to the location and volume of material sampled. Over much longer periods of time, processes like recrystallization and solid-state diffusion homogenize spatial zoning within the solid. This implies that transient isotopic partitioning should be observable over signifi cant periods of time, and that the observed macroscopic fractionation factor is potentially (1) a combination of multiple, distinct mechanisms and (2) variable as a result of parameters such as saturation state and fl ow rate.

Isotopic Evidence for Reductive Immobilization of Uranium Across a Roll-Front Mineral Deposit

“…Our expectation is that the U ore body represents a redox boundary and that the groundwater U concentration, δ 238 U and ( 234 U/ 238 U) will all be affected by reactions such as reduction in the ore zone region. The up-gradient groundwater samples have moderate U concentrations 11–16 μg/L and high ( 234 U/ 238 U) due to the combined effects of α- recoil and the weathering of silicate minerals. ,− The δ 238 U is depleted compared to most known groundwater and silicate minerals. ,,, The up-gradient groundwater ( 234 U/ 238 U) and δ 238 U are tightly clustered varying from 5.12 to 5.61 and −0.88‰ to −1.08‰ respectively, with no discernible correlation with U concentrations.…”

Nernst‐Planck‐based Description of Transport, Coulombic Interactions, and Geochemical Reactions in Porous Media: Modeling Approach and Benchmark Experiments

“…Thus, NP-Phreeqc represents a unique tool to accurately simulate multicomponent ionic transport (from 1-D to 3-D) and to correctly interpret concentration measurements in both homogeneous and heterogeneous domains. The flexibility of the coupling offers the opportunity to extend the proposed multidimensional Nernst-Planck-based approach to many relevant reactive transport processes in groundwater, including mineral precipitation and dissolution, displacement of radionuclides in flow-through systems, transport of tracer isotopes, mobilization of heavy metals and metalloids, propagation of acidic plumes, and degradation reactions (e.g., Druhan et al, 2015;Fakhreddine et al, 2016;Glaus et al 2013;Haberer et al, 2015;Kjøller et al, 2004;Maher et al, 2006;McNeece & Hesse, 2016, 2017Molins et al, 2012;Prigiobbe et al, 2012;Prigiobbe & Bryant, 2014). Besides applications at the continuum scale, further developments are envisioned also at the pore scale where the proposed COMSOL-PhreeqcRM coupling could be extended to different physics and governing equations such as Stokes flow and multidimensional multicomponent ionic diffusion.…”

Kinetic Fractionation of Non-Traditional Stable Isotopes by Diffusion and Crystal Growth Reactions

“…Instead, the focus of this chapter is on the measurement and interpretation of mass dependent diffusivities and reactivities, and how these parameters are implemented in models of crystal growth within a fluid phase. There are, of course, processes aside from crystal growth that give rise to KIEs among non-traditional isotopes, such as evaporation (Young et al 2002;Knight et al 2009;Richter et al 2009a), vapor exsolution (Aubaud et al 2004), thermal diffusion (Richter et al 2009a(Richter et al , 2014bHuang et al 2010;Dominguez et al 2011), mineral dissolution (e.g., Brantley et al 2004;Wall et al 2011;Pearce et al 2012;Druhan et al 2015), and various biological processes (e.g., Zhu et al 2002;Weiss et al 2008;Nielsen et al 2012;Robinson et al 2014). Coverage of these topics, and how they give rise to KIEs, can be found throughout this volume and in the recent literature.…”