Parrotin et al., 2025

The Abinky formation, located in the southern part of the Tim Mersoï Basin (Niger) consists of three distinct facies, each characterized by specific mineralogical and grain organisation, but all containing analcime (~ > 70 wt%), an ANA-type zeolite. Reduced facies contains Fe-chlorite, while oxidized facies includes hematite, and the ‘transition’ facies is intermediate. Due to the low permeability of these analcime-rich rocks, diffusion is expected to be the dominant transport mechanism. However, no data on transport properties in zeolite-rich formations have been reported, and the mobility of water and ionic solutes remains unknown. This study aims to characterize diffusion in the three facies of the Abinky formation, identifying key controls on water and ion transport prior to any anthropogenic perturbation. Through-diffusion experiments using water tracers (HDO and HTO) showed that water diffusion (effective diffusion coefficient from 1.3 to 2 × 10-11 m²/s) is primarily controlled by pore throat size rather than total porosity. A dual-porosity model was required, distinguishing a fast-transport network (open, low-tortuous zones) from a slow one (confined, tortuous zones). Despite variations in diffusion coefficients, all data are interpreted with capacity factors equal to total external porosity measured by water impregnation (excluding crystal water in zeolite micropores). This confirms that water behaves as an inert tracer and that micropores contribute negligibly to migration. For ionic tracers in reduced facies, 36Cl- is partially excluded from external pores (anionic exclusion) and slightly adsorbed with an extent of about 9 × 10-2 meq/100 g) that is a very small value compared to the cation exchange capacities of such rocks (up to ~40 meq/100 g for H+ and NH4+). In contrast, ²²Na⁺ exhibits strong adsorption due to isotopic exchange with 23Na+ in the analcime framework, leading to a diffusivity ~4 times higher than that of water. This dataset is a first step in constraining reactive transport models of water and solutes in zeolite-rich porous media under environmental conditions and prior to anthropogenic disturbances such as in-situ acid leaching.位于尼日尔蒂姆梅尔索伊盆地（Tim Mersoï Basin）南部的阿宾基地层（Abinky Formation）由三种截然不同的岩相组成，每种岩相均具备独特的矿物学特征与颗粒组构，但所有岩相均含有方沸石（analcime，质量占比约＞70 wt%），属于ANA型沸石（ANA-type zeolite）。还原岩相含有铁绿泥石（Fe-chlorite），氧化岩相则以赤铁矿（hematite）为特征，而“过渡”岩相的特征介于二者之间。由于这类富方沸石岩石的渗透率较低，扩散被认定为主导演运机制。然而目前尚无关于富沸石地层输运性质的公开报道，水与离子溶质的运移行为仍未明确。本研究旨在表征阿宾基地层三种岩相中的扩散过程，厘清人为扰动前控制水与离子运移的关键因素。 采用水示踪剂（HDO、HTO）开展的穿透扩散实验（through-diffusion experiments）结果显示，水的扩散过程（有效扩散系数介于1.3~2×10^-11 m²/s）主要受孔喉尺寸（pore throat size）调控，而非总孔隙度（total porosity）。研究需采用双孔隙度模型（dual-porosity model），将输运通道划分为快速输运网络（开放、低迂曲度区域）与慢速输运网络（封闭、高迂曲度区域）。尽管不同岩相的扩散系数存在差异，但所有实验数据均可通过“容量因子等于通过水浸渍法测得的总外部孔隙度（排除沸石微孔内的结晶水（crystal water））”这一规则进行解释。这一结果证实水可作为惰性示踪剂（inert tracer），且沸石微孔对溶质运移的贡献可忽略不计。针对还原岩相中的离子示踪剂（ionic tracers），36Cl-会部分被排斥在外部孔隙之外（阴离子排斥（anionic exclusion）），且仅发生微弱吸附（吸附量约为9×10^-2 meq/100 g），相较于此类岩石的阳离子交换容量（如H+与NH4+的阳离子交换容量可达约40 meq/100 g），该吸附量极低。与之相反，²²Na+会与方沸石骨架中的23Na+发生同位素交换（isotopic exchange），从而表现出强烈吸附，其扩散系数约为水的4倍。本数据集可为环境条件下、原位酸浸（in-situ acid leaching）等人为扰动发生前，富沸石多孔介质（porous media）中水与溶质的反应输运模型（reactive transport models）的约束提供初步依据。