The diffusive exchange of dissolved material between fluid flowing in a fracture and the enclosing wallrocks (rock matrix diffusion) has been proposed as a mechanism by which radionuclides derived from a radioactive waste repository may be removed from groundwater and incorporated into the geosphere. To test the effectiveness of diffusive exchange in igneous and metamorphic rocks, we have carried out an investigation of veins formed at low temperatures (<100°C), comparing the oxygen isotopic composition of vein calcite with that of secondary calcite in the wallrocks. Two examples of veins from the Borrowdale Volcanic Group, Cumbria, and one from the Mountsorrel Granodiorite, Leicestershire, UK, have remarkably similar vein calcite compositions, ca. +20‰(SMOW) or greater, substantially heavier than the probable compositions of the host rocks, and these vein calcite compositions are inferred to reflect the infiltrating fluid and the temperature of vein formation. Calcites from the wallrocks are similar to those in veins, with little evidence for exchange with the wallrocks. The results support existing models for this type of vein which suggest low-temperature growth from formation brines originally linked to Permian or Triassic evaporites. The results are consistent with flow through fractures being attenuated through a damage zone adjacent to the fracture and provide no evidence of diffusional exchange with pore waters from wallrocks.

裂缝中流动的流体与围岩之间溶解物质的扩散交换（岩石基质扩散）已被提议作为一种机制，通过该机制可以将来自放射性废物储存库的放射性核素从地下水中去除并纳入地质圈中。为了测试火成岩和变质岩中扩散交换的有效性，我们对低温（<100°C）下形成的脉进行了研究，比较了脉状方解石与围岩中次生方解石的氧同位素组成。来自坎布里亚郡博罗代尔火山群的两个矿脉实例和来自英国莱斯特郡芒特索雷尔花岗闪长岩的矿脉实例具有非常相似的矿脉方解石成分，约 100%。+20‰ （SMOW）或更大，比主岩的可能成分重得多，并且推断这些脉状方解石成分反映了渗透流体和脉状形成的温度。围岩中的方解石与矿脉中的方解石相似，几乎没有证据表明它们与围岩中存在交换。研究结果支持了此类矿脉的现有模型，该模型表明最初与二叠纪或三叠纪蒸发岩有关的地层盐水的低温生长。结果与通过裂缝的流量通过裂缝附近的损伤区域衰减是一致的，并且没有提供与围岩孔隙水进行扩散交换的证据。