Impact of fault activity on the chemistry of clay-rich rocks in deep geological repositories: Effects of mechanical comminution on Ar and ẟD isotope composition in clay minerals

Clay rocks are prime candidates for host rocks as stable geological formations with self-sealing properties and can provide a permanent barrier for repositories for radioactive waste [Yardley et al., 2016; Zwingmann et al., 2024]. We here report dry and wet clay deformation experiments using the Silurian Rochester Shale [RS] [Brett 1983, Zwingmann et al., 2019] and the Jurassic Opalinus Clay [OC] [Zwingmann et al. 2017] from the Mt. Terri laboratory, CH, which were selected based on different clay mineralogies. The RS consists mainly of illite (60%) and quartz (23%) whereas OC samples are characterized by higher kaolinite (35%), chlorite (11%), lower illite (30%) and quartz (13%). Our study investigates the generally unknown impact of physical deformation (shearing and grinding) on clay mineral composition and isotopic signatures. Laboratory deformation experiments were conducted to assess how mechanical comminution influences the compositional and isotopic signatures of the RS and OC samples. The study utilized different comminution techniques, applying both ball mill [BM] and McCrone mill [MC] for periods of 5 to 60 minutes between room temperature and 300 ºC. This multi-variable approach provides a comprehensive view on how physical and thermal processes influence isotopic signatures (Ar, ẟD) of RS and OC. In addition to dry experiments, pilot wet deformation experiments (5-30 min) involving both mill types were conducted on RS and OC clays with Bern and Fiji water selected because of their different isotopic hydrogen compositions.

Radiogenic Ar loss in the RS was time- and processing-dependent, ranging between ~ 15–56% in BM and ~ 32–80% in MC mill experiments. For the OC, the impact was reversed: BM induced significant loss (9–48%), whereas MC milling had minimal effect (2–14%).

For the RS, ẟD ‰ values decrease with dry MC milling from -61 (5 min), to -72 ‰ (30 min). Wet experiments using Bern water yield similar values (~-61‰) for all three milling times. The values for RS wet experiments with Fiji water range from -57 to -61 ‰ (5-30 min). Regarding the BM RS experiments, ẟD values vary more significantly from -66 to -81 ‰ (5-30 min).

For the dry OC MC experiments, a similar ẟD trend is observed ranging from -71 to -79 ‰ with increasing dry milling time. OC MC wet milling with Bern water shows relatively homogenous ẟD values ranging from -67 to -65 ‰, when using Fiji water from -67 to -70 ‰. The OC BM wet milling experiments with Bern water yield lower values ranging from -72, -to -87 ‰ (5-30 min).

Milling experiment data suggests that the mechanical interaction between hard framework minerals (quartz and feldspar) and the clay fraction drives the variation in Ar loss and affect hydrogen isotope compositions. Under water saturated conditions, the chemical impact of shearing is dimmed, suggesting that these chemical fingerprints may have the potential to indicate the relevance of a fault zone detected within the emplacement drifts of a DGR.