Résumé
Detachment faults in mid‐ocean ridge settings accommodate tectonic extension. These faults are potentially weakened by alteration through phyllosilicate‐ and amphibole‐mineral formation. Seven core samples taken from the Krafla and Reykjanes geothermal fields in Iceland allow for the study of the effect of mineral alteration on the frictional behavior of basic rocks in the context of high‐enthalpy geothermal fields and slow‐spreading mid‐ocean ridges. Gouge samples were prepared from the core samples and tested in a biaxial apparatus. In the core samples from Krafla, the presence of phyllosilicate minerals, which are generally stable at low (<400°C) temperatures, enables water to cause frictional weakening, increased stability, and reduced frictional healing. The core samples from Reykjanes, however, offer a unique opportunity to study the effect of high‐temperature alteration, as they are from a spreading center at 4,600 m depth and >600°C and have not been overprinted by low‐temperature alteration. Although these samples appear frictionally strong and more velocity‐weakening than the Krafla samples, they also exhibit negative frictional healing. This might be associated with a developing crystallographic‐preferred orientation of the amphibole minerals during inter‐seismic periods, potentially implying a new weakening mechanism that may operate along oceanic detachment faults at intermediate temperatures (approximately 400–600°C) in conditions too hot for serpentinization and too cold for the crystal plastic deformation of peridotite. This progressive frictional strength loss along detachment faults may influence detachment‐fault morphology, but further investigation is required to both conclusively link crystallographic‐preferred orientation to negative healing and extrapolate its relevance to crustal conditions.