Subsurface soil and lithology alteration shaping degassing at Krafla caldera, Iceland

Alteration by rock-fluid interaction can significantly impact rock and soil properties influencing fluid flow within hydrothermal aquifers, and shaping surface features and degassing in geothermal settings over time. Hence, the understanding of the spatial distribution of thermal manifestations in correlation with the geological setting and alteration type remains essential for unravelling the processes controlling the transport of fluids from within the hydrothermal aquifers to the surface.

Previous studies focused on the effect of alteration within hydrothermal aquifers on a scale of hundreds of meters. In contrast, limited research has explored the influence of subsurface lithologies – including their intrinsic and altered permeability – and their spatial distribution on degassing activity. This study examines subsoil portions across the active geothermal fields of Krafla caldera in Iceland, to explore the relationship between various soils/lithologies and primary degassing areas. In particular, we investigate how hydrothermal alteration influences the petrophysical characteristics of these lithologies, thereby modulating surface-level fluid circulation.

In two field campaigns carried out in 2022 and 2023, we assessed the in situ petrophysical properties of over 200 samples across 22 sites in the Víti and Hveragil regions. Moreover, we conducted subsoil diffuse CO₂ flux measurements for specific profiles. Field permeability ranged from 10^-11 to 10^-16 m², with CO₂ fluxes varying between 1.25 to 2628.33 g/m² day. Additionally, we examined the grain size distribution and the componentry of selected subsoil layers.

Our findings delineate the presence of both active (thermal) and inactive (non-thermal) regions, depicting the variable impact of hydrothermal alteration on the subsoil properties. In active geothermal sites, the dominance of mineral dissolution/replacement facilitates the formation of preferential pathways for fluid flow. Within inactive areas, mineral cementation of pores and fractures appears to act as barriers to fluid movement. These outcomes offer crucial insights for comprehending and quantifying the effect of hydrothermal alteration on fluid dynamics, shedding light on the progression of surficial manifestations and degassing patterns within active geothermal areas.