Influence of in-situ temperatures and pressures on the permeability of hydrothermally altered basalts in the oceanic crust
Collaborative experiments at the RPMLab, with University of Bern scientists, have uncovered the relationship between the permeability of altered basalts at laboratory conditions and their permeability at formation pressures and temperatures below the seafloor.
When scientists pick up a basalt in an ophiolite, or dredge or drill one from the seafloor, we can measure its ability to transmit fluids that would have circulated in the oceanic crust, but often only at room temperatures and pressures. The permeability of oceanic basalts is a key parameter for thermal-hydraulic-chemical models that describe and improve our understanding of processes at and below the seafloor. However, data have been lacking on the real permeability properties of deeply buried altered basalts, under 1–2 km of seawater and up to 2 km of rock, and how permeability properties vary with pressure and temperature.
In this study published in Tectonophysics, (external page link) the researchers demonstrates relative decreases in permeability of ~45–80% due to temperatures up to 450 °C and effective pressures up to 50 MPa. The study establishes a new temperature-permeability calibration that can be used to correct permeability measurements made at room temperature to in-situ conditions for hydrothermally altered basalts (“spilites” and “epidosites”) abundant in ophiolites. This also provides a means to understand permeability changes in the P–T realm below the seafloor in large scale numerical modelling.
At the same time, the relative changes in permeability are minor compared to heterogeneity of the volcanic oceanic crust. This means that future studies of altered basalts can focus on quantifying the heterogenous nature of permeability in the volcanic oceanic crust.