| Abstract: |
Mechanical properties of rocks at depth control the transport behavior of very high temperature hydrothermal reservoirs. In particular, the brittle to ductile transition in rocks may strongly influence their permeability and the maximum depth and temperature where hydrothermal fluids may circulate. In order to characterize these properties in the context of Icelandic crust, we conducted triaxial compression experiments to investigate the effects of pressure, temperature and strain rate on the rheology of basaltic rocks. The tests were carried out at temperature from 400 to 950°C, confining pressure (Pconf) from 100 to 300 MPa, pore pressure (Pp) from 0 to 50 MPa and strain rate (έ) from 10-6 to 10×10−4 s−1. Mechanical and micro-structural observations at a constant strain rate of 1 × 10− 5 s− 1 and a confining pressure of 100 MPa and 300 MPa indicate that the rocks are brittle and dilatant up to 700 to 800°C. At higher temperatures and effective pressures the deformation mode becomes ductile, in the sense that the deformation is less-localized and that no shear rupture plane develops. No single constitutive law is accepted as definitive of the mechanical behavior of rocks within the semi-brittle regime, but if one assumes that the mechanical behavior can be characterized as Mohr-Coulomb in the brittle field and as a steady-state, power law in the ductile field, then the strength at geological strain rates can be estimated. Such an extrapolation suggests that the brittle to ductile transition will strongly depends on the nature of starting material. Glassy basalts may undergo the transition at about 200°C, whereas the same transition might occur in non-glassy basalts at deeper conditions, i.e., temperatures higher than 600-700°C. The effect of the transition on the permeability of basaltic rocks has not been measured, but the brittle-ductile transition might be considered a limit for the depth at which supercritical hydrothermal fluids may circulate. For basaltic crust with a geotherm similar to that under Iceland, an estimate of depth of the brittle-ductile transition is about 6 to 8 km, results that are consistent with the lower limit of the Icelandic seismogenic zone which seems to be associated with a 750 ± 100 °C isothermal surface. |