| Popis: |
We couple two-dimensional micromechanical models for thermal cracking with fracture mechanics data for olivine to constrain the conditions where diffuse fluid flow could occur in the oceanic lithosphere. In addition, we ran controlled cooling rate experiments on hot-pressed olivine aggregates to test the micromechanical models of thermal cracking in peridotite and study the evolution of permeability due to thermal cracking. In our experiments, impermeable olivine aggregates, formed at elevated temperatures and pressures, are subsequently cooled at constant rates. In situ permeability measurements, made as the aggregate cooled, allow us to determine when an interconnected microcrack network develops. By varying grain size and confining pressure during the experiments, we were able to either enhance or suppress thermal cracking within the olivine samples. When the results of our experiments are coupled with micromechanical models, we estimate a polycrystalline olivine fracture toughness of approximately 0.6 MPa m 1/2 . We scaled the results of our experiments to the Earth using models that account for the influence of grain size, cooling rate, and confining pressure on the onset of thermal cracking in olivine aggregates. The depth extent of thermal cracking is estimated by coupling micromechanical models of stress intensity resulting from anisotropic thermal contraction with thermal models for upwelling mantle at oceanic spreading centers. Our analysis indicates that thermal cracking of peridotite is likely at depths less than 4 to 6 km beneath the seafloor. These predictions agree well with the depth of a transition from serpentinized to unaltered peridotite in the oceanic lithosphere determined from seismic observations. |
