Physical properties of young (3.5 Ma) oceanic crust from the eastern flank of the Juan de Fuca Ridge: Comparison of wireline and core measurements with global data

摘要

We compile and compare petrophysical data from the upper oceanic crust in Hole U1301B, on 3.5 Ma seafloor on the eastern flank of the Juan de Fuca Ridge. Measurements include core-scale (lab-based) and in situ (wireline) measurements of formation porosity, electrical resistivity, bulk density, P wave velocity, and gamma ray emission. A comparison between Hole U1301B data and those from other sites, ranging in age from 5.9 to 167 Ma, demonstrates some important differences. Hole U1301B samples tend to have lower bulk density values for a given porosity than do other young crustal sites. Hole U1301B samples also tend to have higher values of formation factor (the ratio of fluid conductivity to saturated rock conductivity), which may indicate development of tortuous microporosity that is more consistent with samples from older seafloor. Evaluation of global trends of petrophysical properties versus crustal age indicates that Hole U1301B in situ data is consistent with global trends, but core-scale P wave velocity and bulk density values fall below global trend values predicted for young crust. This last observation is likely related to basement topography and the regional exposure, until relatively recently, of basement outcrops around Site U1301. Until most of the exposed basement in this area was buried, hydrothermal circulation extracted much of the lithospheric heat, kept basement fluid temperatures low, and limited the rate and extent of alteration; a subsequent period of higher-temperature fluid circulation with more restricted and less oxidative conditions resulted in enhanced and pervasive alteration of crustal rocks.