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

Abstract

[1] 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.

Expedition 301 synthesis: hydrogeologic studies

“…Logging data help to define the lithostratigraphy of Hole U1301B and establish whether or not conditions are as expected, in comparison to measurements made on core samples and borehole and core data collected from other basement holes (Bartetzko and Fisher, 2008;Tsuji and Iturrino, 2008). Logging data show that upper basement around Hole U1301B is highly layered at a scale of meters to tens of meters, similar to what is observed in other ocean crustal holes (e.g., Bartetzko et al, 2001;Jarrard et al, 2003;Pezard et al, 1992).…”

“…The deepest packer setting depth corresponds to an abrupt change in the character of basement geophysical logs. Above 470 mbsf (205 msb) the borehole diameter is highly irregular and there are intervals 10-50 m thick having very low bulk density, electrical resistivity, and P-wave velocity (Bartetzko and Fisher, 2008). Below ~470 mbsf (~200 msb), the borehole diameter is more consistent with the drill bit diameter, and zones of low bulk density are thinner and more widely spaced (Fig.…”

“…However, Hole U1301B bulk density and P-wave velocity core data fall below global trends--it is as if core samples from Hole U1301B came from much older seafloor, closer to 100 Ma based on measured values (Bartetzko and Fisher, 2008). Basaltic grain densities are also low, and formation factor values (an indication of pore tortuosity) are high.…”

“…Irregular hole conditions continue to ~470 mbsf (~200 msb), below which the borehole diameter is consistent with the drill bit diameter. Wireline measurements of bulk density, P-wave velocity, and resistivity tend to be less than or equal to values determined from Hole U1301B core samples (Bartetzko and Fisher, 2008).…”

“…A comparison of wireline data from a global compilation of crustal holes shows that bulk density, Pwave velocity, and electrical resistivity tend to in-crease with age, whereas core data decrease in bulk density and P-wave velocity with age (there are insufficient resistivity data from core samples for a meaningful evaluation of global age trends) (Bartetzko and Fisher, 2008). These changes in physical properties with crustal age are thought to result mainly from water-rock interaction during off-axis hydrothermal circulation.…”