DENSITIES AND POROSITIES IN THE OCEANIC-CRUST AND THEIR VARIATIONS WITH DEPTH AND AGE

We have used drilling and downhole logging results, measured properties of core samples from the seafloor and from ophiolites, and seismic velocity structure models of the oceanic crust to constrain empirical (statistical) relationship between in situ densities, porosities, and seismic velocities and to estimate densities and porosities in the oceanic crust. Our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration than to its igneous structure. A comparison of reprocessed and corrected downhole logs from holes 395A and 418A indicates that in situ densities and fractional porosities differ by 0.2 and 0.07 Mg m-3, respectively. These differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust. Statistical relationship between in situ density, porosity and slowness are highly linear, and indicate that the density/slowness relation estimated from laboratory samples [ρb = (3.81±0.02)-(6.0±0.1)S] applies to in situ properties as well. We obtained a similar empirical relationship for porosities from the hole 418A log data [Φf = -(0.35±0.03)+(2.37±0.15) S; r2=0.84 S.E. = 0.015]. Applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities: layer 2, 2.62-2.69 Mg m-3, 0.10-0.12 and layer 3, 2.92-2.97 Mg m-3. The estimated average density of the oceanic crust is 2.86±0.03 Mg m-3. -from Authors