Densities and porosities in the oceanic crust and their variations with depth and age

Abstract

We have used drilling and downhole logging results (from Deep Sea Drilling Project holes 395A and 418A), measured properties of core samples from the seafloor and from ophiolites, and seismic velocity structure models of the oceanic crust to constrain empirical (statistical) relationships between in situ densities, porosities, and seismic velocities and to estimate densities and porosities in the oceanic crust, with the following principal results. 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 (or metamorphic grade) than to its igneous structure: the transition from layer 2a (3.7±0.4 km s−1) to 2b (5.2±0.4 km s−1) occurs within the extrusive pile; the top of layer 2c (6.2+0.2 km s−1) is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products; ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies. 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 Mg m−3 and 0.07, respectively. These differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust. Statistical relationships between in situ density, porosity and slowness are highly linear, and indicate that the density/slowness relation estimated from laboratory samples [pb = (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 [Фf = −(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. Densities and porosities estimated from a seismic crustal structure model for old Atlantic crust agree very well with the logging data from hole 418A, suggesting that in situ densities and porosities can be estimated from seismic velocities. High density and porosity gradients are characteristic of the upper oceanic crust; increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth. Based on the seismic properties of layer 2a, we estimate that the layer thins at an approximate rate 20 to 45 m m.y.−1 while the velocity increases by 0.040 km s−1 m.y.−1. The corresponding rates of change of density and porosity are 0.016 ± 0.009 Mg m−3 m.y.−1 and −0.005±0.003 m.y.−1, respectively.