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 crest, with the following principal results. Our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crest 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 'l) occurs within the extmsive pile; the top of layer 2c (6.2 + 0.2 km s 'l) 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 crest. 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 [•f = -(0.35 + 0.03) + (2.37 ñ 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. 'l. The corresponding rates of change of density and porosity are 0.016 ñ 0.009 Mg m '3 m.y. 'l and -0.005 + 0.003 m.y .'l, respectively. Paper number 89JB03583. 0148-227/90/89JB-03583505.00 large-scale porosity of oceanic layer 2 [e.g., Hyndman and Drury, 1976]. In situ densities and porosities can be measured only by downhole logging. Unfor...

[1] Water carried into subduction zones with the downgoing plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge, triggers partial melting and affects subduction zone seismicity. Partially serpentinized peridotite may be a significant reservoir for water in the subducted plate, the mantle wedge and the overriding plate. Here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic P-wave velocities. In partially-serpentinized ultramafic rocks, a 1% decrease in P-wave velocity corresponds to a 2.4% increase in serpentine content, and a 0.3% (0.18 moles/m 3 ) increase in H 2 O content (up to a maximum of 13%). Where there is evidence of serpentinization, mantle serpentine content is typically $15%, corresponding to 4 -5 wt% H 2 O (6 -10 moles/m 3 ).

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[1] Mounting evidence from Holes 504B and 1256D suggests that porosity is the principal factor affecting velocities in the upper oceanic crust. Spheroidal inclusion and asperity compression models based on reprocessed sonic velocity logs and apparent fractional porosities estimated from deep resistivity logs reveal how both porosity and the geometry of the pore space affect seismic velocities in Layer 2. Models that best match the data indicate the following: First, there are three populations of cracks in Hole 504B; most of the transition zone and the dike section are populated by low concentrations of weak cracks, while the upper part of the transition zone and the extrusive pile contain a higher concentration of stiffer cracks. Similarly, the deepest part of the dike section in Hole 1256D can be modeled by a dilute concentration of thin or weak cracks, while the overlying dikes, the transition zone, and the extrusive pile contain a higher concentration of stiffer cracks. Second, in a striking confirmation that porosity controls velocities throughout Layer 2, 90% or more of the variance of sonic velocities logged at both sites is explained by porosity in these models, while third, the effect of pressure on the variation of sonic velocity is essentially negligible. Fourth, while there is no direct correlation between crack populations and igneous lithostratigraphy, some changes of crack population do correspond to metamorphic transitions, and fifth, velocities typical of Layer 3 are reached when the porosity falls to low values (0.2% in Hole 1256D and 0.6% in Hole 504B).

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[1] We have analyzed the relationship between P wave velocities, measured at pressures of 40, 100, and 200 MPa, and modal mineralogy in oceanic gabbro samples from Ocean Drilling Program Holes 735B, 894G, and 923A. At all pressures, increasing velocities correspond to increasing pyroxene contents and decreasing alteration (phyllosilicate and amphibole content) but indicate little or no variation of olivine content, in part because the olivine compositions are in the range Fo 65 to Fo 73 . A Voigt-Reuss-Hill inverse model reveals that the effective bulk density and elastic moduli of olivine in the gabbros are low, even relative to Fo 73 , possibly because the olivine grains contain ubiquitous networks of cracks. Even if the cracks are not present in situ, on average, gabbros with velocities typical of seismic layer 3 (6.7-7.0 km s À1 ) contain 5-15% alteration products, including 5-15% amphibole and 0-5% phyllosilicates. These results suggest that the lower crust is, on average, slightly to moderately altered.

Previous seismic studies suggest that hydrothermal processes are active only within young oceanic crust (<10–16 Ma). However, differences between measured and predicted heat flow at the ocean floor indicate that hydrothermal fluids may be transporting heat advectively in crust up to ages of 65 Ma. We report on seismic velocities of 0–71 Ma slow to intermediate spreading rate upper crust in the western South Atlantic. Thirteen high‐resolution 2‐D velocity models were built using traveltime tomography on downward continued streamer data acquired during the Crustal Reflectivity Experiment Southern Transect. In the Crustal Reflectivity Experiment Southern Transect area, velocities at the top of seismic layer 2A increase rapidly from ~2.4 km/s at 0 Ma to ~4.2 km/s at 6 Ma and then undergo a more gradual increase to ~4.9 km/s at 58 Ma. These new results resolve the long‐standing debate about the duration of interaction between ocean crust and seawater, providing seismic evidence for hydrothermal circulation continuing to crustal ages predicted by heat flow studies. Seismic layer 2B does not show a systematic off‐axis velocity trend but has an average value of 5.15 km/s. We interpret this result to indicate that the hydrothermal system becomes too shallow to affect the physical properties of layer 2B shortly after crustal accretion. Upper crustal heterogeneity in ridge‐parallel profile orientation is more pronounced for crust accreted at slow spreading rates, compared to intermediate rates. This result is consistent with shorter magmatic segments at slower spreading rates, increasing the frequency of tectonic and magmatic accretion alternately occurring along the ridge.