The Petrologic Nature of the Lower Oceanic Crust and Upper Mantle

Abstract

The compositions of the lower oceanic crust and upper mantle are investigated using data from high pressure experiments of compressional and shear wave velocities in rocks. Four compositional models for the lower oceanic crust are considered: 1) partially serpentinized peridotite, 2) gabbro, 3) metabasalt and metagabbro and 4) an ophiolite model consisting of metamorphosed sheeted dikes overlying late differentiates and cumulate gabbros. Comparisons of compressional wave velocities (Vp) from dredged oceanic rocks with layer 3 refraction velocities show that perido-tites 30 to 40% serpentinized, unaltered gabbro, metagabbro and metabasalt all have velocities similar to observed lower crustal velocities. Thus compressional wave velocity measurements alone will not distinguish between the various crustal models. Although only a limited amount of refraction data on shear wave velocities (Vs) are available, it appears that lower crustal Poisson’s ratios, calculated from Vp and Vg, are significantly lower than measured values in partially serpentinized peridotite and unaltered gabbro. In support of models 3 and 4 it is shown that Poisson’s ratios of metabasalt and metagabbro, on the other hand, agree well with seismic data from the upper portion layer 3. The low Poisson’s ratios reported for the lower crust of Iceland (∼ 0.27) suggest that metabasalt and metagabbro are abundant constituents of layer 3. The 74 km/sec layer, often found between layer 3 and oceanic upper mantle in the Pacific Ocean, is interpreted as most likely being composed of peridotite, 10 to 20% serpentinized, or feldspathic peridotite. Compressional and shear wave velocities in eclogite and peridotite at appropriate pressures are similar to oceanic upper mantle velocities. An upper mantle of peridotite or dunite composition is favored, however, in regions where strong upper mantle anisotropy is observed.