The abundance of ultramafic rocks in Atlantic Ocean crust

• Published in: Geophysical Journal International
• Main Author: Carlson, R. L.
• Format: Text
• Language: English
• Published: Oxford University Press 2001
• Subjects: Articles; North Atlantic
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/144/1/37; https://doi.org/10.1046/j.0956-540X.2000.01280.x

Combined P- and S- wave velocity profiles have been widely used to assess the composition of the oceanic crust, but no quantitative estimates of the abundances of mafic and ultramafic rocks which are compatible with the seismic structure of the crust have been made. In this study, a Voigt–Reuss–Hill (VRH) average model based on the average elastic properties of partially serpentinized peridotites and oceanic diabases and gabbros is used to make quantitative estimates of lithological composition from the seismic structure of the crust. The VRH model is applied to several expanding spread profiles from the north Atlantic, in which P- and S- wave velocities are independently constrained. A purely gabbroic composition is sufficient to explain the seismic structure in most cases, but the lower oceanic crust could also be composed of interlayered gabbros and ultramafic rocks, and the average ultramafic content of the crust could be as high 13 per cent. In that case, the magmatic crust would be about 1 km thinner than the seismic crust. A small volume of serpentine (< 5 per cent) could be distributed through the crust without being detected by seismic methods. Even a 1 per cent volume fraction of serpentine would be a significant source of water delivered to the mantle by subduction of the crust. The seismic properties of the upper mantle are consistent with the properties of dunite, as opposed to peridotite. The low-velocity zone in the crust at NAT ESP 5 could be composed of amphibole-rich gabbros, but the VRH model indicates that it is probably composed of a mixture of gabbro with partially serpentinized peridotite, having a total serpentine content of 10–20 per cent. In either case, the low-velocity zone probably represents a zone of hydrous alteration.