A thermal study of the formation of oceanic crust

Abstract

The temperature distribution beneath an ocean ridge with a magma chamber solidifying to form the crustal layer 3 has been investigated by numerical methods. Assuming that water circulation rapidly cools layer 2 but does not penetrate deeper, it is found that a magma chamber forms provided that the half spreading rate exceeds about 0.45 cm y-1; for slower rates instantaneous dyke-like solidification occurs down to the Moho beneath the ridge axis. Numerical models show that the width of the magma chamber and the thickness of the dyke complex depend on half-spreading rate. The width is about 20 km for 3 cm y-1. If there is significant crystal settling to form cumulates, the chamber width is much reduced. The resulting heat flow pattern is as follows: an outer zone where latent heat effects are negligible; a middle zone about twice as wide as the magma chamber where latent heat significantly contributes to the heat loss; a narrow axial zone where quenching of magma to form layer 2 provides the main heat loss. The computations support Cann's petrological model and suggest that the depth of water penetration at the accretion boundary defines the layer 2-layer 3 boundary.