Phase equilibria and elastic properties of a pyrolite model for the oceanic upper mantle

Abstract

The upper-mantle source regions of basaltic magmas in oceanic regions contain both H 2O and CO 2. If the water content of the upper-mantle peridotite is (<0.4%) approx., then its solidus has a distinctive P,T character such that the geotherm for older oceanic regions will enter a zone of incipient (<2%) melting — the low-velocity zone (LVZ) — at depths of 85–95 km. This LVZ is overlain by a lithosphere of subsolidus amphibole-bearing peridotite in which there is a density increase at ~55 km due to the first appearance of garnet. An alternative model in which the LVZ is attributed to the presence of CO 2 fluid phase bubbles is incompatible with experimental data showing high solubility of CO 2 in basaltic magmas at the P,T conditions of the LVZ. The LVZ contains a small melt fraction as an intergranular film (aspect ratio <10 −2); this melt is of olivine melilitite (CO 2, H 2O present) or olivine nephelinite (H 2O only present) character and is interstitial to olivine > orthopyroxene > garnet > clinopyroxene mineralogy. Temperatures at the top of the LVZ are in the range 1000–1150°C. The lithosphere thickens with age and distance from the mid-oceanic ridges, reaching a stable configuration at a thickness of 85–95 km for t > 80 m. y. With increasing age of the oceanic crust, the velocities in the lithosphere increase, the LVZ becomes thinner, and the velocity contrast between the lithosphere and the LVZ decreases. The pyrolite petrological model and its velocity profile satisfactorily account for most of the geophysical data for various age provinces in oceanic regions.