Origin of the Oceanic Lithosphere

Abstract

In a global examination of mid-ocean ridge basalt (MORB) glass compositions, we find that Na8–Fe8–depth variations do not support modeling of MORBs as aggregates of melt compositions generated over a large range of temperature and pressure. However, the Na8–Fe8 variations are consistent with the compositional systematics of solidus melts in the plagioclase–spinel lherzolite transition in the CaO–MgO–Al2O3–SiO2–Na2O–FeO (CMASNF) system. For natural compositions, the P–T range for melt extraction is estimated to be ∼1·2–1·5 GPa and ∼1250–1280°C. This P–T range is a close match with the maximum P–T conditions for explosive pressure-release vaporization of carbonate-bearing melts. It is proposed that fracturing of the lithosphere induces explosive formation and escape of CO2 vapor. This provides the vehicle for extraction of MORBs at a relatively uniform T and P. The upper portion of the CO2-bearing and slightly melted seismic low-velocity zone flows toward the ridge, rises at the ridge axis to lower-lithosphere depths, melts much more extensively during this rise, and releases MORB melts to the surface driven by explosively escaping CO2 vapor. The residue and overlying crust produced by this melting then migrate away from the ridge axis as new oceanic lithosphere. The entire process of oceanic lithosphere creation involves only the upper ∼140 km. When lithospheric stresses shift fracture formation to other localities, escape of CO2 ceases, the vehicle for transporting melt to the surface disappears, and ridges die. Inverse correlations of Na8 vs Fe8 for MORB glasses are explained by mantle heterogeneity, and positive variations superimposed on the inverse variations are consistent with progressive extraction of melts from short, ascending melting columns. The uniformly low temperatures of MORB extraction are not consistent with the existence of hot plumes on or close to ocean ridges. In this modeling, the southern Atlantic mantle from Bouvet to about 26°N is relatively homogeneous, whereas the Atlantic mantle north of about 26°N shows significant long-range heterogeneity. The mantle between the Charlie Gibbs and Jan Mayen fracture zones is strongly enriched in FeO/MgO, perhaps by a trapped fragment of basaltic crust. Iceland is explained as the product of this enrichment, not a hot plume. The East Pacific Rise, Galapagos Ridge, Gorda Ridge, and Juan de Fuca Ridge sample mantle that is heterogeneous over short distances. The mantle beneath the Red Sea is enriched in FeO/MgO relative to the mantle beneath the northern Indian Ocean.