The genesis of the Dunite Transition Zone in the Oman Ophiolite: new insights from Major and Rare Earth Elements.

Abstract:

In most ophiolites, the contact between the mantle peridotites and the gabbroic cumulates from the lower oceanic crust is underlined by a horizon of dunite ranging in thickness from a few meters to a few hundred meters. The genesis of this dunitic transition zone (DTZ) can be attributed to different processes that are not necessarily mutually exclusive. They include olivine accumulation from high-Mg melts, reaction melting between a mantle peridotite and a melt under-saturated with pyroxene, destabilization of pyroxenes by high temperature fluid-rock reaction in conditions of water saturation. To precise and quantify the nature of these reactions and to determine their relative contribution has a major impact on our understanding of basalt genesis and of the chemical budgets at oceanic spreading centers. To reach this objective, we densely sampled the DTZ across several sections in the different massifs of the Oman ophiolites. The chemical composition of the dunites (WR major element and REE concentrations, microprobe data) presents a dramatic but not random variability both along single cross sections and from one cross section to the others. As an example, REE concentrations are a useful tool to investigate the petrogenetic processes behind this organization: in this study we have developed a chemical protocol in order to determine precisely (below 1% RSD) their concentrations, even for very low values (down to 0.1 ng.g^-1). REE concentrations in true dunites are intensely variables: 0.5<[La]<100 ng.g^-1; 0.1<[Eu]<4 ng.g^-1; 8<[Yb]<50 ng.g^-1. These variations are not random, LREE (La/Sm) ratios inversely correlate with HREE (Gd/Yb) ratios, which is a characteristic of melt/rock interaction. The various chemical tracers (major, minor and trace elements) indicate that the deep levels of the DTZ record a complex interaction history between mantle peridotites and various kinds of melts + fluids that migrated, stagnated and crystallized at different levels of this horizon, consistent with the “reactive filter” hypothesis. The shallow levels of the DTZ, however, present chemical trends becoming more and more consistent with an origin by fractional crystallization and olivine accumulation from primitive basalts as the gabbroic cumulates are approached.