Mantle processes during ocean formation: Petrologic records in peridotites from the Alpine-Apennine ophiolites

Abstract

Mantle peridotites were early, exposed at the sea-floor of the Jurassic Tethys derived from the subcontinental mantle of the Europe-Adria system. During continental rifting and oceanic spreading, these lithospheric peridotites were percolated via diffuse reactive porous flow by melt fractions produced by near-fractional melting of the upwelling asthenosphere. Ascending melts interacted with the lower lithosphere, dissolving pyroxenes and precipitating olivine, and crystallized at shallower levels in the mantle column causing melt impregnation. Subsequent focused porous flow formed replacive dunite channels, cutting the impregnated peridotites, which were conduits for upward migration of MORB-type liquids. Melt migration produced depletion/refertilization and significant heating of the percolated/impregnated mantle, i.e the thermochemical erosion of the lithosphere. Impregnated and thermally modified lithospheric mantle was cooled by conductive heat loss during progressive lithosphere thinning and was intruded by MORB magmas, which formed Mg-rich and Fe-rich gabbroic dykes and bodies. Alpine-Apennine ophiolitic peridotites record the deep-seated migration of melts, which changed their compositions and dynamics during the rift evolution. The thermochemical erosion of the lithospheric mantle by the ascending asthenospheric melts, which induces significant compositional and rheological changes in the lower lithosphere, is a major process in the evolution of the continent-ocean transition towards a slow spreading oceanic system.