Percolated mantle peridotites along the Romanche Fracture Zone (Equatorial Atlantic ocean)

Abstract

Pl-peridotites dredged along the Romanche Fracture Zone (FZ) are peculiar rocks. These peridotites derived from weakly deformed upper mantle rocks widespread percolated by melts with gabbroic composition. Mantle peridotite reacted with hot melt percolating through it and trapped melt which crystallised in pockets and in localised conduits at higher levels in the lithosphere. Reaction and crystallisation caused the peculiar microstructures, modal and chemical compositional variations, both in the bulk rocks and in the residual mineral phases. Electron and ion microprobe analyses suggest that percolating melts, characterised by different geochemical signatures, interacted, at deep levels in the lithosphere, with the Romanche peridotites mostly modifying the chemical compositions of Spl and Cpx. These new data show that impregnated mantle peridotites were emplaced all along the Romanche FZ, also in the western Romanche ridge transform intersection, were only fertile Pl-free peridotites were previously described. The cold equatorial upper mantle belt seems to influence strongly the dynamic of melting, melt migration, extraction and its final emplacement on the seafloor. Melts produced in this cold oceanic lithosphere are not able to segregate and join the overlying oceanic crust, but freeze and remain trapped at depth.