Mantle peridotites from continental rifts to ocean basins to subduction zones

Abstract

Some key parameters for mantle-derived spinel peridotites from the North Atlantic, such as reconstructed primary modal and bulk composition, Al 2O 3 content of orthopyroxene (opx), Fo content of olivine, and 100 Cr/(Cr + Al) of spinel, have been compared with the same parameters for peridotite bodies from preoceanic rifts (Zabargad Island in the Red Sea), passive ocean margins (Iberian and Spitsbergen margins in the North Atlantic and southwest Australian margin in the Pacific-Antarctic Ocean) and subduction-related active margins (Mariana, Tonga and Puerto Rico trenches). Going from preoceanic rifts to passive margins to a mature ocean to subduction zones, peridotite bulk and opx Al 2O 3 content and modal clinopyroxene (cpx) content decrease, while olivine Fo, bulk and opx 100 Mg/(Mg + Fe), and spinel 100 Cr/(Cr + Al) increase. These systematic changes, compared with experimental data, suggest an increase of the degree of depletion of the peridotites going from a preoceanic rift to passive margins to mature oceans to subduction-related active margins. Data on Al 2O 3 partition between spinel and opx and application of Wells (1977) cpx-opx geothermometry suggest that peridotites from a preoceanic rift (Red Sea) and from passive margins equilibrated in the mantle under cooler conditions than oceanic peridotites. This is in agreement with the concept of a cooler thermal regime and slower asthenospheric upwelling beneath preoceanic, northern Red Sea-type rifts than beneath mid-ocean ridges. Subduction zone peridotites are more depleted than even the most refractory oceanic peridotites, though they show relatively low temperatures of equilibration, supporting the idea of water-induced melting of slivers of oceanic upper mantle near subduction zones.