The oxygen isotope composition of mantle eclogites as a proxy of their origin and evolution: A review

In the past decade the oxygen isotope composition in rock-forming minerals of mantle eclogites (δ18O = 2-12‰) has been actively discussed as a most reliable proxy of their origin and evolution. In the present study, the possibility of using the oxygen isotope composition as a proxy for the origin of mantle eclogites is carefully tested, and conclusions regarding its application limits are drawn. Contributions of all processes leading to variations in the isotope composition from protolith (oceanic crust) formation to ascent of eclogites in kimberlite to the surface are discussed, including hydrothermal alteration by sea water; fractionation of oxygen isotopes upon metamorphism, dehydration and the removal of volatile components in subduction zone; metasomatism in subduction zone; partial melting; mantle metasomatism; and variation in the oxygen isotope composition upon the exhumation of mantle xenoliths by kimberlitic magma. The total contribution of all processes (excluding mantle metasomatism) typically does not exceed a deviation of ±3.5‰ from the initial δ18O value established in a protolith. Variations of δ18O in mantle eclogites (2-12‰; and <3% out of all samples display δ18O over 8‰), which have inferred basaltic and cumulate (gabbroic) protoliths, do not fully match those in the oceanic crust (0-15‰). This limited overlap could be attributed primarily to the small initial volume of protoliths with δ18O > 8‰, which subducts into the mantle (up to 5%), and the preferential erosion and partial melting of the uppermost layers of the oceanic crust. The statistical data show that eclogite garnet mainly preserves more ancient oxygen isotope signatures (including the initial δ18O of protolith) than clinopyroxene. Garnets with δ18O of <4.5‰ typical of the cumulate portion of oceanic crust, occur mainly in mantle eclogites from the Kaapvaal Craton (22.3% of finds, a total of 157 samples). For all other cratons, the percentage of garnet with δ18O < 4.5‰ is not >1.5% (a total of 451 samples). Oxygen isotope composition in a considerable portion of mantle eclogites that originated from the oceanic gabbro based on their chemical composition (δ18O < 6‰) was re-equilibrated, and they acquired “basalt” isotopic signatures (δ18O > 6‰).