Diamonds from the asthenosphere and transition zone: Remnants of subducted crustal material in the deep Earth's mantle

Abstract

The presence of majoritic garnet inclusions in diamonds from the Jagersfontein kimberlite in South Africa shows that the diamonds formed at depths of up to 550 km in source rocks of basaltic composition. Negative europium anomalies in all recovered majoritic garnets link these basaltic sources to subducted oceanic crust. A narrow range of isotopically light carbon compositions (δ13C: -17 to -24\permil) of the host diamonds suggests that diamond formation in the asthenosphere and transition zone is principally different to the formation of diamonds from the shallower lithosphere, which at Jagersfontein have a broad carbon isotopic range from -1 to -24\permil with a pronounced mode at -4\permil. The narrow range in carbon isotopic composition of diamonds from the asthenosphere and transition zone suggests that they directly reflect the isotopic signature of their carbon source. The isotopically light carbon composition of these diamonds is, therefore, consistent with derivation from organic matter within a subducting slab and most likely related to an early Mesozoic subduction event that transferred oceanic crust into the deep mantle beneath the Kaapvaal craton. Accumulation of subducted oceanic crust in the deep mantle may have also caused the late Cretaceous kimberlite volcanism on the Kaapvaal craton, which ultimately brought the deep diamonds from Jagersfontein to the Earth's surface.