Deep mantle diamonds from South Australia: A record of Pacific subduction at the Gondwanan margin

Abstract

Diamonds from Jurassic kimberlites at Eurelia, South Australia, contain coexisting inclusions of ferropericlase and MgSi-perovskite that provide evidence for their deep (>670 km) lower mantle origin. Eurelia diamonds formed from mixed carbon sources, likely including subducted carbonate, as indicated by a trend toward isotopically heavy carbon compositions (δ¹³C = 0‰) and low nitrogen concentrations (<100 ppm) in highly aggregated states. The discovery of lower mantle diamonds at Eurelia extends the area of known Mesozoic kimberlites carrying sublithospheric diamonds within continental fragments of Gondwana. The alignment of the kimberlite localities with the former Gondwana subduction margin and the presence of crustal signatures in the composition of the sublithospheric diamonds provide evidence that deeply subducted remnants of the proto-Pacific plate are the ultimate source of the diamonds. The kimberlite magmatism and the widespread emplacement of Jurassic to Early Cretaceous large igneous provinces in southern Gondwana are also attributed to this subduction process.