A crustal mineral in a mantle diamond

RECENT studies of diamonds, inclusions in diamonds, and rock fragments from the Earth's mantle have shown the presence of minerals and rocks with a chemical composition that suggests a previous origin in the crust; presumably, this crustal material was transported into the mantle at a subduction zone. The chemical evidence of previous residence of mantle materials in the crust has come principally from determinations of the proportions of the isotopes of oxygen(1,2), sulphur(3,4), lead(4,5) and carbon(6), which typically show a much wider range of composition in the crust than in material of purely mantle origin. Here we report a less subtle crustal signature in a mantle-derived diamond: an inclusion of the mineral staurolite. Staurolite has never previously been found in mantle diamonds or rocks, but is a common mineral in metamorphic rocks in the crust. The characteristics of the staurolite suggest not only the probability that it formed in rocks with a crustal bulk composition, but also the possibility that it formed by metamorphism in the crust or in a subduction zone, before being incorporated in the mantle and, eventually, in a growing diamond.