Unusual upper mantle beneath Guaniamo, Guyana shield, Venezuela: Evidence from diamond inclusions

Abstract

The geochemistry of mineral inclusions in diamond is an important source of information about the composition of the continental lithospheric mantle at depths exceeding 120–150 km. At these depths, two main types of geochemical environment support diamond formation; they are ultramafic (or peridotitic) (U-type) and eclogitic (E-type) environments as shown by minerals that occur as inclusions in diamonds. In primary diamond-bearing kimberlite or lamproite rock the ratio of diamonds from these two geochemical environments varies widely between localities. The U-type environment dominates for the majority of diamond occurrences worldwide, whereas the E-type environment dominates for a very limited number of localities in South Africa, North America, and Australia. The present study shows that an uncommonly high percentage (99.4%) of E-type diamonds with extremely variable inclusion assemblages is found in kimberlites in the northwestern part of the Guyana shield at Guaniamo, Venezuela. These variations range from previously unknown silica-undersaturated corundum eclogite to abundant silica-rich coesite eclogite assemblages representing 22% of all E-type diamonds, some of which contain syngenetic ilmenite and magnetite. The compositional variations of garnet and omphacite inclusions are extremely broad. The wide variability of the eclogitic source in which diamonds formed beneath the Guyana shield indicates a broadly basaltic chemistry in this environment, which may represent ancient subducted oceanic crust. Such specific features have not been reported for any other diamond occurrence and reflect an unusual composition of the deep lithospheric mantle in this area.