Ion microprobe Pb–Pb dating of carbonado, polycrystalline diamond

Abstract

We report in situ ion microprobe Pb–Pb dating of quartz, rutile and clay mineral inclusions and matrix part of polycrystalline diamonds, ‘carbonado’ derived from a Proterozoic conglomerate in diamond-bearing deposit of Minas Gerais in Brazil together with U–Pb ages of zircons also extracted from the deposit. Thirty four spots on seven quartz grains located in the cavity of the GM01 carbonado yield 207Pb/ 206Pb isochron age of 3247±800 Ma (2 σ; MSWD=0.75) and six spots on three rutile inclusions give the age of 3916±1300 Ma (2 σ; MSWD=0.092). Seven spots on three clay mineral inclusions of the DO3 carbonado show common lead compositions while four spots on matrix define 207Pb/ 206Pb isochron age of 3811±1800 Ma (2 σ; MSWD=0.012). Seventy six spots on seventy zircon grains derived from the same conglomerate show three major U–Pb concordant ages of 2151±17 Ma, 2692±20 Ma and 2931±22 Ma with two concordant zircon outliers of 1726±58 Ma and 3599±12 Ma. The youngest age is consistent with the formation age of 1750–1700 Ma in the Proterozoic conglomerate estimated by other workers. The 3.6 Ga zircon is the oldest record in the South American platform. It may be detrital in origin and reflect the remnant of an Archean craton. The Pb–Pb age of quartz and rutile inclusions in GM01 carbonado and that of DO3 carbonado matrix are 3315±720 Ma and 3811±1800 Ma. The matrix age is consistent with Pb–Pb age of 2.6–3.8 Ga of Central African carbonados in the literature, suggesting a close genetic relationship between Brazilian and African carbonados.