Applications of the SHRIMP I ion microprobe to the understanding of processes and timing of diamond formation

Abstract

The ion microprobe is a highly versatile analytical instrument capable of a wide variety of in situ isotopic and trace element studies of solid materials, with a spatial resolution of 15 to 25 mm, consuming only 2 to 5 ng of sample per determination. In a study combining uranium-lead isotope dating, rare earth element fingerprinting, and sulfur plus lead isotope tracing, it has been possible to address critical questions concerning the timing of, sources of materials for, and processes involved in diamond formation in African kimberlites. The zircon uranium-lead data indicate that kimberlites may host two generations of minerals: one crystallizing at the time of kimberlite emplacement and the other significantly older, perhaps relict from a previous kimberlite magma-generating event. Rare earth element analyses of the same zircons suggest that the two generations of zircons are different from each other and both are quite distinct from crustal zircons. Since zircon is not a common phase in the mantle, it may be that some mantle metasomatic event is required to form these minerals and may actually trigger kimberlite generation. The sulfur isotope data from sulfide inclusions in diamonds suggest that two different reservoirs are being sampled during diamond growth: one with essentially pristine mantle characteristics and another with variability more reminiscent of a low-temperature or more oxidizing, possibly crustal, environment. Paragenetic data, based on nickel contents of the sulfides, indicate that it is the eclogitic diamonds which record the crustal signatures. Lead isotope variability parallels that found in sulfur, with perdiotitic sulfides having apparently normal mantle lead and eclogitic sulfides having much more radiogenic lead which in some cases requires very high U/Pb ratio growth environments, more consistent with a crustal source. The data from this study and others, including the carbon isotope heterogeneity of eclogitic diamonds, can be reconciled with a single model in which introduction of crustal material into the mantle triggers both generation of the kimberlite and diamond growth. This makes plate tectonics of paramount importance in locating economic reserves of diamonds.