Diamond from recycled crustal carbon documented by coupled δ18O-δ13C measurements of diamonds and their inclusions

The origin of variability in delta C-13 values of cratonic diamonds is controversial, particularly for diamonds associated with eclogitic source rocks. The variability may be due to distinct primordial reservoirs, high-temperature isotope fractionation, recycling of crustal carbon, or any combination of the three. In contrast, the interpretation of variability in delta O-18 values of silicate minerals in eclogite xenoliths and eclogitic inclusions in diamonds is less contentious the hypothesis that they represent rocks that were exposed to weathering and hydrothermal alteration at the surface of Earth is broadly, but not universally, accepted. Here, we report high-precision SIMS oxygen isotope measurements of 15 eclogitic garnet inclusions in diamond from the Damtshaa mine, which comprises four kimberlites within the Orapa cluster of kimberlites. The results demonstrate a link between delta O-18 values of inclusions and delta C-13 values of their host diamonds. The delta O-18(VSMOW) values range from +4.7 parts per thousand to +8.8 parts per thousand and have a median value of +5.7 parts per thousand, similar to the distribution exhibited by cratonic eclogite xenoliths worldwide, at the nearby Orapa Mine, and of oceanic crust. Contrary to previous suggestions, there is no evidence for a unique oxygen isotope distribution for inclusions in diamond. The oxygen isotope ratios do not correlate with garnet compositions; they have, however, a strong negative correlation with the delta C-13 values of their host diamonds. We interpret this correlation to have geochemical significance. It cannot be due to primordial heterogeneities or high-temperature isotope fractionation, but is most likely due to an association between recycled near-surface crustal rocks and recycled carbon. We suggest that protoliths to the eclogites at Damtshaa that were more strongly affected by low-temperature seafloor weathering (recorded by delta O-18 >= similar to 6 parts per thousand) also have concentrations of primary organic carbon, and, therefore, low-delta C-13 Damtshaa diamonds are associated with recycling of crustal carbon. (C) 2013 Elsevier B.V. All rights reserved.