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

The origin of variability in δ¹³C 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 δ¹⁸O 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 δ¹⁸O values of inclusions and δ¹³C values of their host diamonds. The δ¹⁸O_VSMOW values range from +4.7% to +8.8% and have a median value of +5.7%, 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 δ¹³C 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 δ¹⁸O≥~6%) also have concentrations of primary organic carbon, and, therefore, low-δ¹³C Damtshaa diamonds are associated with recycling of crustal carbon.