DIAMOND ECLOGITES: COMPARISON WITH CARBONACEOUS CHONDRTTES, CARBONACEOUS SHALES, AND MICROBIAL CARBON-ENRICHED MORB

13C-depleted carbon from microbial activity, as sediments and within MORB itself. Subducting MORB with sediments enriched in 13C-depleted microbial carbon can account for both the 13C-depleted nature of some eclogitic diamonds and the basaltic composition of diamond eclogite. The -11 to + 14%e variation in 6 S for sulphide inclusions in eclogitic diamonds can also be explained by subduction of a bacterially-fractionated seawater or igneous sulfur reservoir in the vicinity of seafloor-ridge hydrothermal vents. Microbial organic carbon and bacterially fractionated sulfur is preserved in Proterozoic vent settings and can survive subduction without significant fractionation. Seafloor hydrothermal vents (and associated biota) were more prevalent in the Proterozoic and may explain why most eclogitic diamonds are Proterozoic or younger in age, whereas eclogites of Archaean age have diamonds nearer to the mantle value of -6%c. Some 13C-depleted eclogitic diamond formation may thus be linked with the evolution, and eventual subduction, of the seafloor-ridge vent biosphere. Diamond, eclogite, organic, carbon, sulfur, isotopes