DIAMOND ECLOGITES - COMPARISON WITH CARBO NACEOUS CHONDRITES, CARBONACEOUS SHALES, AND MICROBIAL CARBON-ENRICHED MORB

The similar to 0 to similar to 30 parts per thousand carbon isotopic r ange of eclogitic diamond has been attributed to derivation from carbo naceous chondritic mantle, or to mixing continental carbon with MORB t hrough subduction. Primordial differentiation of carbonaceous chondrit ic mantle is required to produce C-13-depleted diamond and maintain th e basaltic composition of diamond eclogite, whereas continental carbon aceous shales could account for some diamond eclogite when mixed with MORB, but carbonaceous shales are not readily subducted. Seafloor-ridg e hydrothermal vents contain abundant C-13-depleted carbon from microb ial activity, as sediments and within MORB itself. Subducting MORB wit h sediments enriched in C-13-depleted microbial carbon can account for both the C-13-depleted nature of some eclogitic diamonds and the basa ltic composition of diamond eclogite exhibit. The -11 to +14 parts per thousand, variation in delta(34)S for sulfide inclusions in eclogitic diamonds can also be explained by subduction of a bacterially-fractio nated seawater or igneous sulfur reservoir in the vicinity of seafloor -ridge hydrothermal vents. Microbial organic carbon and bacterially fr actionated sulfur is preserved in Proterozoic vent settings and can su rvive subduction without significant fractionation. Seafloor hydrother mal vents (and associated biota) were more prevalent in the Proterozoi c and may explain why most eclogitic diamonds are Proterozoic or young er in age, whereas eclogites of Archaean age have diamonds nearer to t he mantle value of -6 parts per thousand. Some C-13-depleted eclogitic diamond formation may thus be linked with the evolution, and eventual subduction, of the seafloor-ridge vent biosphere.