olatile and metal cycling in subduction zones: Insights from sulphide-bearing diamonds and xenoliths

Sulphide-bearing diamonds and diamondiferous xenoliths from the central Slave craton are used to trace the mobility of carbon, sulphur and trace metals along the slab and mantle wedge. There, eclogitic diamond formed along fluid conduits ca 1.85 Ga ago, penecontem poraneously with subduction beneath the western craton margin and with eclogitisation of gabbroic protoliths. The radiogenic initial 187Os/188Os of the eclogitic sulfide inclusions is consistent with diamond formation in matured, high Re/Os oceanic crust. This may indicate that S in the inclusions was also derived in situ. Conservative behavior of highly siderophile elements in the sulphides and mantle-like d13C of eclogitic diamonds suggests derivation of the carbon from reducing , Cl-poor serpentinite-derived fluid. Subducting oceanic crust may release silicic melts that react with mantle wedge peridotite to form pyroxenite. A low-temperature pyroxenite xenolith has an age and initial 187Os/188Os that is identical to eclogitic inclusions in diamond, indicating that it inherited its radiogenic initial Os and S from oxidising slab-derived melt. Oxidising conditions and formation at shallow depth may explain why pyroxenite or websterite is rarely represented amongst diamonds from the central Slave craton and worldwide. A distinct group of sulphide inclusions in diamond, with Ni content similar to peridotitic inclusions, but 20 x lower Os contents has strongly unradiogenic initial Os and yields a Re-Os isochron age of 1.70±0.26 Ga, within error of the age of accretion at the craton margin. These diamonds may have also formed through interaction of serpentinite-derived, reducing, Os-poor fluids, which were extracted along conduits armoured by a reaction assemblage, with a more oxidised mantle wedge. As the fluid fO2 rose during this interaction, it scavenged Os from the mantle wedge while facilitating isotopic rehomogenisation of pre-existing sulphides. Carbon and N isotopes of host diamonds will be measured at IPGP to constrain the fluid source.