Noble gas and halogen geochemistry of mantle fluids: comparison of African and Canadian diamonds

Volatile-bearing fluids in diamond have been characterised using extension of the 40Ar-39Ar technique to simultaneously measure noble gas isotopes, halogens (Cl, Br and I), K and U. Samples investigated include opaque cubic and fibrous diamonds from the North West Territories, Canada, the Democratic Republic of Congo, (DRC, formerly Zaire), and Jwaneng, Botswana. These are compared with results obtained from metasomatised mantle xenoliths from Bultfontein, South Africa. Diamonds and xenoliths show a narrow range of 40Ar∗/Cl values between 506–1347 × 10−6 molar (M) with mean values that overlap for African diamonds and xenoliths (831 ± 218 × 10−6 M) and Canadian diamonds (965 ± 273 × 10−6 M). These values are consistent with the estimated MORB value and support the presence of a widespread 40Ar and Cl-rich fluid in the mantle. Canadian diamonds have high and variable halogen ratios, with Br/Cl = 1.3–63.0 × 10−3 M and I/Cl = 9.8–1703.5 × 10−6 M. In contrast, African diamonds, have less variable Br/Cl = 1.0–2.0 × 10−4 M and I/Cl = 13.6−176.4 × 10−6 M (with most I/Cl between 20–70 × 10−6 M). Fluids in diamonds from DRC and Botswana, have Ar and halogen compositions close to those estimated for the present-day MORB source. The concentrations of noble gases (Ar, Kr and Xe), halogens, K and U in Canadian coated stones are 10–30 times higher than in African coated stones probably due to a higher inclusion population density. The large variation in halogen ratios measured in Canadian diamonds is the first evidence for significant halogen fractionation in the mantle. The Br/Cl ratios are notably above the range reported for crustal fluids. The high halogen ratios in Canadian diamonds are consistent with crystallisation of a Cl-bearing mineral, possibly involving apatite, from a fluid with starting composition similar to that in African diamonds.