Sulphur isotope variations in the mantle from ion microprobe analyses of micro-sulphide inclusions
Résumé
21 samples of sulphide trapped either as liquid globules or grains in various minerals (olivine, pyroxenes, ilmenite and garnet) or rocks (basalt glasses, peridotites, eclogites and kimberlites) of mantle origin, have been analysed for their sulphur isotope, and their Cu, Ni, Fe compositions by ion microprobe. The results show a wide range of δ34S values between −4.9 ± 1 and +8 ± 1‰. Sulphides with high nickel contents (up to 40% pentlandite), corresponding mostly to residual peridotites, have δ34S values ranging from −3.2‰ to +3.6‰ with a mode of +3 ± 1‰, compared to low Ni content sulphides, mostly contained in pyroxenites, OIB and kimberlites, ranging from −3.6‰ to +8‰ with a mode of +1 ± 1‰.
The δ34S of sulphides originating from within the mantle are variable. The sulphide globules with high Ni contents and δ34S values close to +3‰, are probably produced by 10–20% partial melting of a mantle source containing 300 ppm sulphur as an upper limit and having a δ34S value of +0.5 ± 0.5‰. This difference in δ34S values suggests a high-temperature S-isotope fractionation of ≈ +3‰ between liquid sulphide and the sulphur dissolved in the silicate liquid. The sulphur isotopes balance in the system upper mantle + oceanic crust + continental crust + seawater requires a mean δ34S value of the primitive upper mantle of +0.5‰, slightly but significantly different from that of chondrites (+0.2 ± 0.2‰).