Osmium isotope disequilibrium between mantle minerals in a spinel-lherzolite

Rhenium (Re)–osmium (Os) isotope and elemental data have been obtained for coexisting silicates and sulphide from a spinel-lherzolite from Kilbourne Hole, New Mexico. These results confirm that sulphide dominates the Os budget in mantle rocks, but a significant proportion of the Re is located in the silicates. The silicates and interstitial sulphide yield indistinguishable Os isotope compositions suggesting these phases were in equilibrium at the time of xenolith eruption. However, sulphide inclusions in silicate minerals preserve significantly less radiogenic Os isotope compositions, suggesting that they have been shielded from reaction or diffusion by their silicate hosts. Diffusion rates in silicates and sulphides are likely to be rapid at mantle temperatures. However, for the sulphide inclusions diffusion through the silicate host is likely to be impaired by the high partition coefficient for Os between sulphide and silicate phases, in the range of 104 to 106. Consequently, sulphide inclusions trapped in silicates may be unable to equilibrate either with other minerals or with an intergranular melt, and in principle may preserve significantly older Re–Os age information than coexisting phases.