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 H ole, New Mexico. These results confirm that sulphide dominates the Os budge t 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 minera ls presence significantly less radiogenic Os isotope compositions, suggesti ng that they have been shielded from reaction or diffusion by their silicat e hosts. Diffusion rates in silicates and sulphides are likely to be rapid at mantle temperatures. However, for the sulphide inclusions diffusion thro ugh the silicate host is likely to be impaired by the high partition coeffi cient for Os between sulphide and silicate phases, in the range of 10(4) to 10(6). Consequently, sulphide inclusions trapped in silicates may be unabl e to equilibrate either with other minerals or with an intergranular melt, and in principle may preserve significantly older Re-Os age information tha n coexisting phases. (C) 1999 Elsevier Science B.V. All rights reserved.