The effects of liquid immiscibility and thermal diffusion on oxygen isotopes in silicate liquids

Differences between the delta(18)O values of Si- and Fe-rich immiscible liq uids in the system Fe2SiO4-KAl-Si2O6-SiO2 (Fa-Lc-Q) in isothermal experimen ts at 0.1 MPa have been determined experimentally to be 0.6 permil. The obs erved partition of O-18 into the Si-rich liquid is consistent with previous experience with the preferential partition of O-18 into Si-rich minerals i n isothermal equilibrium with minerals of less polymerized structure. Cryst allochemical principles affect the distribution of oxygen isotopes in coexi sting isothermal liquids in the same way as they apply to isothermally coex isting crystals. The effects of Soret (thermal) diffusion on the distributi on of oxygen isotopes in silicate liquids above the solvus in the system Fa -Lc-Q under conditions of an imposed temperature gradient of ca. 250 degree s C over 4 mm and at 2 GPa have also been investigated experimentally. Both the magnitude and the direction of separation of oxygen isotopes as a resu lt of Soret diffusion are unexpected. For each of the silicate liquids, the cold end of the charge is enriched in O-18 by up to 4.7 permil. and the hi ghest delta(18)O values are associated with the most silica-poor compositio ns. The distribution of oxygen isotopes appears to be similar in each liqui d, regardless of their chemical compositions, which is in contrast to the b ehaviour of cations whose distributions are compositionally dependent and c haracterized by strong crystallochemical effects wherein network-forming sp ecies such as Si and Al separate to the hot end and Mg, Fe and Ca are segre gated preferentially to the cold end. Structural units in the melts are evi dently less selective between oxygen isotopes than between cations, because oxygen redistribution over all possible sites in these units proceeds acco rding to mass. Self-diffusion coefficients of oxygen in basaltic liquids es timated from the Soret experiments are in accord with those from other isot ope tracer experiments, and comparable to those of Si. The possible effects of Soret diffusion on the oxygen isotopic composition of metasomatic veins in the mantle are examined in light of these data, and indicate that decay of the thermal gradients in the veins exceeds that of the diffusion of oxy gen needed to produce variations in the delta(18)O values of mantle mineral s. Variations in oxygen isotope ratios in most natural systems as a result of Soret effects are unlikely.