The effects of liquid immiscibility and thermal diffusion on oxygen isotopes in silicate liquids |
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Authors: | T K Kyser C E Lesher D Walker |
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Institution: | (1) Department of Geological Sciences, Queen's University, Kingston, ON Canada, K7L 3N6; Fax: 613 545-6592; E-mail: kyser@geol.queensu.ca, CA;(2) Department of Geological Sciences, University of California, Davis, CA 95616, USA, US;(3) Lamont-Doherty Geological Observatory, Palisades, NY 10964, USA, US |
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Abstract: | Differences between the δ18O values of Si- and Fe-rich immiscible liquids in the system Fe2SiO4-KAlSi2O6-SiO2 (Fa-Lc-Q) in isothermal experiments at 0.1 MPa have been determined experimentally to be 0.6 permil. The observed partition
of 18O into the Si-rich liquid is consistent with previous experience with the preferential partition of 18O into Si-rich minerals in isothermal equilibrium with minerals of less polymerized structure. Crystallochemical principles
affect the distribution of oxygen isotopes in coexisting isothermal liquids in the same way as they apply to isothermally
coexisting crystals. The effects of Soret (thermal) diffusion on the distribution 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 °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 result
of Soret diffusion are unexpected. For each of the silicate liquids, the cold end of the charge is enriched in 18O by up to 4.7 permil, and the highest δ18O values are associated with the most silica-poor compositions. The distribution of oxygen isotopes appears to be similar
in each liquid, regardless of their chemical compositions, which is in contrast to the behaviour of cations whose distributions
are compositionally dependent and characterized by strong crystallochemical effects wherein network-forming species such as
Si and Al separate to the hot end and Mg, Fe and Ca are segregated preferentially to the cold end. Structural units in the
melts are evidently less selective between oxygen isotopes than between cations, because oxygen redistribution over all possible
sites in these units proceeds according to mass. Self-diffusion coefficients of oxygen in basaltic liquids estimated from
the Soret experiments are in accord with those from other isotope 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 oxygen needed to
produce variations in the δ18O values of mantle minerals. Variations in oxygen isotope ratios in most natural systems as a result of Soret effects are
unlikely.
Received: 6 January 1997 / Accepted: 28 June 1998 |
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