Structure and Composition Effects on the Oxygen Isotope Fractionation in Silicate Melts

• Published in: Petrology 2018-07, Vol.26 (4), p.414-427
• Main Authors: Dubinina, E. O., Borisov, A. A.
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Bridging; Composition effects; Data processing; Earth and Environmental Science; Earth Sciences; Fractionation; Geochemistry; High temperature; Isotope fractionation; Isotopes; Mathematical models; Melts; Mineralogy; Oxygen; Oxygen isotopes; Silica; Silicates; Silicon dioxide; Temperature; Temperature effects; Titanium dioxide
• ISSN: 0869-5911; 1556-2085
• DOI: 10.1134/S0869591118040021

The influence of melt composition and structure on the oxygen isotope fractionation was studied for the multicomponent (SiO 2 ± TiO 2 + Al 2 O 3 ± Fe 2 O 3 + MgO ± CaO) system at 1500°C and 1 atm. The experiments show that significant oxygen isotope effects can be observed in silicate melts even at such high temperature. It is shown that the ability of silicate melt to concentrate 18 O isotope is mainly determined by its structure. In particular, an increase of the NBO/T ratio in the experimental glasses from 0.11 to 1.34 is accompanied by a systematic change of oxygen isotope difference between melt and internal standard by values from–0.85 to +1.29‰. The obtained data are described by the model based on mass-balance equations and the inferred existence of O 0 , O – , and O 2– (bridging, non-bridging, and free oxygen) ions in the melts. An application of the model requires the intra-structure isotope fractionation between bridging and non-bridging oxygens. Calculations show that the intra-structure isotope fractionation in our experiments is equal to 4.2 ± 1.0‰. To describe the obtained oxygen isotope effects at the melts relatively to temperature and fraction of non-bridging oxygen a general equation was proposed.