Redox control on iron isotopic fractionation in magmas
Abstract
The causes of iron isotope variations in terrestrial igneous rocks are uncertain but the redox state of iron may play a role (Dauphas et al. 2009). To develop a quantitative understanding of iron isotopic fractionation at high temperature, it is critical to establish equilibrium isotopic fractionation factors between Fe2+ and Fe3+ in minerals and melts. The reduced partition function ratio (β-factor) controlling equilibrium isotopic fractionation can be derived from Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements at a synchrotron facility. For the 56Fe/54Fe ratio at high temperature, we simply have 1,000 ×ln β=2,904<F>/T2 with <F> the average force constant of the bonds holding iron in position (in N/m), which is calculated by taking the 3rd moment of the NRIXS scattering probability function (Dauphas et al. 2012). We have measured the force constants of olivine and silicate glasses of basalt, andesite, dacite, and rhyolite compositions synthesized under controlled redox conditions. XANES and conventional Mössbauer measurements were also performed on the same glasses to characterize the Fe3+/Fetot ratio and coordination environment of iron. In basalt, andesite, and dacite, we find that <F> increases with the Fe3+/Fetot ratio. Extrapolation to pure Fe2+ and Fe3+ gives equilibrium isotopic fractionation between these two redox states of ~+0.2 to +0.3 ‰ at magmatic temperatures (~1,300 °C). For comparison, the precision of stable iron isotope analyses by MC-ICPMS is ~±0.03 ‰, so redox state can influence the iron isotopic composition of magmatic rocks. However, this fractionation is insufficient to explain the heavy iron isotopic composition of MORBs and we are currently investigating whether pressure or other parameters can influence iron β-factor in magmas. We have also found that the force constant of Fe2+ in rhyolitic melt is high, which explains the heavy Fe isotopic compositions of some granitoids. Dauphas N. et al. 2009. Earth Planet. Sci. Lett. 288, 255-267 Dauphas N. et al. 2012. Geochim. Cosmochim. Acta, in press (10.1016/j.gca.2012.06.013)
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMDI13B2427D
- Keywords:
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- 1038 GEOCHEMISTRY / Mantle processes;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry