Author's personal copy Equilibrium high-temperature Fe isotope fractionation between fayalite and magnetite: An experimental calibration Anat Shahar a, ⁎ , Edward D. Young a,b , Craig E. Manning a a Department of Earth & Space Sciences, University of California, Los Angeles, CA 90095 USA b Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095 USA Received 20 October 2007; received in revised form 14 January 2008; accepted 17 January 2008 Editor: R.W. Carlson Available online 6 February 2008 Abstract The iron isotopic fractionation factor between magnetite and fayalite was measured at high temperature in a piston–cylinder apparatus. For the first time, the three-isotope method [Matsuhisa, J., Goldsmith, J. R., and Clayton, R. N., 1978. Mechanisms of hydrothermal crystallisation of quartz at 250 °C and 15 kbar. Geochim. Cosmochim. Acta 42, 173–182.] has been used to determine equilibrium fractionation between two minerals directly. The experimentally-determined temperature-dependent iron isotope fractionation between fayalite and magnetite is described by 10 3 ln α 57 Mag–Fa ≅ Δ 57 Fe Mag–Fa = 0.30 (+/- 0.024) × 10 6 / T 2 . The equation predicts measurable fractionation at magmatic temperatures. Our work bears on the magnitude of Fe isotope fractionation attending differentiation in magmatic systems and provides a new isotope thermometer for co- existing fayalite and magnetite. © 2008 Elsevier B.V. All rights reserved. Keywords: iron isotope; thermometer; igneous rocks; high-temperature; three-isotope exchange; magnetite 1. Introduction Iron isotope ratios have proven to be especially useful tracers in a variety of geochemical settings due to the ubiquitous nature of Fe and to its variable oxidation states. Differences in iron isotope ratios have been used to address a myriad of questions including, but not limited to, those relating to: biosignatures (Beard and Johnson, 1999), early solar system processes (Zhu et al., 2001), and planet formation processes (Poitrasson et al., 2004). Each of these applications depends critically on under- standing factors that fractionate Fe isotopes. Most work involving Fe isotopes has focused on low- temperature processes because differences between minerals equilibrated at high temperature are expected to be small due to decreasing fractionation with increasing temperature (T ). But, with the advent of the multiple-collector inductively coupled plasma-source mass spectrometer (MC-ICPMS), these smaller differences in δ 56 Fe and δ 57 Fe (1) can now be measured more precisely. Differences between δ 56(57) Fe values of mantle minerals have been measured, but no consensus has been reached as to whether there are consistent iron isotopic fractionations amongst them. Zhu et al. (2002) found that olivine separates from lherzolites are isotopically lighter (by N 0.2‰ in 57 Fe/ 54 Fe) than co-existing orthopyroxene and clinopyroxene. Beard and Johnson (2004) found that olivine and orthopyroxene sepa- rates from spinel peridotites have indistinguishable 57 Fe/ 54 Fe, whereas co-existing clinopyroxene and olivine were found to show some measurable differences in δ 57 Fe. A compilation of Δ 56 Fe mineral-olivine (Δ 56 Fe i - j = δ 56 Fe i - δ 56 Fe j ) representing sepa- rates of co-existing minerals in igneous rocks (Williams et al., Available online at www.sciencedirect.com Earth and Planetary Science Letters 268 (2008) 330 – 338 www.elsevier.com/locate/epsl ⁎ Corresponding author. E-mail address: ashahar@ess.ucla.edu (A. Shahar). 1 δ 56(57) Fe=( 56(57) R smp / 56(57) R IRMM-014 - 1) ⁎ 1000 where 56(57) R= 56(57) Fe/ 54 Fe and IRMM-014 is an international Fe standard. In what follows reference will be made to both δ 56 Fe and δ 57 Fe since both values are reported by various authors. In all cases considered here Fe isotope fractionation is mass dependent and so there is no significance to choosing one ratio over the other. 0012-821X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.01.026