HR: 1330h
AN: V52A-0423    [PDF]
TI: Iron Isotope Fractionation in Metamorphic Rocks
AU: * von Blanckenburg, F
EM: fvb@mineralogie.uni-hannover.de
AF: Institute for Mineralogy, University of Hannover, Callinstrasse 3, Hannover, 30167 Germany
AU: Berger, A
EM: berger@geo.unibe.ch
AF: Institute for Geology, University of Berne, Baltzerstrasse 1, Berne, 3012 Switzerland
AB: Polyakov and Mineev [1] predict high-T equilibrium fractionation favouring incorporation of heavy Fe isotopes into Fe(III)-containing minerals. We have confirmed this prediction for metamorphic rocks. A prerequisite was the development of highly accurate and precise protocols for Fe isotope analysis (Delta 56Fe/54Fe to 50ppm at 2 sigma of repeatedly separated natural samples; Delta 57Fe/54Fe to 70ppm; and Delta 58Fe/54Fe to 280ppm, n=40) using a Finnigan Neptune MC-ICP-MS at the University of Hannover. In greenschist facies rocks, Delta 56Fe of magnetite and pyrite is 0.3 to 0.6permil heavier than Fe in Fe(II) silicate minerals. In migmatites formed at 800C Delta 56Fe of magnetite is 0.1 to 0.3 permil above Fe(II) silicate minerals (Chlorite, Biotite). These observations confirm that (a) high-T equilibrium fractionations of Fe do indeed exist; (b) strongly-bound Fe(III) oxides and sulfides do indeed favour heavier isotopes; and that (c) a temperature-dependency exists, favouring higher fractionation at lower temperature as predicted [1]. It will now be important to explore the potential of this new isotope system for of high-T thermometry and in terms of redox conditions in metamorphic rocks and melts. 1 V.B. Polyakov and S.D. Mineev, The use of M”ssbauer spectroscopy in stable isotope geochemistry, Geochimica Cosmochimica Acta 64, 849-865, 2000.
DE: 4870 Stable isotopes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting