Applications of Mössbauer Spectroscopy to Studies of the Earth’s Interior C. McCammon Bayerisches Geoinstitut Universität Bayreuth D-95440 Bayreuth, Germany E-mail: catherine.mccammon@uni-bayreuth.de http://www.bgi.uni-bayreuth.de

Introduction The major mineralogy within the Earth’s mantle has primarily been determined through (1) laboratory synthesis and characterisation of high-pressure phases for the relevant chemical systems (e.g., MgO-FeO-SiO2), (2) measurement of their elastic properties, and (3) comparison of these properties with seismic data (figure right). Although iron is only ~ 6 atom % of the mantle, it is the most abundant transition element, and changes in oxidation and spin state influence a broad spectrum of physical and chemical properties. 57Fe Mössbauer spectroscopy is one of the most important tools for studying iron in mantle phases, particularly because it can be applied at conditions of both high pressure and high temperature. For recent reviews of iron behaviour in the Earth’s mantle see: - C. A. McCammon, Science 308, 807-808 (2005). - C. McCammon, in Earth's Deep Mantle: Structure, Composition, and Evolution R.D. van der Hilst, J. Bass, J. Matas, J. Trampert, Eds. (American Geophysical Union, Washington D.C., 2005) pp. 221-242. - C. McCammon, J. Mineral. Petrol. Sci. 101, 130-144 (2006). - D.J. Frost, C.A. McCammon, Ann. Rev. Earth. Planet. Sci. 36, 389-420 (2008). Several examples of 57Fe Mössbauer investigations relating to the Earth’s interior are given in the next slides.

Oxygen fugacity during metasomatism Mössbauer spectra taken of different zones of metasomatised garnet peridotite (figure right) show differences in Fe3+ concentration (figure far right) that can be converted to oxygen fugacity values using the olivine-orthopyroxene-garnet oxybarometer. Results (figure bottom) show a progressive increase in oxygen fugacity during metasomatism to nearly reach the breakdown curve between graphite/diamond and carbonate. Diamond is therefore unlikely to survive extended metasomatising events in the upper mantle. metasomatism 2 FMQ = fayalite-magnetite-quartz OG = olivine-graphite OD = olivine-diamond IW = iron-wüstite EM = enstatite-magnesite C.A. McCammon, W.L. Griffin, S.H. Shee, H.S.C. O'Neill, Contrib. Mineral. Petrol. 141, 287-296 (2001).

Depth profile of oxygen fugacity in the upper mantle Numerous studies in the past two decades have shown that the dominant assemblage in the top part of the upper mantle, spinel peridotite, equilibrated at oxygen fugacities ranging from -2 to +2 FMQ, depending on factors such as tectonic environment and metasomatism. Oxygen fugacity at greater depths has been determined through measurement of Fe3+ in garnet using Mössbauer spectroscopy (figure left) combined with the olivine-orthopyroxene-garnet oxybarometer. Results show a strong decrease in oxygen fugacity with depth to nearly Fe metal equilibrium (figure right). Dlog fO2 (FMQ) depth (km) pressure (kbar) MORB subduction continent spinel peridotite garnet peridotite garnet 25-4 MORB = Mid-ocean rich basalt A.B. Woodland, M. Koch, Earth Planet. Sci. Lett. 214, 295-310 (2003). C.A. McCammon, M.G. Kopylova, Contrib. Mineral. Petrol. 148, 55-68 (2004).

Iron oxidation state in transition zone phases The dominant phases of the transition zone can be synthesised at high P,T in the laboratory and quenched to ambient conditions for study using Mössbauer spectroscopy (figure bottom). When these phases are synthesised at their minimum oxygen fugacity stability limit (i.e., in equilibrium with SiO2 and Fe metal), Mössbauer spectra (figure right) show the presence of measureable Fe3+ in all phases, implying elevated Fe3+ in the transition zone despite relatively low oxygen fugacity conditions. ringwoodite wadsleyite ringwoodite wadsleyite majorite majorite DJ Frost H.S.C. O'Neill, C.A. McCammon, D.C. Canil, D.C. Rubie, C.R. Ross II, F. Seifert, Amer. Mineral. 78, 456-460 (1993).

Iron oxidation state in (Mg,Fe)(Si,Al)O3 perovskite (Mg,Fe)(Si,Al)O3 perovskite can be synthesised at high P,T in the laboratory (figure bottom left). In the absence of Al, the phase incorporates 15-20 % Fe3+ (figure top right), while the presence of Al stabilises much higher concentrations (figure bottom right and bottom centre). To balance charge in the lower mantle, the following reaction takes place: 3 Fe2+ → Fe23+ + Fe0. 200 µm C.A. McCammon, Nature 387, 694-696 (1997). S. Lauterbach, C.A. McCammon, P. van Aken, F. Langenhorst, F. Seifert, Contrib. Mineral. Petrol. 138, 17-26 (2000). D.J. Frost, C. Liebske, F. Langenhorst, C.A. McCammon, R. Trønnes, D.C. Rubie, Nature 428, 409-411 (2004).

Diamonds from the lower mantle Mössbauer spectra of inclusions in diamonds from the lower mantle (figure below) show that (Mg,Fe)(Si,Al)O3 perovskite contains a large fraction of Fe3+, while (Mg,Fe)O contains only a small amount (figure right). This is consistent with a low oxygen fugacity in the lower mantle, and the stability of large amounts of Fe3+ in the perovskite phase, even under reducing conditions. (Mg,Fe)(Si,Al)O3 (Mg,Fe)O C.A. McCammon, M. Hutchison, J. Harris, Science 278, 434-436 (1997).

Iron spin state in (Mg,Fe)O Mössbauer spectra (figure right) of (Mg,Fe)O subjected to high pressure in a diamond anvil cell show the appearance of a new component (red singlet) which corresponds to Fe2+ in the low-spin state. Spin crossover is expected to occur in (Mg,Fe)O over a broad region in the lower part of the lower mantle (figures below). S. Speziale, A. Milner, V.E. Lee, S.M. Clark, M.P. Pasternak, R. Jeanloz, Proc. Natl. Acad. Sci. 102, 17918-17922 (2005). I.Y. Kantor, L.S. Dubrovinsky, C.A. McCammon, Phys. Rev. B 73, 100101 (2006).

Iron spin state in (Mg,Fe)(Si,Al)O3 perovskite Pressures given in GPa Pressures given in GPa Mössbauer spectroscopy (figure right) and NFS (figure far right) data of (Mg,Fe)(Si,Al)O3 perovskite subjected to high pressure and temperature in an externally heated diamond anvil cell show the appearance of a new component (red doublet) which corresponds to Fe2+ in the intermediate-spin (IS) state. IS Fe2+ is observed to be stabilised by high temperature; hence lower mantle perovskite contains Fe2+ predominantly in the IS state. C. McCammon, I. Kantor, O. Narygina, J. Rouquette, U. Ponkratz, I. Sergueev, M. Mezouar, V. Prakapenka, L. Dubrovinsky, Nature Geoscience 1, 684-687 (2008).