Irradiation-induced stabilization of zircon (ZrSiO 4 ) at high pressure Maik Lang a , Fuxiang Zhang a , Jie Lian a, b , Christina Trautmann c , Reinhard Neumann c , Rodney C. Ewing a, ⁎ a Department of Geological Sciences, University of Michigan,1100 N University Avenue, Ann Arbor, MI 48109-1005, USA b Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA c Gesellschaft für Schwerionenforschung (GSI), Planckstr. 1, 64291 Darmstadt, Germany article info abstract Article history: Received 17 January 2008 Received in revised form 8 February 2008 Accepted 12 February 2008 Available online 4 March 2008 Editor: L. Stixrude Zircon (ZrSiO 4 ), the most important accessory mineral in the Earth's crust, transforms under high pressure to reidite, a scheelite-structured polymorph. Recently, reidite was found in association with meteorite impact structures. Here, we show that the zircon-to-reidite transition, and thus the amount of reidite produced during high-pressure events, strongly depends on the microstructure of the initial zircon. Our results clearly demonstrate that radiation damage, present in natural zircon due to radioactive decay, dramatically modifies the phase stability of crystalline zircon at high pressure. By simulating this radiation damage with ion beams, we show that zircon, pre-irradiated with 1.47-GeV Xe ions, formed only minor amounts of reidite up to 36GPa; whereas, an unirradiated zircon was almost completely transformed to reidite under the same conditions. By means of Raman scattering, X-ray diffraction, and transmission electron microscopy, we confirmed that the stability field of the irradiated zircon is expanded to higher pressures as a result of the interplay between pressure, ion beam-induced amorphization, and the formation of nanoscale damage domains. These results provide insight into the formation-conditions of reidite in nature and illustrate how pressure-induced phase transitions may be affected by defects, in this case those caused by radioactive decay. © 2008 Elsevier B.V. All rights reserved. Keywords: zircon reidite high-pressure phase transitions radiation damage ion beams nanocrystals 1. Introduction Zircon (ZrSiO 4 ) is the most important accessory mineral in the Earth's crust. It is the main mineral used in U/Th/Pb age-dating and also provides geochemical and isotopic signatures of the Earth's earliest formed rocks (Hanchar and Hoskin, 2003). Among other remarkable properties (e.g., physical and chemical durability), zircon exhibits an anomalous phase-transformation to reidite at high pressure leading to a 10% denser phase with the scheelite structure (Liu, 1979; Kusaba et al., 1985; Knittle and Williams,1993; Leroux et al., 1999; Glass and Liu, 2001; Glass et al., 2002; Gucsik et al., 2004; Ono et al., 2004; Van Westrenen et al., 2004). Based on the structural similarities between zircon and reidite, a special displacive mechan- ism has been proposed (Kusaba et al., 1986). This first-order transition is the result of simple shearing, followed by small atomic adjustments (Kusaba et al., 1986). At elevated temperatures (~ 1000°C), reidite begins to form at pressures above ~ 10 GPa (Ono et al., 2004). Due to hampered kinetics, the critical pressure at room temperature has to be significantly overstepped in excess of 20 GPa (Knittle and Williams, 1993; Van Westrenen et al., 2004). Above this threshold, the transformation takes place gradually, with both phases coexisting, up to pressures of 30 to 40 GPa (Gucsik et al., 2004; Van Westrenen et al., 2004). However, once the scheelite-structured phase is formed, it persists after pressure release and does not revert to zircon unless the temperature is above 1200°C (Kusaba et al., 1985). Recently, metastable reidite was found in naturally occurring shock-metamor- phosed zircon in the vicinity of a meteorite impact structure (Glass and Liu, 2001; Glass et al., 2002). Thus, the zircon-reidite phase relation at elevated pressures has been proposed as a new peak- pressure indicator of such impact events (Kusaba et al., 1985; Leroux et al., 1999; Glass and Liu, 2001). The zircon structure can incorporate and retain up to a few wt.% uranium and thorium (Finch and Hanchar, 2003). The decay of nuclides in the 238 U, 235 U, and 232 Th decay series leads to structural damage, the metamict state, mainly caused by alpha-decay events (Weber et al., 1994; Ewing et al., 2003). Hence, many natural zircons are partially amorphous or metamict. The use of zircon in geochro- nology and thermochronology (Davis et al., 2003; Reiners and Ehlers, 2005), as well as its possible application as a nuclear waste form for plutonium (Ewing, 1999), requires a detailed knowledge of the damage-formation process. For this reason, radiation-damage studies of zircon, particularly using ion irradiations, have been systematically completed using a variety of ion beams at different energies up to temperatures of nearly 1000°C (Weber et al., 1994; Ewing et al., 2000, 2003). Partially amorphous, natural zircons have recently been the subject of moderate (up to 9 GPa) compression experiments focusing on the structural details of the defective crystalline (Ríos and Boffa- Ballaran, 2003) and the amorphous phase (Trachenko et al., 2007). The question of whether radiation effects may modify the phase stability of zircon under subsequent compression has never been addressed. Here, we report the first experiments that investigate the Earth and Planetary Science Letters 269 (2008) 291–295 ⁎ Corresponding author. E-mail address: rodewing@umich.edu (R.C. Ewing). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.02.027 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl