Discovery of whitlockite in mantle xenoliths: Inferences for water- and halogen-poor fluids and trace element residence in the terrestrial upper mantle Dmitri A. Ionov a,b, ⁎ , Albrecht W. Hofmann a , Claude Merlet c , Andrey A. Gurenko a , Eric Hellebrand a , Gilles Montagnac d , Philippe Gillet d , Vladimir S. Prikhodko e a Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany b Institut für Mineralogie, J.W. Goethe-Universitaet, Senckenberganlage 28, 60054 Frankfurt/Main, Germany c Laboratoire de Tectonophysique (UMR 5568 CNRS), Université Montpellier 2, 34095 Montpellier, France d Laboratoire de Sciences de la Terre, Ecole Normale Supérieure de Lyon, 46 allèe d'Italie, 69364 Lyon, France e ITIG, Russian Academy of Sciences, 680063 Khabarovsk, Russia Received 24 November 2005; received in revised form 7 February 2006; accepted 7 February 2006 Available online 13 March 2006 Editor: R.W. Carlson Abstract Chemical analyses and Raman micro-spectroscopy have identified whitlockite (water- and halogen-free phosphate) in mantle xenoliths from Siberia. Whitlockite has not previously been reported from terrestrial mantle-derived rocks, but is the most common accessory phosphate in meteorites and igneous rocks from Mars and the Moon. The presence of the ‘anhydrous’ whitlockite, together with the breakdown of ‘hydrous’ accessory minerals (amphibole, phlogopite) in the same xenoliths, indicates that portions of the terrestrial upper mantle may be nearly as low in water and halogens as in inner parts of the smaller solar system bodies, regardless of enrichments of other highly mobile components (phosphorus, alkalis). Whitlockite may be an important host for some lithophile trace elements in those portions of the terrestrial mantle. It contains up to 3 times more rare earth elements than coexisting apatite but less Sr, Ba and U while Th abundances are similar. Thus, trace element abundances, patterns and ratios (e.g. Sr/Nd, Th/U) in whitlockite-bearing mantle rocks and coexisting fluids may be distinct from those for mantle rocks containing only apatite and/or other ‘hydrous’ minerals. Several generations of both whitlockite and apatite with different textural positions and abundances of Na, Mg, Sr and LREE were identified in some of the xenoliths. Furthermore, precipitation of the phosphates was accompanied by the formation of a distinct generation of clinopyroxene, which contains much less Zr (up to 10 times) but more LREE than clinopyroxenes formed in previous metasomatic episodes. © 2006 Elsevier B.V. All rights reserved. Keywords: whitlockite; apatite; terrestrial mantle; metasomatism; trace elements; volatiles 1. Introduction There is no doubt that water plays a fundamental role in the formation and evolution of terrestrial continents and oceans, and it is widely believed that water is also an essential prerequisite for the development of plate Earth and Planetary Science Letters 244 (2006) 201 – 217 www.elsevier.com/locate/epsl ⁎ Corresponding author. Institut für Mineralogie, J.W. Goethe- Universitaet, Senckenberganlage 28, 60054 Frankfurt/Main, Germany. Tel./fax: +49 6131 305281. E-mail address: ionov@mpch-mainz.mpg.de (D.A. Ionov). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.02.012