Author's personal copy Lithium isotope fractionation during magma degassing: Constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano (Réunion Island) I. Vlastélic a,b,c, ⁎, T. Staudacher d , P. Bachèlery e , P. Télouk f , D. Neuville g , M. Benbakkar a,b,c a Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France b CNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France c IRD, R 163, LMV, F-63038 Clermont-Ferrand, France d Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, CNRS UMR 7154, 14 RN3, le 27 ème km, 97418, La Plaine des Cafres, La Réunion, France e Laboratoire GéoSciences Réunion, Université de La Réunion, Institut de Physique du Globe de Paris, CNRS UMR 7154, 15 Avenue René Cassin, 97715 Saint-Denis cedex 09, La Réunion, France` f Laboratoire des Sciences de la Terre, Ecole Normale Supérieure de Lyon, CNRS UMR 5570 46 Allée d'Italie, 69364 Lyon cedex 07, France g Laboratoire de Géochimie et Cosmochimie, Université Paris Diderot, Institut de Physique du Globe de Paris, CNRS UMR 7154, 1 rue Jussieu, 75238 Paris cedex 05, France abstract article info Article history: Received 30 August 2010 Received in revised form 31 January 2011 Accepted 2 February 2011 Available online 5 March 2011 Edited by: R.L. Rudnick Keywords: Lithium isotopes Isotopic fractionation Magma degassing Piton de la Fournaise volcano, Réunion Island Recent volcanic products from the Piton de la Fournaise Volcano, Reunion, show pronounced depletion or enrichment in lithium and significant isotopic fractionation related to degassing. (1) trachytic pumices from the April 2007 eruption show extreme Li depletion (90%) and isotopic fractionation (δ 7 Li of −21‰). The depletion of water and volatiles (Cl, F, B, Cs) in these samples suggests that Li loss occurred in response to degassing, which most likely occurred as the small, isolated volume of magma underwent extensive differentiation near the surface. Because the pre-degassing composition is relatively well known, the composition of the degassed pumice constrains the partition coefficient to 60 b D V–M b 135 and the isotopic fractionation factor, α V–M , to 1.010 at magmatic temperatures. Unlike D V–M , α V–M does not depend on whether crystallization and degassing occurred successively or concomitantly. (2) basaltic samples from the interior wall of the long-lived 1998 Piton Kapor were extensively altered by acidic gas. They also show extreme Li depletion, but barely significant isotopic fractionation (δ 7 Li=+4.5‰), suggesting that high- temperature leaching of Li by volcanic gas does not significantly fractionate Li isotopes. (3) high-temperature (400–325 °C) gas condensates formed during degassing of the thick lava flow of April 2007 display high Li contents (50–100 ppm), which are consistent with Li being as volatile as Zn and Sn. Their isotopically light Li signature (average of −1.7‰) is consistent with their derivation from isotopically heavy vapor (+ 13.5‰) if the factor of isotopic fractionation between condensate and vapor is less than 0.985. A degassing- crystallization model accounts for the evolution of trace species, which, like lithium, are volatile but also moderately incompatible. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Alkali metals are only moderately volatile, in the sense that only a small proportion (b 0.1%) of the initial budget of mantle derived melts is ultimately lost by magmas (Rubin, 1997). On the other hand, alkali are largely mobilized during exsolution of H 2 O-rich vapor phase in magmatic systems (Sakuyama and Kushiro, 1979). Selective vapor transport of potassium has been suggested during the late stage of crystallization of oceanic magmas (Sinton and Byerly, 1980), and from bottom to top of lava lakes (Richter and Moore, 1966) although the Kilauea data on which this idea is based were subsequently questioned (Helz et al., 1994). There has been recently a growing interest in lithium, the lightest alkali metal, but above all one of the fastest diffusing elements in silicates (Jambon and Semet, 1978; Giletti and Shanahan, 1997; Richter et al., 2003). Indeed, lithium seems to be particularly sensitive to vapor transfer, either in shallow magma conduit shortly before eruption (Berlo et al., 2004; Kent et al., 2007) or during post-eruptive degassing of thick lava flows (Kuritani and Nakamura, 2006). While the use of lithium abundance as tracer of degassing processes is still in its infancy, new perspectives arise from lithium stable isotopes ( 6 Li and 7 Li) geochemistry. The main reason is that our knowledge of the laws governing lithium isotopes fractionation in nature has improved significantly in recent years. First, it is now well established that heavy lithium partitions preferentially into aqueous Chemical Geology 284 (2011) 26–34 ⁎ Corresponding author at: Laboratoire Magmas et Volcans, Observatoire de Physique du Globe de Clermont-Ferrand, UMR 6524, 5 Rue Kessler, 63038 Clermont- Ferrand, France. Tel.: +33 4 73 34 67 10; fax: +33 4 73 34 67 44. E-mail address: I.Vlastelic@opgc.univ-bpclermont.fr (I. Vlastélic). 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.02.002 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo