Magnesium isotopes constraints on the origin of Mg-rich olivines from the Allende chondrite: Nebular versus planetary? Johan Villeneuve a, ⁎, Marc Chaussidon a , Guy Libourel a,b a Centre de Recherches Pétrographiques et Géochimiques - Nancy Université, CNRS, UPR 2300, 15 Rue Notre-Dame des Pauvres, BP20, 54501 Vandoeuvre-lès-Nancy, France b Ecole Nationale Supérieure de Géologie-Nancy Université, Rue du Doyen Marcel Roubault, BP40, 54501 Vandoeuvre-lès-Nancy, France abstract article info Article history: Received 22 March 2010 Received in revised form 13 October 2010 Accepted 19 October 2010 Available online 26 November 2010 Editor: R.W. Carlson Keywords: refractory olivines Mg isotopic composition Allende chondrite Eagle station pallasite early solar system chronology chondrules High precision Mg isotope measurements by multi-collector ion microprobe show that refractory olivines from the Allende chondrite, either olivines isolated in the matrix (2 samples studied) or olivines in type I chondrules (6 samples studied), have variable δ 26 Mg* enrichments and deficits (calculated in permil as the 26 Mg deviation from the instrumental mass fractionation line) relative to the Earth. Most average δ 26 Mg* (noted δ 26 Mg* av ) values (between 10 and 20 analyses per chondrule) are negative but the total range is from -0.029 (±0.010) ‰ (2 sigma errors) to +0.011 (±0.011) ‰ with an exception of one olivine at +0.043 (± 0.023) ‰. These variations in δ 26 Mg* av reflect the formation of the olivines from reservoirs enriched in various amounts of 26 Mg by the decay of short-lived 26 Al (T 1/2 = 0.73 Ma). Similarly, 30 analyses of olivines from the Eagle Station pallasite show a δ 26 Mg* av value of -0.033 ± 0.008‰, as negative as some olivines from Allende chondrules and the Solar system initial δ 26 Mg* value of -0.038 ± 0.004‰ (defined at the time of formation of type B Ca–Al-rich inclusions – CAIs – when 26 Al/ 27 Al = 5.23 × 10 -5 , Jacobsen et al., 2008). Because olivines are Al-poor and because their Mg isotopic compositions are not reset during the chondrule forming events, their δ 26 Mg* av can be used to calculate model crystallization ages relative to various theoretical Mg isotope growth curves. The two end-member scenarios considered are (i) a “nebular” growth in which the Al/Mg ratio remains chondritic and (ii) a “planetary” growth in which a significant increase of the Al/Mg ratio can be due to, for instance, olivine magmatic fractionation. The low δ 26 Mg* av value of olivines from the Eagle Station pallasite demonstrate that metal-silicate differentiation occurred as early as ~ 0. 15 - 0. 23 + 0. 29 Ma after CAIs in either of the growth scenarios. Similarly the variable δ 26 Mg* av values of refractory olivines can be understood if they were formed in planetesimals which started to differentiate as early as the Eagle Station parent body. Accretion of these planetesimals must have been coeval to the formation of CAIs and their disruption could explain why their fragments (Mg-rich olivines) were distributed in the chondrule forming regions of the disk. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Although extensively studied, the origin of chondrules, the major high-temperature component of primitive meteorites (i.e. chon- drites), remain highly debated (see review by Zanda, 2004). Textures and chemical compositions indicate that chondrules were formed by varying degrees of melting of solid precursors during brief, possibly repetitive, high temperature events (e.g. Jones et al., 2005). Melting has been proposed to occur either in a nebular setting or during collisions between planetesimals (see reviews by Boss, 1996; Desch et al., 2005; Hutchison et al., 2005; Sanders and Taylor, 2005). The shock-wave model (Ciesla and Hood, 2002; Desch and Connolly, 2002) is currently the most popular to explain repetitive melting events in the accretion disk, although the nature of shock waves is still under debate (Boss and Durisen, 2005). Also debated is the nature of chondrule precursors and the conditions of melting, i.e. open- or closed-system relative to the nebula gas (Hewins et al., 2005; Jones et al., 2005). The process that formed chondrules is exemplary of the complex- ity of the processes which took place in the accretion disk. The question of the origin of olivines in chondrules is an example of the kinds of questions that cosmochemists ask about the early solar system. While it is clear that Fe-rich olivines in porphyritic type II chondrules (for which iron is oxidized) crystallized from the chondrule melt, there is good chemical, isotopic and petrographic arguments to indicate that type II chondrules can also contain relict Mg-rich olivines (Pack et al., 2004; Ruzicka et al., 2007). Mg-rich olivines are widespread in porphyritic type I chondrules (for which iron is reduced) and it has been shown from textural, chemical and isotopic observations that they did not crystallize from the melt corresponding to the chondrule mesostasis (Chaussidon et al., 2008; Libourel et al., 2006). It has been recently suggested that porphyritic Earth and Planetary Science Letters 301 (2011) 107–116 ⁎ Corresponding author. Tel.: +33 3 83 59 42 11; fax: +33 3 83 51 17 98. E-mail addresses: johanv@crpg.cnrs-nancy.fr (J. Villeneuve), chocho@crpg.cnrs-nancy.fr (M. Chaussidon), libou@crpg.cnrs-nancy.fr (G. Libourel). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.10.025 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl