Constraints on the loss of matrix-sited helium during vacuum crushing of mafic phenocrysts P.-H. Blard a, * , N. Puchol b , K.A. Farley a a Division of Geological and Planetary Sciences, MC 100-23, California Institute of Technology, Pasadena, CA 91125, USA b Ecole Normale Supe ´rieure de Lyon, 69364 Lyon, France Received 21 December 2007; accepted in revised form 24 May 2008; available online 5 June 2008 Abstract Vacuum crushing is an efficient technique to selectively release the mantle-derived helium component trapped within oliv- ine and pyroxene phenocrysts. However, contrary to previous assumptions, recent studies have shown that this method may liberate significant matrix-sited cosmogenic 3 He ( 3 He c ) or radiogenic 4 He ( 4 He * ). Because this loss may bias both the deter- mination of magmatic 3 He/ 4 He ratios and the accuracy of 3 He c measurements, it is essential to understand what mechanism is responsible and under what conditions matrix helium loss is manifest. To address this question, olivines and pyroxenes with various amounts of matrix-sited 3 He (from 10 7 to 10 11 at. g 1 ) were crushed in air or in vacuum using several crushing devices. Sample temperature was controlled during each crushing experiment, and ranged from 25 to 325 °C. The resulting powders were then sieved to obtain several homogeneous grain fractions ranging between <10 and >300 lm. The 3 He c concentrations measured in each fraction clearly show that significant 3 He c loss (>20%) affects only the finest fraction (<10 lm) and, impor- tantly, only under hot conditions (here T P300 °C). Even the smallest fractions (<10 lm) quantitatively retain matrix-sited 3 He c when crushed under cold conditions (T 625 °C), regardless of the duration and energy of crushing. These results inval- idate the model previously proposed by (Yokochi R., Marty B., Pik R. and Burnard P. (2005) High 3 He/ 4 He ratios in peri- dotite xenoliths from SW Japan revisited: evidence for cosmogenic 3 He released by vacuum crushing. Geochem. Geophys. Geosyst. 6, doi:10.1029/2004GC000836) that involved spallation tracks and implied that the magnitude of loss was mainly controlled by the grain size. Moreover, new diffusion experiments were carried out to constrain the diffusivity of matrix-sited helium in crushed olivines. When used to model diffusive 3 He c loss as a function of grain size during crushing, these new data predict the observed release fairly well. Therefore, we conclude that temperature-enhanced volume diffusion is one of the main mechanisms controlling the release of 3 He c during crushing. For future applications, special attention should thus be paid to control the grain size, the crushing duration, and the temperature of the sample. Ó 2008 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Helium isotopes trapped in inclusions in mafic pheno- crysts are a very useful geochemical proxy for characterizing mantle sources (e.g., Kurz et al., 1982; Stuart et al., 2003). In addition, cosmogenic 3 He in the matrix of mafic phases (Kurz, 1986a) is a useful tool for understanding Earth-sur- face processes, not only because its nuclear stability permits quantitative constraints over several million years of expo- sure (Blard et al., 2006a), but also because its low detection limit compared to its high production rate allow quantifica- tion of young exposure events (<10 ka) (e.g., Kurz et al., 1990). However, accurate determination of magmatic helium isotopic ratios and cosmogenic 3 He ( 3 He c ) abundances re- quire very specific analytical conditions to isolate each of these helium components. In the case of olivines and pyrox- enes, which are the most common minerals used for such applications, vacuum crushing has been widely adopted be- cause of its supposed ability to selectively release the mag- matic helium contained within fluid and melt inclusions, 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.05.044 * Corresponding author. E-mail address: blard@gps.caltech.edu (P.-H. Blard). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 3788–3803