Diffusive reequilibration of quartz-hosted silicate melt and fluid inclusions: Are all metal concentrations unmodified? Zoltan Zajacz a,c, * , Jacob J. Hanley b , Christoph A. Heinrich c , Werner E. Halter c , Marcel Guillong c a Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland, College Park, MD 20742, USA b Department of Geology, Saint Mary’s University, Halifax, Canada c Institute of Isotope Geochemistry and Mineral Resources, ETH Zu ¨ rich, Switzerland Received 8 September 2008; accepted in revised form 17 February 2009; available online 4 March 2009 Abstract Experiments were conducted to determine the extent and mechanism by which the composition of quartz-hosted silicate melt inclusions (SMI) and aqueous fluid inclusions (FI) can undergo post-entrapment modification via diffusion. Quartz slabs containing assemblages of SMI and FI were reacted with synthetic HCl bearing and metalliferous aqueous fluids at T = 500– 720 °C and P = 150–200 MPa. SMI from the single inclusion assemblages were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and electron probe microanalysis (EPMA) before and after the experiments. Anal- yses revealed that rapid diffusion of the univalent cations Na + , Li + , Ag + , Cu + and H + occurred through the quartz from the surroundings, resulting in significant changes in the concentrations of these elements in the inclusions. Concentrations of other elements with an effective ionic radius larger than that of Ag + , or multiple valence states were not modified in the inclu- sions during the experiments. Our results warn inclusion‘‘ researchers that the interpretation of Na, Li, Cu and Ag concen- trations from quartz-hosted SMI and FI should be treated critically. Ó 2009 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Quartz-hosted fluid inclusions (FI) and silicate melt inclu- sions (SMI) are commonly considered to preserve the compo- sition of hydrothermal fluids and evolved silicate melts, allowing natural processes that lead to the generation of ore deposits and volcanic eruptions to be traced and charac- terized chemically. However, observations of anomalously Na-deficient SMI compositions in natural samples (Aude ´tat et al., 2000; Aude ´tat and Pettke, 2003; Student and Bodnar, 2004; Zajacz et al., 2008) and modification of the Na, Cu and Ag content of natural SMI during experimental reheat- ing (Kamenetsky and Danyushevsky, 2005; Zajacz et al., 2008) raise the question as to whether inclusions in quartz reliably represent the composition of fluids and silicate melts at the time of their entrapment. No systematic experimental study has been carried out to determine if such fluid and melt inclusions can be compositionally modified after entrapment by diffusion-controlled cation exchange through the quartz host between the inclusions and the surroundings of their host crystals. Previous studies focused only on the diffusion of molecular H 2 and H 2 O in and out of the inclusions (Qin et al., 1992; Mavrogenes and Bodnar, 1994; Vityk et al., 2000; Severs et al., 2007). Aude ´tat and Gu ¨ nther (1999) stud- ied the compositional changes of fluid inclusions during their migration related to the deformation of quartz in natural sys- tems and found that besides water loss from the inclusions, the ratios of the analyzed cations remained constant with the exception of Li. The present study, describes a series of experiments con- ducted on natural samples to determine which element con- centrations can be significantly modified in quartz-hosted FI and SMI after their entrapment at P–T conditions typi- 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.02.023 * Corresponding author. Address: Laboratory for Mineral Deposits Research, Department of Geology, University of Mary- land, College Park, MD 20742, USA. E-mail address: zajacz@umd.edu (Z. Zajacz). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 3013–3027