Magnesium isotopic systematics of metapelite in the deep crust and implications for granite petrogenesis

doi: 10.7185/geochemlet.1508 Fluid-absent reactions that involve the breakdown of hydrous minerals produce granites in the continental crust. The minerals in the deep continental crust thus play a significant control on the chemical compositions of granites, but how and to what extent it may influence their isotopic signatures is poorly understood. Here we present Mg isotope data for a suite of amphiboliteto granulite-facies metapelites as well as the biotite and garnet minerals therein from the Ivrea Zone, NW Italy. The mineralogy of metapelites changes from biotitedominated in the amphibolite-facies to garnet-dominated in the granulite-facies. The bulk Mg isotopic compositions (δ26Mg = -0.23 to +0.20 ‰) do not correlate with metamorphic grade, indicating negligible Mg isotopic variation caused by metamorphism of metapelites. By contrast, the δ26Mg values of biotite vary widely from -0.08 to +1.10 ‰, and increase with metamorphic grade. Correspondingly, coexisting garnets become isotopically heavy (-1.22 to +0.10 ‰) as metamorphism proceeds, in order to equilibrate with the biotite, with a fractionation equation of 103 lnαbiotite-garnet = 0.96 × 106/T2, which can be used as a novel geothermometer. Our results indicate a nearly closed system for Mg isotopes of metapelites during metamorphism, and the bulk Mg isotopic compositions are therefore reconciled by the shifting garnet and biotite modes accompanied by increasing mineral δ26Mg values. The systematic Mg isotopic variation in the biotite and garnet implies a possible Mg isotope fractionation between melts and residues during biotite dehydration melting, which makes Mg isotopes a potential monitor of crustal melting and a tracer of granite petrogenesis. Received 17 April 2015 | Accepted 5 June 2015 | Published 22 June 2015 1. Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195-1310, USA * Corresponding authors (emails: S.-J. Wang sxw057@gmail.com; F.-Z. Teng fteng@u.washington.edu) 2. Department of Mineralogy and Petrology, Campus Fuentenueva, University of Granada, 18002 Granada, Spain Geochemical Perspectives Letters Letter Letter Geochemical Perspectives Letters Geochem. Persp. Let. (2015) 1, 75-83 | doi: 10.7185/geochemlet.1508 Geochem. Persp. Let. (2015) 1, 75-83 | doi: 10.7185/geochemlet.1508 77 78 Montero, 1999; Luvizotto and Zack, 2009; Redler et al., 2011). The modal abundance of garnet in metapelites increases at the expense of biotite with increasing metamorphic grade following the metamorphic reaction: biotite + sillimanite + plagioclase + quartz → garnet + K-feldspar + rutile + melt. This reaction proceeds until the biotite is almost entirely consumed in granulite-facies metapelites (Bertolani and Garuti, 1970; Schmid and Wood, 1976). The metapelites in the Ivrea Zone are divided into three zones from low to high metamorphic grade based on the biotite/garnet ratio (Bea and Montero, 1999): the kinzigite zone that corresponds to metapelites with modal biotite/garnet >2, the transition zone in which the modal biotite/garnet is between 0.5 and 2, and the stronalite zone where modal biotite/ garnet is <0.5 (Fig. S-1). Twelve representative samples of metapelites and garnetbiotite mineral pairs from the three zones were studied for Mg isotopes (Fig. S-1).