Earth and Planetary Science Letters, 113 (1992) 397-409 397 Elsevier Science Publishers B.V., Amsterdam [CH] Closure temperatures of the Sm-Nd system in metamorphic garnets K. Mezger, E.J. Essene and A.N. Halliday Department of Geological Sciences, C.C. Little Building, University of Michigan, Ann Arbor, MI 48109-1063, USA Received December 17, 1991; revision accepted August 5, 1992 ABSTRACT Garnet-whole rock and garnet-mineral isochrons were determined on granulite facies gneisses and amphibolites from the Archean Pikwitonei Granulite Domain of the Superior Province, and the Proterozoic Central Gneiss Belt and Adirondack Highlands of the Grenville orogen. The Sm-Nd ages obtained from Archean garnets 0.1-0.5 cm in length are 30-110 Ma younger than the U-Pb ages obtained on the same garnets and also younger than the time of the last regional metamorphism, as determined by the growth ages of the youngest metamorphic garnets and zircons. Similarly, the Sm-Nd ages obtained from Proterozoic garnets with a diameter of 0.1-5 cm are younger than the time of the last regional metamorphism and similar or younger than cooling ages obtained on sphenes from the same sample or from the same geologic setting. Only the core of a garnet with a diameter of ca. 30 cm and without abundant inclusions may record the time of garnet growth. Comparison of the Sm-Nd ages with other geochronologic data and temperature estimates leads to the conclusion that the closure temperature for the Sm-Nd system in garnets analyzed in this study is ca. 600 + 30°C. Only garnets with radii much larger than 5 cm may record Sm-Nd growth ages in upper amphibolite facies rocks from slowly cooled terranes. Garnets from higher grade terranes yield cooling ages that define the retrograde history of metamorphic terranes. 1. Introduction Garnet is the single most important mineral for obtaining information on pressures and tem- peratures in medium to high grade metamorphic rocks [e.g., 1,2]. Garnet is also an important min- eral in many isogradic reactions and is one of the most widely used minerals for determining meta- morphic P-T paths [e.g., 3-9]. In addition, gar- net can be used to obtain geochronological infor- mation with the Sm-Nd [e.g., 10-15], U-Pb [16,17] and Rb-Sr [18] decay schemes. Thus, gar- net is the only major rock-forming mineral in metamorphic rocks that can yield high precision ages and, at the same time, provide information on P-T conditions during its formation. In this respect dating of garnet has the potential to pro- Correspondence to: K. Mezger, Max-Planck Institut fiir Chemie, SaarstraBe 23, D-6500 Mainz, Germany. vide a better understanding of metamorphic pro- cesses than dating of accessory minerals. This information is critical for the construction of quantitative P-T-t paths and to obtain informa- tion on rates of metamorphic processes. P-T-t paths are essential for constraining important tectonothermal processes involved in the evolu- tion of metamorphic terranes and thus large parts of the continental crust. In order to utilize garnet chronometers mean- ingfully it is necessary to obtain ages which are highly precise and to evaluate whether an age represents the time of mineral growth or some time along the cooling path. As a result of ad- vances in geochronological techniques, including sample preparation and mass spectrometry, it is now possible to obtain high precision Sm-Nd ages that have an uncertainty of less than 1% for most Proterozoic and Archean samples. Such precision is essential for defining events within a single metamorphic episode. Currently the major problem in the interpretation of mineral ages is 0012-821X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved