Can magmatic zircon be distinguished from hydrothermal zircon by trace element composition? The effect of mineral inclusions on zircon trace element composition Shihua Zhong a,b, ⁎, Chengyou Feng b, ⁎⁎, Reimar Seltmann c , Daxin Li b , Hongying Qu b, ⁎⁎ a Key Lab of Submarine Geosciences and Prospecting Techniques, Ministry of Education, College of Marine Geosciences, Ocean University of China, Qingdao 266100, China b MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China c Centre for Russian and Central EurAsian Mineral Studies, Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK abstract article info Article history: Received 19 March 2018 Accepted 27 June 2018 Available online 03 July 2018 Mineral inclusions, e.g., apatite, titanite, monazite, K-feldspar, are common in magmatic zircons. Although many studies mention that light rare earth element (LREE) contents of zircons could be compromised by an analytical artefact of the accidental sampling of mineral inclusions, how and to what degree these inclusions influence anal- ysis of zircon composition is still not well constrained. Here we report U–Pb ages and trace element abundances for zircon crystals, where apatite and K-feldspar inclusions are observed, from diorite porphyry in the Weibao de- posit, East Kunlun Mountains, Northern Tibetan Plateau. Although zircon morphological and chronological evi- dence consistently advocates a magmatic origin without undergoing significant hydrothermal alteration, 7 of 15 analytical spots show LREE-enriched patterns and low Ce/Ce* ratios which are comparable to those for pub- lished “hydrothermal” zircon. Quantitative modelling in this study manifests that these LREE-enriched patterns and low Ce/Ce* ratios can be achieved with only 0.1 to 2 vol% contamination from sub-micrometer apatite inclu- sions, which in practice are hard to monitor under the LA–ICP–MS (normally with large pit diameter and depth) and conventional microscopes. Titanite, monazite, xenotime, and allanite have similar roles to apatite, and LREE contents of zircon can be significantly elevated with only 0.05 vol% contamination from these inclusions. We therefore suggest that the widely used geochemical discrimination criteria for magmatic and hydrothermal zir- con, e.g., (Sm/La) N vs. La and Ce/Ce* vs. (Sm/La) N diagrams and the degree of Ce anomalies, are ambiguous since they are extremely susceptible to contamination by mineral inclusions, and that within single samples only Ce 4+ / Ce 3+ values calculated from zircons of low LREE values probably represent the oxidation state of magmas. © 2018 Elsevier B.V. All rights reserved. Keywords: Magmatic zircon Hydrothermal zircon Mineral inclusion REE concentration Ce/Ce* 1. Introduction Zircon is a common but important accessory mineral in natural rocks. It is particularly of great value in geochronology owing to its high U and Th contents and relatively high closure temperature for U–Th–Pb sys- tems (Cherniak and Watson, 2001; Lee et al., 1997), as well as its dura- bility to chemical and physical weathering (Burnham and Berry, 2012). Its Hf–O isotopic composition can provide a robust indicator for many geological processes, such as crustal assimilation (Kemp et al., 2007), in- teraction with meteoric waters (Westhues et al., 2017), and crustal recycling (Van Kranendonk and Kirkland, 2013). Its trace element com- position can provide a proxy for melt and/or fluid oxygen fugacity from which they precipitate and therefore it is of great significance in mineral exploration of intrusion-related deposits (e.g., porphyry Cu and skarn deposits). Besides, it is also a potential host material for the immobiliza- tion of weapons-grade plutonium (Ewinga et al., 1995). Zircon can be of magmatic, hydrothermal or metamorphic origin. Magmatic zircon provides an excellent and detailed record of the near-solidus crystallization history of silicic magmas (e.g. Szymanowski et al., 2017), whereas hydrothermal zircon (or, zircons which crystallized from fluid saturated melts during the magmatic–hy- drothermal transition) can be used to date subsequent fluid-infiltration events and water-rock interaction (Hoskin, 2005). Metamorphic zircon formed under metamorphic conditions by sub-solidus or other pro- cesses that commonly record oxygen isotope exchange with the host rock or fluids (Cavosie et al., 2011). In this study, our focus is exclusively on the discrimination between magmatic and hydrothermal zircon, Lithos 314–315 (2018) 646–657 ⁎ Corresponding author at: College of Marine Geosciences, Ocean University of China, Qingdao 266100, China. ⁎⁎ Corresponding authors at: Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China. E-mail addresses: li.zhong@ouc.edu.cn, (S. Zhong), fengchy@cags.ac.cn, (C. Feng), hongyingqu@126.com (H. Qu). https://doi.org/10.1016/j.lithos.2018.06.029 0024-4937/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos