REVISION 3 1 Age discordance and mineralogy 2 3 Igor M. Villa 1,2 , John M. Hanchar 3 4 1 Institut für Geologie, Universität Bern, Baltzerstrasse 3, 3012 Bern, Switzerland; igor@geo.unibe.ch 5 2 Centro Universitario Datazioni e Archeometria, Università di Milano Bicocca, 6 piazza della Scienza 4, 20126 Milano, Italy 7 3 Department of Earth Sciences, Memorial University of Newfoundland, 9 Arctic Ave., St John's, NL A1B 3X5 8 Canada; jhanchar@mun.ca 9 10 11 Abstract 12 13 Observations of discordant ages, meaning that an age given by one mineral geochronometer is 14 different from the age given by another geochronometer from the same rock, began in the early days of 15 geochronology. In the late 1950s and 1960s, discordant U-Pb zircon ages were unquestioningly 16 attributed to Pb diffusion at high temperature. Later, the mineralogical properties and the petrogenesis 17 of the zircon crystals being dated was recognized as a key factor in obtaining concordant U-Pb ages. 18 Advances in analytical methods allowed the analysis of smaller and smaller zircon multigrain fractions, 19 then the analysis of individual grains, and even pieces of grains, with higher degrees of concordancy. 20 Further advances allowed a higher analytical precision, a clearer perception of accuracy, and a better 21 statistical resolution of age discordance. As for understanding the cause(s) of discordance, belief 22 revision followed the coupling of imaging, cathodoluminescence (CL) and back-scattered electrons 23 (BSE), to in situ dating by secondary ion mass spectrometry (SIMS), or by laser ablation inductively 24 coupled plasma mass spectrometry (LA-ICPMS). Discordant zircon and other accessory minerals (e.g., 25 This is a preprint, the final version is subject to change, of the American Mineralogist (MSA) Cite as Authors (Year) Title. American Mineralogist, in press. (DOI will not work until issue is live.) DOI: http://dx.doi.org/10.2138/am-2017-6084 Always consult and cite the final, published document. See http:/www.minsocam.org or GeoscienceWorld