Bias in detrital zircon geochronology and thermochronometry Marco G. Malusà a, ⁎, Andy Carter b , Marta Limoncelli a , Igor M. Villa a,c , Eduardo Garzanti a a Department of Earth and Environmental Sciences, University of Milano-Bicocca, Milan, Italy b Department of Earth and Planetary Sciences, Birkbeck College, University of London, UK c Institute of Geological Sciences, University of Bern, Switzerland abstract article info Article history: Received 30 October 2012 Received in revised form 27 September 2013 Accepted 29 September 2013 Available online 9 October 2013 Editor: L. Reisberg Keywords: Zircon fission track dating Zircon U–Pb dating Radiation damage U concentration bias Etching bias European Alps Detrital studies that utilize zircon U–Pb geochronology and fission-track (FT) thermochronometry are subject to a range of potential sources of bias that should be properly evaluated and minimized. Some of them are common to any single-grain mineral analysis (e.g., variable bedrock mineral fertility, hydraulic sorting during transport, selective grain loss during sample processing), whereas others are intrinsic to zircon, and are related to radiation damage and age discordance. In this article, we quantify the impact of intrinsic bias on detrital studies thanks to the analysis of modern detritus shed from the European Alps, and illustrate the general implications on geological interpretations. We show that detrital zircon U–Pb age distributions based on statistically robust datasets are highly reproducible and representative of the parent bedrock ages in the catchment. Arbitrary or selective removal of discordant grain ages can be minimized by using the Kolmogorov–Smirnov test to identify an appropriate cutoff level. Loss of metamict (α-damaged) zircon has a minor impact on data representativeness, and is mainly controlled by regional metamorphism rather than by mechanical abrasion during river transport. Zircon FT grain-age distri- butions were found to have poor reproducibility, although age spectra are consistent with bedrock data. Howev- er, unlike the U–Pb datasets, U-rich zircon grains (N 1000 ppm) are systematically missed, and undatable grains may exceed 50%. We identify two major sources of distribution bias specific to zircon FT datasets: (i) sediment sources dominated by U-rich zircon grains are markedly underrepresented in the detrital record, because such grains often have uncountable high densities of fission tracks (“U concentration bias”); (ii) sediment sources that shed zircon grains with high levels of α-damage are underrepresented, because these grains are lost during chemical etching for FT revelation (“etching bias”). In the case of multimethod dating on the same grains (e.g., FT and U–Pb double dating), bias affecting detrital zircon FT dating propagates to the entire dataset. These effects may not impact on exhumation-rate studies that utilize the youngest grain ages (i.e., lag-time approach). How- ever, they represent a limiting factor for conventional provenance studies, and generally preclude application of zircon FT dating to sediment budget calculations. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Geochronological analyses of minerals extracted from clastic rocks are increasingly employed to solve a wide range of tectonic and landscape-evolution problems (e.g., Clift et al., 1996; Brandon et al., 1998; DeCelles et al., 2004; Miller et al., 2006; Whitchurch et al., 2011; Cawood et al., 2012). Zircon is the most frequently used mineral, be- cause it is found in a wide range of igneous, sedimentary and metamor- phic rocks, it is relatively resistant to weathering and abrasion, and can be dated by various isotopic methods, including U–Pb and fission-track (FT) dating (Košler and Sylvester, 2003; Bernet and Garver, 2005). Dif- ferent methods can be used to double date single grains (Carter and Bristow, 2003; Bernet et al., 2006), whereby the FT system generally constrains exhumation (Malusà et al., 2011a) and the U–Pb system chiefly dates magmatic crystallization or metamorphic growth (Dahl, 1997). Typical detrital zircon studies use geochronological data to detect (paleo)sources of detritus (Gehrels et al., 2011), constrain patterns of orogen erosion (Spiegel et al., 2004; Amidon et al., 2005), and quantify variations in mineral ages through a stratigraphic succession to infer the long-term exhumation history of eroded rocks (Garver et al., 1999; Bernet et al., 2001; Malusà et al., 2011a). The efficacy of such stud- ies rests on the assumption that detrital data are truly reproducible and representative of the parent bedrock. For example, detrital zircon FT studies that use the youngest age modes to monitor changes in source region erosion rate, i.e., the lag-time approach (Garver et al., 1999; Bernet et al., 2001) assume that any given dataset has indeed captured the youngest age population present in the source (Bernet et al., Chemical Geology 359 (2013) 90–107 ⁎ Corresponding author. E-mail address: marco.malusa@unimib.it (M.G. Malusà). 0009-2541/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2013.09.016 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo