Portrait of a reference material: Zircon production in the Middledale Gabbroic Diorite, Australia, and its implications for the TEMORA standard Kieran A. Iles a, ⁎, Janet M. Hergt a , Keith N. Sircombe b , Jon D. Woodhead a , Simon Bodorkos b , Ian S. Williams c a School of Earth Sciences, The University of Melbourne, Parkville, Victoria 3010, Australia b Minerals and Natural Hazards Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2609, Australia c Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia abstract article info Article history: Received 17 October 2014 Received in revised form 26 February 2015 Accepted 27 February 2015 Available online 8 March 2015 Editor: K. Mezger Keywords: Zircon O–Hf isotopes U–Pb geochronology Element mapping SHRIMP LA-MC-ICPMS The Middledale Gabbroic Diorite (MGD) in New South Wales, Australia, is host to the internationally distributed TEMORA 2 zircon reference material and its prototype, TEMORA 1. The original characterisation of the source of the reference zircons revealed that the more altered TEMORA 2 host rock contains an order of magnitude more zircon than the TEMORA 1 host, despite similar bulk-rock Zr concentrations. Although TEMORA 1 and 2 preserve the same U–Pb age, they are variable in oxygen isotope composition. In this study, petrographic observations in concert with bulk-rock and mineral geochemistry and zircon U–Pb geochronology have been applied to new samples of the MGD to investigate the link between alteration and zircon abundance. Trace element maps reveal that the products of late-stage, deuteric alteration (particularly actinolite after hornblende) are depleted in Zr, and many other trace elements, relative to the unaltered mafic, magmatic phases. It is posited that the conversion of hornblende to secondary amphiboles in the latter stages of magma solidification liberated Zr, permitting the crystallisation of additional zircon. New high resolution SIMS U–Pb determinations on four samples confirm the age homogeneity of the zircon across the pluton and reaffirm the value of TEMORA 2 as a valuable geochronological reference material. Zircon oxygen isotope data have been acquired for these same samples and the mean δ 18 O values encompass the accept- ed values for TEMORA 1 and 2. Likewise, the Hf-isotope determinations are similar to the accepted TEMORA 2 composition. Together with petrographic observations, these data reveal the TEMORA 2 zircon and its host to be broadly reflective of the relatively coarse-grained portions of the MGD, and that the isotopically less evolved compositions (i.e. the lower δ 18 O of TEMORA 1 and low δ 18 O, high 176 Hf/ 177 Hf of one sample from this study) are associated with a relatively fine-grained, marginal lithology. Given δ 18 O values greater than typical mantle- derived zircon and the broad correlation between O- and Hf-isotopic compositions, the data imply the pluton evolved by crustal contamination of a primitive magma. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Well characterised zircon reference materials are required as primary calibrants in the acquisition of zircon U–Pb ages and Hf- and O-isotope data, as well as for assessment of acquired data quality as sec- ondary reference materials. There are three main requirements of an ideal zircon reference material as follows: (1) accurately and precisely established data (age, Pb/U, Hf isotope ratio etc., according to purpose), determined independently; (2) homogeneity from the intergranular to submicron scale of the relevant characteristic; and, (3) sufficient abun- dance and availability for indefinite use (Black et al., 2003; Woodhead and Hergt, 2005; Sláma et al., 2008). Depending upon the isotope system being used, other criteria may also be required; therefore, the use of multiple reference zircons by a single laboratory is often necessary (Black, 2005; Woodhead and Hergt, 2005; Stern et al., 2009). Historically, potential zircon reference materials have been rigorous- ly characterised to assess their suitability for U–Pb dating, and subse- quently co-opted for use as multi-isotope reference materials to meet the demands of the rapid advancement of in situO- and Hf-isotopic anal- ysis. Unfortunately, characterisation of the O and Hf isotope systems has not always involved rigorous testing to ensure that the same level of homogeneity determined for the U–Pb system is preserved. The TEMORA zircons (TEMORA 2, and its predecessor, TEMORA 1) are an ex- ample of such reference materials that have been shown to be valuable based on the three criteria summarised above (see the following for comparisons of reference zircons: Black et al., 2003, 2004; Ickert et al., 2008; Woodhead and Hergt, 2005). Hence, these reference materials have been utilised widely in the geological research community Chemical Geology 402 (2015) 140–152 ⁎ Corresponding author. Tel.: +61 83449596. E-mail addresses: kiles@student.unimelb.edu.au (K.A. Iles), jhergt@unimelb.edu.au (J.M. Hergt), keith.sircombe@ga.gov.au (K.N. Sircombe), jdwood@unimelb.edu.aua (J.D. Woodhead), simon.bodorkos@ga.gov.au (S. Bodorkos), ian.williams@anu.edu.au (I.S. Williams). http://dx.doi.org/10.1016/j.chemgeo.2015.02.036 0009-2541/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo