Portrait of a reference material: zircon production in the Middledale Gabbroic Diorite, Australia, and its implications for the TEMORA standard

Abstract

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 δ18O values encompass the accepted 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 δ18O of TEMORA 1 and low δ18O, high 176Hf/177Hf of one sample from this study) are associated with a relatively fine-grained, marginal lithology. Given δ18O 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.