GZ7 and GZ8 – Two Zircon Reference Materials for SIMS U‐Pb Geochronology

Abstract

Here, we document a detailed characterisation of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U‐Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean 206Pb/238U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure‐related parameters correspond well with the calculated alpha doses of 1.48 × 1018 g−1 (GZ7) and 2.53 × 1018 g−1 (GZ8), respectively, and the (U‐Th)/He ages of 438 Ma ± 3 Ma (2s) for GZ7 and 426 Ma ± 9 Ma (2s) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U mass fractions are 680 μg g−1 (GZ7) and 1305 μg g−1 (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U‐Pb geochronology. In addition, GZ7 (Ti mass fractions 25.08 μg g−1 ± 0.18 μg g−1; 95% confidence uncertainty) may prove useful as reference material for Ti‐in‐zircon temperature estimates.

SA01 – A Proposed Zircon Reference Material for Microbeam U‐Pb Age and Hf‐O Isotopic Determination

“…Although the seller informed us that the SA01 zircon megacryst was collected from the KwaZulu‐Natal Province, South Africa, zircons with similar geochemical features have not been found in KwaZulu‐Natal so far. As shown in Table , SA01 is quite different from the now fairly large suite of similar aged Sri Lankan zircons that have been published [SL7 (Kinny et al ), CZ3 (Pidgeon et al , Xu et al , Cavosie et al ), M257 (Nasdala et al , Li et al , Hu et al ), BR266 (Stern , Woodhead et al , Liu et al ), BB9 (Santos et al ), GZ7/GZ8 (Nasdala et al ), M127 (Nasdala et al ) and LGC‐1 (Tian et al )]. In this regard, the genesis of the SA01 zircon megacryst may be different from those of the Sri Lankan zircon megacrysts.…”

“…In spite of significant differences in trace element mass fractions, different SA01 zircon shards are characterised by consistent chondrite‐normalised REE patterns, namely depletion in LREE, enrichment in HREE, positive Ce anomaly and negligible Eu anomaly. Compared with the widely used zircon reference materials 91500 (Wiedenbeck et al ), Mud Tank (Woodhead and Hergt ), CZ‐3 (Pidgeon et al ), OG1 (Stern et al , Kemp et al ), Temora (Black et al ), Plešovice (Slama et al , Ver Hoeve et al ), GJ‐1 (Jackson et al , Piazolo et al ), GZ7/GZ8 (Nasdala et al ) and Qinghu (Li et al ), SA01 zircon is distinct with high Ti mass fractions and Th/U ratios (Figure f; Kirkland et al ). It is very common that zircons crystallised in alkaline magmas have high Ti mass fractions and high Th/U ratios (Fu et al , Peng et al , Zhu et al ).…”

“…Chondrite‐normalised REE patterns of intra‐grain analyses of three shards in (a–d) session 1 and (e) 2; (f) Th/U ratios versus Ti contents (μg g −1 ) of different zircon reference materials and SA01 zircon (data for zircon reference materials are from Li et al , Kirkland et al , Piazolo et al , Nasdala et al , and Ver Hoeve et al ; alkaline rocks data from Peng et al ). [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Zircon (ZrSiO 4 ), a ubiquitous accessory mineral in a wide variety of sedimentary, igneous and metamorphic rocks, is extremely resistant to later geological processes and can survive most post‐crystallisation thermal disturbances (e.g., Cherniak and Watson , Yang et al , Wang et al ). It is a mineral well suited not only for U‐Pb geochronology, but also for trace element and Li, O and Hf isotopic studies using microbeam techniques such as LA‐(MC)‐ICP‐MS (Longerich et al , Norman et al , Jackson et al , Klötzli et al , Arevalo , Schaltegger et al , Sylvester and Jackson ) and SIMS (Compston et al , Williams , Cherniak and Watson , Valley , Nasdala et al ). However, microbeam analysis, including LA‐(MC)‐ICP‐MS and SIMS, is a comparative method.…”

“…A suitable natural reference material should be homogeneous in both chemical and isotopic compositions at the scale of analysis. Previous studies have identified a number of natural zircons as reference materials suitable for geochronology and isotope geochemistry, including SL7 (Kinny et al ), CZ3 (Pidgeon et al ), SL13 (Claoué‐Long et al ), 91500 (Wiedenbeck et al ), Temora (Black et al ), BR266 (Stern ), GJ‐1 (Jackson et al , Morel et al ), Mud Tank (Woodhead and Hergt ), M257 (Nasdala et al ), Plešovice (Slama et al ), Sri Lanka (Gehrels et al ), OG1/OGC (Stern et al , Kemp et al ), Penglai (Li et al ), Qinghu (Li et al ), OD‐3 (Iwano et al ), M127 (Nasdala et al ), BB (Santos et al , Lana et al ), LGC‐1 (Tian et al ), GZ7/GZ8 (Nasdala et al ), LKZ‐1 (Cheong et al ) and GHR1 (Eddy et al ). However, most of them are either exhausted or have limited availability against the increasing demands of the microbeam laboratories.…”