GHR1 – A NEW EOCENE NATURAL REFERENCE MATERIAL FOR U-PB AND HF ISOTOPIC MEASUREMENTS IN ZIRCON

We present chemical abrasion-isotope dilution-thermal ionization (CA-ID-TIMS) U-Pb zircon geochronology and solution multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) Hf isotopic data from a proposed natural zircon reference material for use during in situ analyses of U-Pb and Hf isotopic ratios. The sample, GHR1, was collected from the rapakivi intrusive phase of the Eocene Golden Horn batholith in Washington, USA. Zircons separated from this sample range up to 250-300 μm in length and have moderate aspect ratios. A weighted mean of 15 Th-corrected ²⁰⁶Pb/²³⁸U zircon dates from GHR1 produced at the Massachusetts Institute of Technology is 48.132 ± 0.023 Ma (2σ analytical and tracer uncertainties only, MSWD=1.70) confirming that there is little or no inter-crystal age heterogeneity at the scale of a few 10 kyr. Solution MC-ICP-MS measurements of chemically purified aliquots give a ¹⁷⁶Hf/¹⁷⁷Hf weighted mean of 0.283050 ± 17 (2σ, n=10), corresponding to a εHf₀ of ca. +9.3. The 2σ variability of these measurements is comparable to our reproducibility of the JMC-475 Hf isotopic standard 0.282160 ± 14 (n= 13), suggesting that GHR1 zircons are homogenous with respect to ¹⁷⁶Hf/¹⁷⁷Hf. In situ ²⁰⁶Pb/²³⁸U dates from collaborating secondary ion mass spectrometry (SIMS), sensitive high-resolution ion microprobe (SHRIMP), and laser ablation ICP-MS (LA-ICP-MS) laboratories are in excellent agreement with the CA-ID-TIMS date and illustrate the reproducibility and potential value of this reference zircon. The mean values of ¹⁷⁶Hf/¹⁷⁷Hf measurements from two LA-ICP-MS laboratories are in agreement with the solution MC-ICP-MS value, but show slightly greater dispersion and higher (Lu+Yb)/Hf values. We attribute this discrepancy to apatite inclusions that are high in REE and may lead to greater isobaric interferences on ¹⁷⁶Hf. These inclusions and potential isobaric interferences from REE were removed during the chemical abrasion step prior to bulk dissolution and ion-exchange purification in the solution data. Nevertheless, the apparent isotopic homogeneity and reproducibility of ²⁰⁶Pb/²³⁸U and ¹⁷⁶Hf/¹⁷⁷Hf ratios and the potentially unlimited reserves of GHR1 suggest that it is a promising reference material. We plan to distribute GHR1 to interested laboratories at the meeting.