Zircon M127 – A Homogeneous Reference Material for SIMS U–Pb Geochronology Combined with Hafnium, Oxygen and, Potentially, Lithium Isotope Analysis

Abstract

IntroductionThe development of the SIMS (secondary ion mass spectrometry) technique opened the world of high spatial resolution determination of U-Pb ages from polished zircon surfaces (e.g., Andersen and Hinthorne 1972; Compston et al. 1984; Williams 1998). The SIMS technique is a comparative method and the determination of accurate isotopic ratios in zircon requires calibration of the unknown results against the results of a well-characterized calibrant material measured under identical conditions during the same analytical session. In principle either a natural or synthetic reference material can be used as such a calibrant; unfortunately wellperforming synthetic reference standards for the U-Pb analysis of zircon are not available. This is due to two main reasons. First, no synthesis technique is currently available that allows one to produce homogeneous Pb-bearing zircon crystals, owing to the exceptionally low partition coefficient of Pb in zircon. Second, glass reference materials cannot be used due to their different sputtering behavior under the oxygen-ion beam, compared to that of the zircon crystals to be analysed. Consequently, high-quality natural zircon reference materials must be used, and are therefore increasingly sought.Any potentially suitable reference zircon needs to be characterized thoroughly in order to provide users with the best currently obtainable values, with regards to precision and accuracy, for the isotope ratios and concentrations of the calibrant material. Also, a well-performing reference zircon needs to be exceptionally homogeneous (Pidgeon 1997, Kennedy 2000, because each individual chip, and micro-areas within chips, of the reference material must have the chemical, physical and isotopic properties of the entire, previously characterized, bulk material. Another important characteristic that determines the quality of a reference material is its reliability during analysis. In U-Pb geochronology, higher count rates -in particular for the radiogenic Pb isotope, which is always lower in abundance than the parent U isotope -lead to better Poisson statistics. This may allow the operator to shorten the analysis time and/or to decrease the spot diameter without loss of precision. A suitable SIMS U-Pb reference should therefore contain sufficiently high concentrations of U and radiogenic Pb (whereas the concentration of non-radiogenic Pb should be negligible). However, higher levels of radiogenic Pb correlate with elevated radiation damage and this leads to an analytical difficulty, as the secondary ion sputtering behaviour under the ion beam may be different for the unknowns and reference if the structural states are sufficiently different. In searching for suitable U-Pb zircon references it is therefore advantageous to find gem-quality specimens that have some, but not too much radiation damage. Zircon M127 fulfils all of these important prerequisites. Zircon M127contains notably higher levels of U and radiogenic Pb isotopes than most other SIMS references, resulting in a somewhat ...

Quantifying Isotopic Heterogeneity of Candidate Reference Materials at the Picogram Sampling Scale

“…) or 0.092‰ (1 s , n = 100, Nasdala et al . ) on silicates, which is reasonably typical of SIMS of this specification. Briefly, a 2.5 nA 133 Cs + primary beam, operating in Gaussian mode, was focused to a ~ 5‐μm‐diameter spot on the surface of the highly polished sample.…”

“… and Nasdala et al . ). When the results from a single analysis are based on an integrated mean for the full 80 s of data acquisition, variations in secondary ion emission conditions (sample charging, primary ion flickering, etc.)…”

“…, δ 18 O repeatability of 0.07‰ on n = 10 zircon measurements; Nasdala et al . , δ 18 O repeatability of 0.07‰ on n = 39 zircon measurements). Such fine‐scale sampling is often crucial for understanding geochemical processes in complex natural materials; however, this fundamental advantage of in situ microanalysis is accompanied by a need for well‐characterised reference materials (RMs) that are fit‐for‐purpose when operating at micrometre sampling dimensions.…”

Contrasting sources of Late Paleozoic rhyolite magma in the Polish Lowlands: evidence from U–Pb ages and Hf and O isotope composition in zircon

“…We also used the Potsdam CAMECA 1280-HR instrument to perform δ 18 O determinations on zircon, largely following the method described in Nasdala et al (2016). Briefly, the SIMS analyses employed a 2.5 nA, 133 Cs + primary ion beam with a Gaussian intensity distribution with a total impact energy of 20 keV which was rastered over a 10 µm area with the dynamic transfer circuitry activated.…”

GGR Biennial Critical Review: Analytical Developments Since 2014

“…Nasdala et al . () proposed the use of a new zircon reference material, M127, which was chemically characterised by electron probe and laser ICP‐MS. The homogeneity of δ 18 O was checked by multiple measurements by SIMS.…”