Zircon Reference Research Articles

Evaluation of a Novel Femtosecond Laser Ablation System for In Situ Analysis Based on Two-Dimensional Galvanometer Scanners.

Uneven energy distribution of femtosecond lasers presents a significant challenge for single-spot analysis, which often leads to concave ablation craters. This study assesses the performance of a femtosecond laser ablation system for in situ analysis using novel galvanometer scanners. A galvanometer rapidly moved the laser beam focus to create craters with a small beam spot. We first examined the inductively coupled plasma mass spectrometry (ICP-MS) signal sensitivity to laser parameters, establishing a strong linear correlation with the laser energy, repetition rate, scanning ablation area, and galvanometer scanning frequency. Elemental fractionation analysis of NIST SRM 610 suggests minimal bias, with fractionation indices of different elements approaching unity. Subsequently, the elemental concentration of six reference material glasses was measured by fsLA-ICP-MS to evaluate the elemental quantification capabilities of the Galvo-femtosecond laser (Galvo-fsLA). The laser's capability for in situ U-Pb dating was demonstrated by concordant U-Pb ages of five zircon reference materials that are highly consistent with the ID-TIMS ages reported previously. Finally, the reliability of the new Galvo-fsLA for isotope analysis was verified by the accurate determination of radiogenic Hf, Pb isotopes, and stable Cu isotopes, all agreeing well with their reference values within uncertainties. These assessments underscore the significant potential of Galvo-fsLA for enhanced accuracy and precision of single-spot in situ analysis.

Effect of chemical abrasion of zircon on SIMS U–Pb, δ18O, trace element, and LA-ICPMS trace element and Lu–Hf isotopic analyses

Abstract. This study assesses the effect of chemical abrasion on in situ mass spectrometric isotopic and elemental analyses in zircon. Chemical abrasion improves the U–Pb systematics of SIMS (secondary ion mass spectrometry) analyses of reference zircons, while leaving other isotopic systems largely unchanged. SIMS 206Pb/238U ages of chemically abraded reference materials TEMORA-2, 91500, QGNG, and OG1 are precise to within 0.25 % to 0.4 % and are within uncertainty of chemically abraded TIMS (thermal ionization mass spectrometry) reference ages, while SIMS 206Pb/238U ages of untreated zircons are within uncertainty of TIMS reference ages where chemical abrasion was not used. Chemically abraded and untreated zircons appear to cross-calibrate within uncertainty using all but one possible permutation of reference materials, provided that the corresponding chemically abraded or untreated reference age is used for the appropriate material. In the case of reference zircons QGNG and OG1, which are slightly discordant, the SIMS U–Pb ages of chemically abraded and untreated material differ beyond their respective 95 % confidence intervals. SIMS U–Pb analysis of chemically abraded zircon with multiple growth stages is more difficult to interpret. Treated igneous rims on zircon crystals from the S-type Mount Painter Volcanics are much lower in common Pb than the rims on untreated zircon grains. However, the analyses of chemically abraded material show excess scatter. Chemical abrasion also changes the relative abundance of the ages of zircon cores inherited from the sedimentary protolith, presumably due to some populations being more likely to survive the chemical abrasion process than others. We consider these results from inherited S-type zircon cores to be indicative of results for detrital zircon grains from unmelted sediments. Trace element, δ18O, and εHf analyses were also performed on these zircons. None of these systems showed substantial changes as a result of chemical abrasion. The most discordant reference material, OG1, showed a loss of OH as a result of chemical abrasion, presumably due to dissolution of hydrous metamict domains or thermal dehydration during the annealing step of chemical abrasion. In no case did zircon gain fluorine due to exchange of lattice-bound substituted OH or other anions with fluorine during the HF partial dissolution phase of the chemical abrasion process. As the OG1, QGNG, and TEMORA-2 zircon samples are known to be compositionally inhomogeneous in trace element composition, spot-to-spot differences dominated the trace element results. Even the 91500 megacrystic zircon pieces exhibited substantial chip-to-chip variation. The light rare earth elements (LREEs) in chemically abraded OG1 and TEMORA-2 were lower than in the untreated samples. Ti concentration and phosphorus saturation ((Y + REE) / P) were generally unchanged in all samples.

A preliminary investigation into the feasibility of laser ablation U-Pb isotope ratio measurement via all-faraday cup detection with 1011- and 1013-Ω amplifiers on the Neoma multicollector-inductively coupled plasma-mass spectrometer.

Signal detection for uranium-lead (U-Pb) dating of zircon is typically performed via ion counters. Here, we develop a preliminary understanding of the strengths and limitations of faraday-cup-based detection. A suite of zircon reference materials and the NIST-610 glass were sampled using laser ablation followed by U-Pb isotope ratio measurement on a Neoma multicollector-inductively coupled plasma-mass spectrometer. We were able to produce geologically accurate 207Pb/206Pb, 206Pb/238U, and 207Pb/235U ratios for the NIST-610 glass and the zircon standards, with ages ranging from ~2.5Ga to ~337 Ma (TanBrown A, Oracle, 91550, Mud Tank, Temora, and Plešovice). Two of the younger zircon standards examined (94-35, ~55.6Ma, and Fish Canyon, 28.6Ma) yielded accurate 206Pb/238U but not 207Pb/235U or 207Pb/206Pb ratios, whereas the youngest zircon standard (Penglai, ~4.4Ma) failed for all three ratios of interest. The accuracy and precision of the all-faraday method are directly tied to signal intensity, with reliable data capable of being produced even when both isotopes in a ratio have signals below ~0.001 V (equivalent to ~62 500 cps on an ion counter). The all-faraday cup multicollection method provides sufficient sensitivity to obtain geologically meaningful U-Pb data, with possible advantages being that laser pit depth-dependent changes in the observed interelemental fractionation behavior may be easier to correct using a static collector configuration compared to when the ion beam is swept across a single detector while also removing the need for an interdetector-type calibration. Further work is needed to refine the all-faraday cup method (e.g., application of background subtraction and common Pb corrections, outlier removal, and interelement as well as down-hole fractionation corrections), but our initial results demonstrate that the faraday detector method has sufficient sensitivity to warrant further study.

Simultaneous U–Pb and U–Th Dating Using LA-ICP-MS for Young (<0.4 Ma) Minerals: A Reappraisal of the Double Dating Approach

Simultaneous U–Pb and U–Th dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was performed on the ca. 0.1 Ma Toya tephra and the ca. 0.08 Ma SS14-28 U–Th zircon reference material. In U–Pb dating, both Th/U and Pa/U partitioning between magma and minerals were considered. In U–Th dating, both abundance sensitivity and molecular interferences on 230Th were reevaluated. As a result, the Toya tephra yielded an accurate weighted mean U–Pb age of 0.103 ± 0.029 Ma (2σ) using zircon and monazite. Conversely, the SS14-28 zircon yielded an inaccurate U–Pb age (0.25 ± 0.10 Ma), which was attributed to low 206Pb signal intensity. Both the Toya tephra zircon and the SS14-28 zircon yielded accurate U–Th model ages of 0.108 ± 0.014 Ma and 0.078 ± 0.007 Ma, respectively. The agreement of U–Pb and U–Th ages for Toya indicates that simultaneous U–Pb and U–Th dating is possible and viable. The inappropriate age of SS14-28 U–Pb age and appropriate U–Th model age also indicates it is preferable to apply both U–Pb and U–Th dating simultaneously for young (<0.4 Ma) zircons to check internal consistency. The proposed double dating approach may be especially useful for small grains when it otherwise would be impossible to obtain multiple ages from a single grain. By adopting simultaneous U–Pb and U–Th dating using LA-ICP-MS, zircon crystallization ages as old as 4.5 Ga to as young as 0.1 Ma (or even younger) can be obtained in a quick and cost-effective manner with a reasonable (~5% at 1σ) uncertainty.

Laser ablation MC-ICPMS U-Th and U-Th-Pb dating of Quaternary zircons from Jeju Island, Korea

BackgroundLaser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) emerged in the mid-1980s and rapidly became a crucial dating tool. The advent of femtosecond LA systems has substantially reduced volatility-dependent mass fractionation. This study showcases U-Th and U-Th-Pb dating results of Quaternary zircons collected from Jeju Island, Korea, utilizing an advanced femtosecond laser-connected multi-collector (MC)-ICPMS.FindingsZircon grains from trachyte samples near the Baeknokdam lake (JJ616-1), Yeongsil (JJ08-1), Chunwangsa (JJ09-1), and Oraidong (JJ09-3) provided weighted mean 238U-230Th ages of 28.7 ± 1.6 ka (n = 56/64, MSWD = 3.8), 81.8 ± 10.9 ka (n = 11/12, MSWD = 1.6), 92.6 ± 4.6 ka (n = 49/51, MSWD = 2.2), and 117.6 ± 8.2 ka (n = 48/50, MSWD = 3.2), respectively. The age determination for JJ08-1 zircon aligned well with the recommended value (82 ± 6 ka). Zircons from Sanbangsan (JJ615-1) and Wonmansa (JJ08-2) trachytes yielded common Pb and radioactive disequilibrium-corrected weighted mean 238U-206Pb ages of 785 ± 5 ka (n = 27/28, MSWD = 0.90) and 743 ± 8 ka (n = 28/30, MSWD = 0.79), respectively. The weighted mean 238U-206Pb ages of Penglai and 61.308 reference zircons were determined to be 4226 ± 21 ka (n = 22/25, MSWD = 3.8) and 2488 ± 20 ka (n = 19/20, MSWD = 1.8), respectively. These ages are concordant with the recommended values.ConclusionsOur data provides additional evidence of trachyte magmatism occurring in Jeju Island during the transitional period between the Early and Middle Pleistocene and the Late Pleistocene. The zircon samples analyzed in this study could serve as reference age data for Quaternary geochronology research.

Optimization of standard zircon U–Pb dating: insights into high-temperature thermal annealing

The weighted average 206Pb/238U ages for unannealed and annealed Plešovice zircons (photomicrographs are shown in (a) and (b)) using LA-ICP-MS are shown in (c) and (d) and Sri Lanka zircons using SIMS are shown in (e) and (f).

Novel Methods for Concomitant Determination of the Stable Zr Isotope Composition, Lu‐Hf Isotope Systematics and U‐Pb Age of Individual Zircons

The stable Zr isotope ratios in zircon yield a novel geochemical tracer that, together with the Lu‐Hf and U‐Pb radiogenic isotope systems, allows for a better understanding of the magmatic evolution of silicate melts. We present a solution‐based procedure for coupled stable Zr, Lu‐Hf and U‐Pb isotope ratio determinations for individual zircons using a 91Zr‐96Zr tracer purified from Hf and a late‐spiking protocol for Zr. This method yields high‐precision Zr and Hf isotope results while maintaining low blank levels for U‐Pb isotope and Lu/Hf ratio determinations. With a two‐fold improvement on the precision relative to previous solution‐based work, we report δ94ZrIPGP‐Zr values (deviation of the 94Zr/90Zr ratio in the sample relative to the IPGP‐Zr reference material) and associated intermediate precisions (2s) for the zircon reference materials 91500, Mud Tank, Plešovice and Penglai of ‐0.041 ± 0.015‰, 0.018 ± 0.013‰, 0.089 ± 0.020‰ and ‐0.117 ± 0.021‰, respectively. Furthermore, this method yields un‐biased δ94ZrIPGP‐Zr and 176Hf/177Hf results for 25‐ng Zr and 0.56‐ng Hf aliquots of the Mud Tank zircon with intermediate precisions (2s) of 0.027‰ and 1.7 ε (parts per ten thousand), respectively. Thus, the presented method is applicable for the analysis of extremely small and rare zircon grains.

Exploring the CAM18 Crystal as a Potential Reference Material for U–Pb Analysis of Zircon

In the process of in situ zircon U–Pb dating, it is an effective means to overcome the matrix effect by using a matrix-matched external reference material. However, the limited number of available zircon reference materials still makes it difficult to meet the research needs. In this paper, we performed a preliminary analysis of the gemological characteristics, trace elements and U–Pb ages of natural zircon CAM18 to assess its suitability as a reference material for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating. This tawny, gem-quality zircon has no visible inclusions and weighs approximately 0.55 g. Its density, full width at half maximum (FWHM) of the Raman peak and alpha flux (Dα) indicate that the sample has suffered mild-to-moderate radiation damage without any thermal treatment. The LA-ICP-MS U–Pb dating results reveal that the trace elements content and U–Pb ages of the sample are fairly homogeneous at the 50 μm scale, and there is no obvious loss of radiogenic Pb. The 206Pb/238U age (571.0 ± 3.0 Ma, 2s) and 207Pb/235U age (573.4 ± 6.0 Ma, 2s) are consistent within the analytical uncertainty, and the calculated concordia age is 571.4 ± 1.4 Ma (2s, n = 20). The variation in the 206Pb/238U ages is small, with a measurement repeatability of 0.46% (RSD), which is within the uncertainty of the age accuracy obtained by LA-ICP-MS. The oscillatory zoning, Th/U ratio (0.2) and chondrite-normalized rare-earth element (REE) pattern imply a magmatic origin of zircon CAM18. The Ti-in-zircon temperature ranges from 714 to 742 °C, and the oxygen fugacity ranges from ΔFMQ−2.87 to ΔFMQ−3.17, suggesting that it is crystallized in a reducing environment. All the results show that zircon CAM18 may has great potential in LA-ICP-MS U–Pb dating.

Characterisation of Three Sri Lankan Zircon Megacrysts as Potential Reference Materials for Microbeam U‐Pb Geochronology and Hf‐O‐Zr Isotope Measurements

In situ U‐Pb geochronology and hafnium, oxygen and zirconium isotope measurements in zircons using laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and ion microprobe techniques can provide essential isotopic data to constrain geological evolutionary histories. Developing reliable zircon reference materials is the cornerstone for in situ zircon chronology and isotopic studies. In this study, the homogeneity of U‐Pb ages and Hf‐O‐Zr isotope ratios in three Sri Lankan zircon megacrysts (SLZA, SLZB and SLZC) were investigated using multiple analytical methods. The obtained U, Th, Pb and Hf mass fractions of the SLZA zircon were 839 ± 56 μg g‐1 (1s), 151 ± 15 μg g‐1 (1s), 198 ± 28 μg g‐1 (1s) and 8635 ± 286 μg g‐1 (1s), respectively. The mass fractions of U, Th, Pb and Hf in the SLZB zircon were 1106 ± 106 μg g‐1 (1s), 331 ± 61 μg g‐1 (1s), 376 ± 57 μg g‐1 (1s) and 9673 ± 976 μg g‐1 (1s), respectively. The U, Th, Pb and Hf mass fractions determined in the SLZC zircon were 551 ± 35 μg g‐1 (1s), 111 ± 8 μg g‐1 (1s), 129 ± 18 μg g‐1 (1s) and 7881 ± 393 μg g‐1 (1s), respectively. The chemical abrasion isotope dilution thermal ionisation mass‐spectrometry (CA‐ID‐TIMS) method yielded a Th‐corrected weighted mean 206Pb/238U age of 556.94 ± 1.29 Ma (95% conf., n = 5) for the SLZA zircon, 552.90 ± 1.29 Ma (95% conf., n = 7) for the SLZB zircon and 560.83 ± 1.29 Ma (95% conf., n = 7) for the SLZC zircon. The obtained Hf isotopic compositions of the SLZA, SLZB and SLZC zircons determined with the solution MC‐ICP‐MS method were 0.281651 ± 0.000014 (2s, n = 10), 0.281704 ± 0.000008 (2s, n = 10) and 0.281676 ± 0.000006 (2s, n = 10), respectively. The obtained O isotopes of the SLZA and SLZB zircons measured with the laser fluorination method were 12.14 ± 0.56‰ (2s, n = 4) and 11.91 ± 0.30‰ (2s, n = 4), respectively. The Zr isotopes of the SLZA, SLZB and SLZC zircons determined with double spike TIMS analysis yielded mean δ94/90ZrSRM3169 values of ‐0.03 ± 0.06‰ (2s, n = 10), ‐0.03 ± 0.04‰ (2s, n = 10) and 0.00 ± 0.07‰ (2s, n = 8), respectively. The SLZA zircon can be used as a primary reference or quality control material for microbeam U‐Pb, Hf and Zr isotope measurements because of its slight heterogeneity. The U‐Pb, Hf and Zr isotopic compositions of the SLZB and SLZC megacrysts were homogeneous. The O isotopic compositions in the SLZA and SLZB zircon were slightly dispersed, indicating that these two megacrysts can only serve as secondary reference materials for microbeam O isotope measurements.

Multiproxy sediment provenance analysis of two megafans in the Owambo Basin, northern Namibia

The sediments in two stacked megafans in the Owambo Basin of northern Namibia and southern Angola were made accessible by a c. 400 m long, continuously cored borehole. Previous studies have indicated that the lower buried Paleocene–Eocene Olukonda Megafan was deposited by a palaeo-Kunene River transporting material from the NW (i.e. from the Kunene Intrusive Complex and the adjacent Angola Shield). The morphology of the overlying Eocene–Pliocene Cubango Megafan suggests the input of sediments from the north via the Cubango River. Mineralogical and geochemical data from the upper fan indicate felsic metamorphic and granitoid sources. Previous studies, however, did not provide a unique provenance identifier. Combining detrital zircon U–Pb data from both megafans with previously published and newly obtained mineralogical and geochemical data confirms two distinct provenances. The Olukonda Megafan can now be uniquely attributed by means of its detrital zircon ages to the Kunene Intrusive Complex and the surrounding Epupa Metamorphic Complex. In good agreement with the geochronologically more varied geology of the source region in the north, the Cubango Megafan detrital zircon record shows a wide distribution of Paleoproterozoic to Archean ages, but also a younger age range, probably related to a later Damaran/Pan-African source, which is absent in the Olukonda detrital zircon record. Supplementary material: Data obtained for this study, e.g. geochemical data, heavy mineral composition, heavy mineral morphology, zircon morphology, U-Th-Pb isotopes and ages on zircon, instrument settings, and U-Th-Pb isotope data of analysed reference zircon are available at https://doi.org/10.6084/m9.figshare.c.6280517

A LA‐ICP‐MS Comparison of Reference Materials Used in Cassiterite U‐Pb Geochronology

Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) is used to compare the suitability of four cassiterite (SnO2) materials (SPG, Yankee, AY‐4 and Jian‐1), and three matrix‐mismatched reference materials (NIST SRM 612, NIST SRM 614 and 91500 zircon) for normalisation of U‐Pb and Pb‐Pb isotope ratios in cassiterite. The excess variance of ages determined by LA‐ICP‐MS is estimated to be ±0.33% for 207Pb/206Pb vs. 208Pb/206Pb isochron ages and ± 1.8% and for U‐Pb ages. Incorporation of this excess variance in cassiterite ages is necessary for realistic uncertainties. 207Pb‐206Pb ages are advantageous for dating Precambrian cassiterite such as SPG compared with U‐Pb ages as matrix effect on instrumental mass fractionation of Pb isotopes are generally considered to be minor. We note minor bias in 207Pb/206Pb vs. 208Pb/206Pb isochron ages (~ 0.6%) when using either the NIST SRM 614 or 91500 zircon reference materials and emphasise the requirement for uncertainty propagation of all sources of error and reference materials with comparable U and Pb mass fraction to the cassiterite. The 238U/206Pb isotopic ratios from normalisation to matrix‐mismatched reference materials show varied results, which emphasises the need to use matrix‐matched reference materials for calculating U‐Pb ages. When cross‐calibrated against each other, LA‐ICP‐MS U‐Pb ages of the ca. 1535 Ma SPG, ca. 245 Ma Yankee and ca. 155 Ma Jian‐1 cassiterites are all consistent with their ID‐TIMS values.

Ban‐1 Zircon: A New Natural Zircon Reference Material for LA‐MC‐ICP‐MS Zr and Hf Isotopic Determinations

New zircon reference materials for in situ zircon radiogenic Hf isotope and stable Zr isotopic determinations made by laser ablation multi‐collector inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS) are required due to high data productivity and consequently high reference material consumption rate. This study examines a new natural zircon for Zr isotope ratios by double spike thermal ionisation mass spectrometry (TIMS), and for Hf isotopes by bulk solution nebuliser (SN)‐MC‐ICP‐MS with both Zr and Hf determined by LA‐MC‐ICP‐MS. A total of five zirconium isotope measurements from drilled zircons, determined by TIMS, yield a mean δ94/90ZrIPGP‐Zr value of ‐0.09 ± 0.06‰ (2s). Five and eight hafnium isotope measurements for powders from the drilled zircons and Ban‐1‐4 by SN‐MC‐ICP‐MS, yield mean 176Hf/177Hf ratios of 0.282985 ± 0.000011 (2s) and 0.282982 ± 0.000007 (2s), respectively. The mean δ94/90ZrIPGP‐Zr value and 176Hf/177Hf ratio determined by LA‐MC‐ICP‐MS analyses are ‐0.06 ± 0.09‰ (2s, n = 504) and 0.282985 ± 0.000035 (2s, n = 327), respectively. The isotopic homogeneities suggest that the Ban‐1 zircon is a suitable reference material for microbeam Zr and Hf isotopic measurements.

Zirconium isotopic composition of zircon reference materials by laser ablation multi-collector inductively coupled plasma mass spectrometry

Zirconium isotopes have been suggested as a new geochemical fingerprint for tracing magmatic evolution and continental crust differentiation. In this study, we optimized the Zr isotope measurement by laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) and reported stable Zr isotope compositions for different zircon reference materials (ZRMs). The RESOLution laser, coupled with MC-ICP-MS, yielded better results than the New Wave laser when comparing crater shape and depth indices, signal intensity, and isotopic ratio variations. The best working conditions for Zr isotope measurements were found to be with a laser spot size of 23 μm, a repetition rate of 2 Hz, and energy density of 3 J cm−2 which resulted in a crater depth of ∼8 μm. The 90Zr intensities increased by a factor of 3.2, and the raw 94Zr/90Zr ratio remained constant within a broader range of sample gas flow rate with the addition of 8 mL min−1 N2 compared to the normal mode. Negligible interferences were identified from the gas blank and also from the different integrated cycles between the bracketing standard and samples. Using the above-mentioned parameters, we obtained a long-term δ94/90ZrGJ-1 stability of 0.08‰, comparable with femtosecond laser-ablation (fsLA)-MC-ICP-MS methods. ZRMs (e.g., 91,500, Plešovice, Penglai, SA01, Tanz, and Ban-1) yielded homogeneous Zr isotope compositions, consistent with double spiked solution methods within error. Furthermore, zircon megacrysts of LKZ-1 and Cam-1, synthetical MUN-zircon with high Yb content, and MTUR1 zircon standard from carbonatite also had homogeneous Zr isotope compositions that indicate they could be used as tuning and monitoring standards. On the contrary, GHR1, Temora-2, Qinghu, and Jilin zircons were not ideal ZRMs for Zr isotope measurement. Zr isotope variations among homogeneous ZRMs (except Plešovice) further prove that Zr isotopes are a potentially powerful geochemical tracer.

Sub‐Micrometre Resolution FIB‐SEM‐based ToF‐SIMS Used to Map Geochemical Zoning in Four Zircon Reference Materials

Zircon geochemistry can vary over micrometre scales; therefore, natural reference materials need to be well characterised before being used to calculate trace element mass fractions in unmeasured samples. Moreover, reference material homogeneity needs to be ensured with the accelerating rate of geoanalytical developments to map mineral chemistry at increasingly finer scales. Here, we investigate trace element zoning in four widely used zircon reference materials: 91500, Mud Tank, Temora and Plešovice, as well as zircon crystals from the Mount Dromedary/Gulaga Igneous Complex, Australia. Sub‐micrometre resolution focused ion beam scanning electron microscope (FIB‐SEM) based time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and 5 μm resolution LA‐ICP‐MS mapping show that trace elements are zoned in all reference materials, though 91500 exhibited the least zonation. We demonstrate that FIB‐SEM‐based ToF‐SIMS can rapidly resolve variations in trace elements (e.g., U, Th, Sc, Y, Gd, Dy, Yb and Li) at sensitivities down to the μg g‐1 level with a spatial resolution of 195 nm for areas 100 × 85 μm to 959 × 828 μm. Zircon 91500 is recommended for future quantitative analyses provided that (1) the spatial distribution of elements is imaged before analysis of unknown samples and (2) it is used in conjunction with a doped glass as the primary reference material.

Development of an In SituU‐Th Disequilibrium Dating Method Utilising Multiple‐Spot Femtosecond Laser Ablation‐CRC‐ICP‐MS

In situ 238U‐230Th dating was conducted using an ICP‐mass spectrometer coupled with a laser ablation sampling system (LA‐ICP‐MS). To minimise the contribution of mass spectrometric interferences on 230Th, a kinetic energy discrimination technique using a collision/reaction cell (CRC) system was employed. The major problem associated with the CRC‐ICP‐MS system is the lower transmission efficiencies of ions than sector field mass spectrometers. To improve the signal‐to‐background ratio for analytes, a combination of a multiple‐spot laser ablation (msLA) system together with a sensitivity‐enhanced dry plasma cone was employed. Analytical accuracy was evaluated through analyses of three zircon reference materials (GJ‐1, OD‐3 and Plešovice) and the resulting 230Th/238U ratios for GJ‐1 and Plešovice zircons agreed with the secular equilibrium value. The developed technique was applied to Quaternary volcanic zircon and monazite samples from Japan. The resulting ages are 113.5 +5.4‐5.2 ka for the Toya tephra, 105.1 +6.4‐6.1 ka for the Sambe Kisuki tephra, 55.2 +7.7‐7.2 ka for the Hikageyama lava and 16.2 +4.2‐4.1 ka for the Susaki pumice (95% confidence levels). The resulting ages are consistent with previous chronological constraints including zircon fission track ages. The data obtained here demonstrate clearly that the combination of the CRC‐ICP‐MS system and the msLA protocol can become a powerful analytical tool for in situ U‐Th disequilibrium dating.

On the Statherian–Calymmian palaeogeography of northwestern Laurentia

The c. 1.75 to 1.27 Ga Hornby Bay intracontinental basin, exposed in northwestern Canada, NE of Great Bear Lake, includes the Big Bear, Mountain Lake and Dismal Lakes groups. The Big Bear group comprises mainly immature clastic rocks deposited by high-energy rivers in restricted fault-bounded basins formed during the late-stage assembly of supercontinent Nuna. Detrital zircon analysis of the Big Bear group suggests provenance from a combination of local sources including recycling of sedimentary rocks, which were originally derived from sources further east, such as the Thelon orogen. The overlying Mountain Lake group was deposited by westerly flowing rivers over a much broader region, with less evidence for syndepositional faulting, suggesting thermal subsidence. It displays provenance via recycling of strata from the proximal Coronation Supergroup and the more distal and correlative Goulburn Supergroup of Kilohigok Basin. Following tectonic uplift and erosion, basal clastic rocks of the Dismal Lakes Group were deposited in fluvial and then shallow-marine to paralic environments. Detrital zircon from these strata displays prominent late Archean peaks demonstrating provenance from adjacent rocks of the Slave craton and recycling of the Mountain Lake group. Detrital zircon geochronology of sandstones from the Hornby Bay Basin supports stratigraphic correlation with the Wernecke Supergroup in the Wernecke Mountains and the Muskwa assemblage, located further southward along the ancient western margin of Laurentia. Our data, along with Nd and C isotopes from corresponding shale and carbonate units, provide robust evidence for a palaeogeographical model whereby these successions represent the terrestrial and marine components of a west-facing, passive-margin clastic wedge, considered the westward extension of an Amazon-scale, braided fluvial system that originated in the foreland of the Trans-Hudson orogen, to the east. Elements of this system have been identified from inliers in northeastern Australia, which was attached to northwestern Laurentia prior to the break-up of supercontinent Nuna. Supplementary material: U–Pb and Hf results, and Hf isotopic compositions of reference zircons are available at https://doi.org/10.6084/m9.figshare.c.5980940

Quantifying Long‐Term Reproducibility of Zircon Reference Materials by U‐Pb LA‐ICP‐MS Dating

Inter‐session excess variance of U/Pb and Pb/Pb ratios in LA‐ICP‐MS zircon dating is the largest contributor to systematic errors, which in turn limit the accuracy of age determinations. Quantifying long‐term excess variance of reference materials allows for the estimation of excess variance in samples, but such compilations are not available in the literature. Here, we present the results of over 100 measurement sessions over 6 years for seven common zircon reference materials and calculate a characteristic excess variance for each. For 206Pb/238U ages, these values (2s) are as follows: AusZ7‐1 = 1.7% (115 sessions), AusZ7‐5 = 0% (74), OD‐3 = 0.6% (19), Temora2 = 1.2% (86), Plešovice = 1.1% (100), 91500 = 1.0% (146) and Mud Tank = 3.0% (12). For 207Pb/206Pb ages, smaller excess variances are observed: Temora2 = 0.3%, Plešovice = 0.4%, Mud Tank = 1.7% and 91500 = 0.4%. These values are well‐constrained estimates of inter‐session excess variance for the ETH Zürich LA‐ICP‐MS laboratory using a sector‐field ICP‐MS, and may provide either a first‐order estimate for other laboratories or a value to compare against their own reference material compilations.

CATHODOLUMINESCENCE AND RAMAN SPECTROSCOPY AS A BASIS FOR THE SELECTION OF REFERENCE SAMPLES FOR LA-ICP-MS ANALYSIS OF ZIRCON

In order to develop express methods for the quantitative assessment of properties and the selection of reference samples close to investigated sample, to ensure similar conditions for the evaporation of substances and parameters of fractionation of trace elements in LA-ICP-MS, a comparative analysis of the U-Pb isotopic composition, Raman and cathodoluminescent spectra of zircon GJ-1, Plesovice, 91500, Temora-2, Mud Tank reference samples has been performed. Variations in the position and width of the full width at half maximum (FWHM) mode of asymmetric stretching vibrations ν3(SiO4) B1g, as well as the values of the accumulated autoradiation dose Dα and the equivalent dose Dαed have been analyzed. It has been shown that in the series Mud Tank→91500→Temora-2→GJ-1→Plesovice, an increase in the degree of autoradiation damage is recorded: the first three samples are slightly damaged; Plesovice is a moderately damaged difference. The analyzed reference samples can be correctly used for LA-ICP-MS analysis of samples of weakly damaged zircons from magmatites, as well as fragments (zones) of zircon grains from metamorphic rocks of weak and medium degree of structural damage. For the first time, using a Jeol JSM6390LV SEM equipped with a Horiba H-CLUE iHR500 attachment, cathodoluminescence spectra of zircon reference samples were obtained in the 200–800 nm range. It has been found that the integral luminescence brightness varies for samples by more than an order of magnitude; the spectra are of a complex superposition nature; their decomposition into elementary components is ambiguous; a large number of components is detected, which are combined into three main Аi–Вi–Сi groups of broad bands in the near-ultraviolet (UV), blue-green and yellow regions with Emax=4.3–5.0, 2.6–3.5, and 2.1–2.3 eV, respectively. For the first time, it wasproposed to use a triple Аi–Вi–Сi diagram to discriminate zircons by their luminescent properties. It has been found that the position of zircon standards on it significantly differs, which is proposed to be used as a basis for an express assessment of their properties and selection. A comparative analysis was performed on the U-Pb isotopic composition, discordance of age determinations, features of Raman and cathodoluminescence spectra for a large sample of accessory zircon from kimberlites and diamond-bearing placers of Yakutia, as well as metamorphic rocks of the Urals – from granite of the final phase of the Neplyuevsky pluton, high-magnesian diorite massif, from high-pressure garnetites of the Mindyaksky lherzolite massif, from leptinites of the Taldyk block Mugodzhar. The Аi–Вi–Сi diagram is compared with similar data on the cathodoluminescence of the reference samples.

An Improved In Situ Zircon U‐Pb Dating Method at High Spatial Resolution (≤ 10 μm Spot) by LA‐MC‐ICP‐MS and its Application

In situ small‐spot U‐Pb geochronology is essential for measuring zircon crystals containing micrometre‐scale heterogeneities, abundant inclusions, inevitable cracks and narrow growth zones, all of which are especially common in metamorphic zircons. We present a high spatial resolution (≤ 10 μm diameter) laser ablation‐multi‐collector‐inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS) U‐Pb dating method using a Nu Plasma II mass spectrometer coupled with a GeoLas HD laser ablation system, which is capable of analysing zircon crystals with a wide range of U contents through two analytical modes (mixed‐mode for 10 μm ablation and IC‐mode for 5 μm ablation). We examined Pb/U down‐hole fractionation (DHF) as a function of different laser settings under small‐spot ablation, and how it controls the accuracy and precision of U‐Pb ages of zircon reference materials. After DHF correction using Iolite software (version 3.71), five measurement sessions (with different laser fluence and/or laser pulse settings) for the two analytical modes yielded accurate and precise U‐Pb ages for four zircon reference materials (Plešovice, 91500, GJ‐1 and Temora‐2). Further determinations of U‐Pb ages of three metamorphic zircon samples (~ 14 to 2000 Ma) from the Himalaya, North China Craton and North Qaidam were conducted, the results of which confirm the effectiveness and feasibility of this small‐spot ablation method for natural zircons with multiple growth zones.

Jilin zircon – a new natural reference material for microbeam U–Pb geochronology and Hf–O isotopic analysis

This study presented a new natural zircon reference material for microbeam U–Pb geochronology and Hf–O isotopic analysis.