Zircon Reference Materials Research Articles

Concurrent Determination of U‐Pb Age and REE Mass Fractions of Zircon by High Mass Resolution SIMS

A combined geochronological and geochemical investigation for the same domain of zircon provides valuable information on timing and genesis, particularly in the case of multi‐growth metamorphic zircon. A high spatial resolution concurrent analytical method for zircon U‐Pb age and rare earth element content was successfully achieved in this study, using a multi‐collector secondary ion mass spectrometer (SIMS) at a ~ 8 μm diameter scale. Special instrument parameters were employed, including a high mass resolution of approximately 15000 applied to replace the previous energy filter method, and a dynamic multi‐collector mode used to reduce the measurement time to 18 min per analysis. Six zircon reference materials yielded precise and accurate 206Pb/238U ages, which are comparable to those obtained by the ordinary mono‐collector method, but with 2–3 times higher spatial resolution. All zircon grains measured in this study showed enriched heavy‐REE (HREE) contents consistent with previously reported values determined by LA and solution ICP‐MS methods. The light‐REE (LREE) mass fractions measured using both SIMS and LA‐ICP‐MS methods in this study, although with quite different volume, show consistent results within uncertainties.

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

A new natural zircon reference material SA01 is introduced for U‐Pb geochronology as well as O and Hf isotope geochemistry by microbeam techniques. The zircon megacryst is homogeneous with respect to U‐Pb, O and Hf isotopes based on a large number of measurements by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and secondary ion mass spectrometry (SIMS). Chemical abrasion isotope dilution thermal ionisation mass spectrometry (CA‐ID‐TIMS) U‐Pb isotopic analyses produced a mean 206Pb/238U age of 535.08 ± 0.32 Ma (2s, n = 10). Results of SIMS and LA‐ICP‐MS analyses on individual shards are consistent with the TIMS ages within uncertainty. The δ18O value determined by laser fluorination is 6.16 ± 0.26‰ (2s, n = 14), and the mean 176Hf/177Hf ratio determined by solution MC‐ICP‐MS is 0.282293 ± 0.000007 (2s, n = 30), which are in good agreement with the statistical mean of microbeam analyses. The megacryst is characterised by significant localised variations in Th/U ratio (0.328–4.269) and Li isotopic ratio (−5.5 to +7.9‰); the latter makes it unsuitable as a lithium isotope reference material.

Further Characterization of the BB Zircon via SIMS and MC-ICP-MS for Li, O, and Hf Isotopic Compositions

In this contribution, we report the results for the characterization of the BB zircon, a newly developed zircon reference material from Sri Lanka, via secondary ion mass spectrometry (SIMS) and multiple-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The focus of this work was to further investigate the applicability of the BB zircon as a reference material for micro-beam analysis, including Li, O, and Hf isotopes. The SIMS analyses reveal that BB zircon is characterized by significant localized variations in Li concentration and isotopic ratio, which makes it unsuitable as a lithium isotope reference material. The SIMS-determined δ18O values are 13.81‰ ± 0.39‰ (2SD, BB16) and 13.61‰ ± 0.40‰ (2SD, BB40), which, combined with previous studies, indicates that there is no evidence of conspicuous O isotope heterogeneity within individual BB zircon megacrysts. The mean 176Hf/177Hf ratio of BB16 determined by solution MC-ICP-MS is 0.281669 ± 0.000012 (2SD, n = 29) indistinguishable from results achieved by laser ablation (LA)-MC-ICP-MS. Based on the SIMS and MC-ICP-MS data, BB zircon is proposed as a reference material for the O isotope and Hf isotope determination.

High‐Precision, High‐Accuracy Oxygen Isotope Measurements of Zircon Reference Materials with the SHRIMP‐SI

Analytical protocols for SHRIMP‐SI oxygen isotope analysis (δ18O) of a suite of zircon reference materials (RMs) are presented. Data reduction involved a robust estimate of uncertainties associated with the individual spot as well as for groups where the spot data are combined. The repeatability of δ18O measurements is dependent on both the analytical conditions and the choice of the primary reference material. Under optimised conditions, repeatability was often better than 0.4‰ (2s) allowing sample uncertainties to be obtained to better than 0.2‰ (at 95% confidence limit). Single spot uncertainty combined the within‐spot precision with the scatter associated with repeated measurements of the primary zircon reference material during a measurement session. The uncertainty for individual spots measured under optimised conditions was between 0.3 and 0.4‰ (at 95% confidence). The analytical protocols described were used to assess a variety of zircon RMs that have been used for geochronology and for which laser fluorination oxygen isotope data are available (Temora 2, FC1, R33, QGNG and Plešovice), as well as zircons that have been used as RMs for trace element or other types of determination (Mud Tank, Monastery, 91500, AS57, AS3, KIM‐5, OG1, SL13, CZ3 and several other Sri Lankan zircons). Repeated analyses over nine sessions and seven different mounts show agreement within analytical uncertainty for Temora 2, FC1, R33, QGNG, Plešovice and 91500, when normalised to Mud Tank. For existing ion microprobe mounts with these materials, an appropriate δ18O can be determined. However, care should be taken when using zircons from the Duluth Complex (i.e., FC1, AS57 and AS3) as reference materials as our data indicated an excess scatter on δ18O values associated with low‐U zircon grains.

LKZ-1: A New Zircon Working Standard for the In Situ Determination of U–Pb Age, O–Hf Isotopes, and Trace Element Composition

This study introduces a new zircon reference material, LKZ-1, for the in situ U–Pb dating and O–Hf isotopic and trace element analyses. The secondary ion mass spectrometric analyses for this gem-quality single-crystal zircon yielded a weighted mean 206Pb/238U age of 572.6 ± 2.0 Ma (2σ, n = 22, MSWD = 0.90), with moderately high U concentrations (619 ± 21 ppm, 1 SD), restricted Th/U ratios (0.146 ± 0.002, 1 SD), and negligible common Pb content (206Pbc < 0.2%). A comparable 206Pb/238U age (570.0 ± 2.5 Ma, 2σ) was produced by the isotope dilution-thermal ionization mass spectrometry. The secondary ion mass spectrometric and laser ablation-assisted multiple collector inductively coupled plasma mass spectrometer analyses respectively showed that LKZ-1 had little variation in O (δ18OV-SMOW = 10.65 ± 0.14‰; laser fluorination value = 10.72 ± 0.02‰; 1 SD) and Hf (176Hf/177Hf = 0.281794 ± 0.000016, 1 SD) isotopic compositions. LKZ-1 was also fairly homogeneous in its chemical composition (RSD of laser ablation ICPMS data ≤ 10%), displaying a relatively uniform chondrite-normalized rare earth element pattern ((Lu/Gd)N = 31 ± 3, Eu/Eu* = 0.43 ± 0.17, Ce/Ce* = 44 ± 32; 1 SD). These consistencies suggest that the LKZ-1 zircon is a suitable working standard for geochronological and geochemical analyses.

A NanoSIMS 50 L Investigation into Improving the Precision and Accuracy of the 235U/238U Ratio Determination by Using the Molecular 235U16O and 238U16O Secondary Ions

A NanoSIMS 50 L was used to study the relationship between the 235U/238U atomic and 235U16O/238U16O molecular uranium isotope ratios determined from a variety of uranium compounds (UO2, UO2F2, UO3, UO2(NO3)2·6(H2O), and UF4) and silicates (NIST-610 glass and the Plesovice zircon reference materials, both containing µg/g uranium). Because there is typically a greater abundance of 235U16O+ and 238U16O+ molecular secondary ions than 235U+ and 238U+ atomic ions when uranium-bearing materials are sputtered with an oxygen primary ion beam, the goal was to understand whether use of 235U16O/238U16O has the potential for improved accuracy and precision when compared to the 235U/238U ratio. The UO2 and silicate reference materials showed the greatest potential for improved accuracy and precision through use of the 235U16O/238U16O ratio as compared to the 235U/238U ratio. For the UO2, which was investigated at a variety of primary beam currents, and the silicate reference materials, which were only investigated using a single primary beam current, this improvement was especially pronounced at low 235U+ count rates. In contrast, comparison of the 235U16O/238U16O ratio versus the 235U/238U ratio from the other uranium compounds clearly indicates that the 235U16O/238U16O ratio results in worse precision and accuracy. This behavior is based on the observation that the atomic (235U+ and 238U+) to molecular (235U16O+ and 238U16O+) secondary ion production rates remain internally consistent within the UO2 and silicate reference materials, whereas it is highly variable in the other uranium compounds. Efforts to understand the origin of this behavior suggest that irregular sample surface topography, and/or molecular interferences arising from the manner in which the UO2F2, UO3, UO2(NO3)2·6(H2O), and UF4 were prepared, may be a major contributing factor to the inconsistent relationship between the observed atomic and molecular secondary ion yields. Overall, the results suggest that for certain bulk compositions, use of the 235U16O/238U16O may be a viable approach to improving the precision and accuracy in situations where a relatively low 235U+ count rate is expected.

Optimization of a Laser Ablation‐Single Collector‐Inductively Coupled Plasma‐Mass Spectrometer (Thermo Element 2) for Accurate, Precise, and Efficient Zircon U‐Th‐Pb Geochronology

AbstractMany applications specific to detrital mineral U‐Th‐Pb geochronology in the Earth sciences necessitate large numbers of age observations to be made from samples and require accurate and precise isotope measurements across wide dynamic ranges in elemental concentrations and signal intensities. This implies that the laser system and mass spectrometer cannot be tuned between individual analyses as to optimize measurements based on the isotope composition and concentrations of samples and that intensity matching between the unknowns to be dated and the reference material(s) used for fractionation correction is impossible to ensure. We describe methodologies for optimization of laser ablation‐single collector‐inductively coupled plasma‐mass spectrometer for the accurate determination of initial‐Pb‐corrected (using measured 204Pb) U‐Th‐Pb zircon ages, taking full advantage of the high sensitivity provided by the Thermo Element 2 ICP‐MS instruments fitted with a high‐performance low ultimate vacuum Jet interface. “We describe an approach that corrects for nonlinearity of the detector—the primary obstacle avoided with sample‐specific tuning—as well as element‐ and mass‐dependent fractionation and instrumental drift by using a suite of three zircon reference materials with known isotopic ratios from isotope dilution‐thermal ionization mass spectrometry measurements but with differing U and Pb concentrations.” This approach allows for (experimentally) determining an instrumental fractionation versus ion beam intensity curve used for standard‐sample bracketing, thus taking into consideration an important instrumental variable that is commonly ignored in most applications of U‐Pb dating using laser ablation‐single collector‐inductively coupled plasma‐mass spectrometer. We show that these methodologies yield uncertainties and age offsets typically better than ±2.0% for individual measurements of small (e.g., 10‐μm depth × 20‐μm diameter) volumes of material.

Isotope-dilution anchoring of zircon reference materials for accurate Ti-in-zircon thermometry

Abstract The temperature-dependence of Ti incorporation into zircon can be used to estimate crystallisation conditions and to make inferences about the petrogenesis of host rocks or parental melts. However, the foundation for such temperature estimates – Ti concentration in zircon – is currently determined by a variety of techniques with no common reference. Analyses of Ti in zircon commonly utilise in situ microbeam techniques such as secondary ion mass spectrometry (SIMS), laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS), and electron probe microanalysis (EPMA). These techniques require external calibrations that are prone to matrix effects, potentially imposing significant errors on Ti concentrations and the derived model temperatures. To improve the accuracy of these determinations, we present a new framework of zircon reference materials for Ti built around two thoroughly characterised zircons, the widely distributed zircon 91500 and the new reference zircon GZ7. Laser ablation ICP-MS and SIMS analyses reveal that the homogeneity with respect to Ti/Si is on the order of 4.1–8.2% (relative standard deviation, RSD) for zircon 91500 and 1.1% for zircon GZ7, which is considered sufficient for their use as primary calibration materials. We present independent determinations of Ti concentration in multiple fragments of these two zircon crystals using isotope dilution (ID)-ICP-MS employing a precisely calibrated 47Ti–49Ti double spike. The recommended ID Ti concentration values are 4.73 ± 0.15 μg g−1 for zircon 91500 and 25.08 ± 0.18 μg g−1 for zircon GZ7 (95% confidence level). A set of complementary secondary reference materials is also characterised using LA-ICP-MS with Ti concentrations anchored to the new isotope dilution values, thereby permitting comparisons of Ti-in-zircon datasets whenever data for these secondary reference materials are reported.

Evaluating downhole fractionation corrections in LA-ICP-MS U-Pb zircon geochronology

Abstract Among the most significant challenges in maximizing the precision and accuracy of U-Pb zircon geochronology by LA-ICP-MS is minimizing the impact of downhole fractionation, the time-dependent evolution of Pb/U ratios caused mainly by complex differences in the volatility and chemical properties of elements as they are excavated from the ablation site. To produce meaningful dates for unknown materials, downhole fractionation is typically quantified in a reference zircon and a time-based correction factor subsequently employed to yield constant Pb/U in both standard and unknown zircons. This assumes that both the reference and unknown zircon exhibit similar downhole behaviour. As a test of this assumption, downhole fractionation trends were characterized and quantified in three common zircon reference materials (Plesovice, 337 Ma; Temora-2, 417 Ma; 91500; 1065 Ma) and in three low-U (

Improved accuracy of LA-ICP-MS U-Pb ages of Cenozoic zircons by alpha dose correction

While Laser Ablation Inductively-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) is the method of choice for U-Pb dating of zircons in provenance analysis, its application to young (<100Ma) zircons is hindered by systematic analytical bias. In magmatic petrology, where the majority of studied units are young, this often places zircon dates at odds with established 40Ar/39Ar eruption ages or high-precision ID-TIMS crystallization ages. Zircon lattice properties, particularly the degree of lattice damage caused by the radioactive decay of U and Th, impart analytical bias by causing differential ablation rates and therefore differential fractionation of U and Pb throughout each analysis. Although it is possible to normalize the zircon lattice strengths to calibration reference zircons by thermal annealing to some extent, this may not entirely alleviate the problem. In this study, the effects of alpha decay dose (i.e., degree of radiation damage) on analytical biases in age determination are examined by analyzing a number of zircon reference materials under well-constrained analytical parameters. A regression-based, multi-standard correction method is demonstrated, which improves the accuracy of age data, particularly in young (Cenozoic) zircons. A novel data reduction scheme (Dose_Corrector.ipf) is introduced, which runs in conjunction with the widely-used Igor Pro/Iolite platform and performs a correction for alpha dose and Th disequilibrium. This scheme improves the accuracy of age data for unannealed zircons, and its utility is demonstrated by applying it to zircons from several well-studied units.

A New Appraisal of Sri Lankan BB Zircon as a Reference Material for LA‐ICP‐MS U‐Pb Geochronology and Lu‐Hf Isotope Tracing

A potential zircon reference material (BB zircon) for laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U‐Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5–1.5 cm3) from Sri Lanka was studied. Within‐grain rare earth element (REE) compositions are largely homogeneous, albeit with some variation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains, the variation in U content for all analysed crystals ranged from 227 to 368 μg g−1 and the average Th/U ratios were between 0.20 and 0.47. The Hf isotope composition (0.56–0.84 g/100 g Hf) is homogeneous within and between the grains – mean 176Hf/177Hf of 0.281674 ± 0.000018 (2s). The calculated alpha dose of 0.59 × 1018 g−1 for a number of BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the same crystal (analysed by ID‐TIMS in four different laboratories) gave consistent U‐Pb ages with excellent measurement reproducibility (0.1–0.4% RSD). Interlaboratory assessment (by LA‐ICP‐MS) from individual crystals returned results that are within uncertainty equivalent to the TIMS ages. Finally, we report on within‐ and between‐grain homogeneity of the oxygen isotope systematic of four BB crystals (13.16‰ VSMOW).

Trace element homogeneity from micron- to atomic scale: Implication for the suitability of the zircon GJ-1 as a trace element reference material

The quality of a chemical reference material relies on the fact that the composition of the material is homogeneous across all scales. A series of different techniques have been used to evaluate the trace element homogeneity of the GJ-1 reference zircon from the micron- to atomic scale. Cathodoluminescence imaging was conducted along with quantitative crystallographic orientation analysis and trace element analysis using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The nanometre-scale homogeneity was evaluated by analysing five mineral tips using atom probe tomography, which provides atomic scale three dimensional chemical reconstructions with unprecedented spatial resolution. Results show that the GJ-1 reference zircon is homogeneous at all scales, both structurally and chemically. Crystallographic orientation data confirms that this gem quality zircon has no detectable internal crystallographic orientation changes such as crystal-plastic deformation features or cracks. No mineral inclusions were found. Atom probe tomography shows that there is a lack of any chemical clustering or other modes of spatially defined elemental accumulation or depletion for the most abundant trace elements such as Y, Yb and Hf. This finding is supported by LA-ICPMS data revealing homogeneity within the analytical precision. Trace elements of significant abundance include P, Yb, Y, U and Hf, with contents of 30±6, 65±2, 238±5, 284±14 and 6681±57ppm, respectively. Hence, the GJ-1 zircon used as a reference zircon for UPb and Hf-isotopic studies is also a suitable zircon reference material for trace element analyses.

LGC-1: A zircon reference material for in-situ (U-Th)/He dating

A pairwise in-situ (U-Th)/He dating method has been proposed for mitigating matrix-related bias in U and Th measurements using synthetic reference materials. This method requires a natural zircon reference material whose (U-Th)/He age should be homogeneous on the scale (~10–100μm) to be used in such dating experiments. A newly characterized zircon LGC-1 megacryst fulfils this requirement. This pale-yellowish, flawless Sri Lanka gem specimen is about 1.2∗0.8∗0.8cm in size. Optical microscopy, cathodoluminescence-imaging, X-ray elemental mapping, and Raman spectroscopy on a large number of random shards did not reveal any detectable textural and compositional heterogeneity. Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) analyses on a large number of randomly selected fragments yield 266 measurements of U, Th and Pb concentrations, which are within the corresponding experimental uncertainties. The weighted mean U, Th and Pb concentrations are 357.7±1.8ppm, 740.9±5.0ppm, and 39.06±0.18ppm, respectively, with a weighted mean Th/U ratio of 2.07±0.01, indistinguishable from Isotope Dilution ICP-MS (ID-ICP-MS) and Thermal Ionization Mass Spectrometry (ID-TIMS) results. ID-TIMS U/Pb ages are concordant within uncertainties of decay constants, with a concordia age of 541.70±0.70Ma. Conventional (U-Th)/He dating on 28 random shards from the crystal in different laboratories gives a central age of 476.4±5.7Ma. Six in-situ (U-Th)/He analyses yield consistent 4He concentrations and ages with weighted mean values of 1248±46nmol/g and 462±21Ma, respectively. Fractions of this zircon have been shared with several laboratories in the Australia, China, UK and US, and are expected to serve as a reference for both in-situ and conventional (U-Th)/He analyses. The combination of analytical methods used to characterize LGC-1 zircon may be used as a template for future age reference calibration.

Zircon from the Anorthosite zone II of the Stillwater Complex as a U–Pb geochronological reference material for Archean rocks

Well-characterized reference materials are essential for precise and accurate geochronology. The growing use of in situ techniques, especially U–Pb zircon dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), requires the availability of natural reference materials for matrix-matched calibration, age normalization, and quality control. Although there are a number of zircon reference materials available for U–Pb LA-ICP-MS (e.g., Plešovice, 91500, Temora, FC-1), there is currently no widely available Archean zircon reference material for U–Pb geochronology. Zircon from a leucogabbro in the Anorthosite zone II (AN2) of the Neoarchean Stillwater Complex (Montana, USA) is a candidate for an Archean reference material. The AN2 leucogabbro contains abundant zircon with simple sector zoning typical of mafic igneous rocks and with no inheritance of older grains; accessory baddeleyite and secondary rutile and titanite are also present. Trace element concentrations and ratios determined by LA-ICP-MS are variable in Stillwater AN2 zircon (e.g., U=54–585ppm, Hf=5500–9800ppm, Ti=9–22ppm, Th/U=0.38–0.70, Ce/Nd=0.76–18) and consistent with crystallization from highly fractionated interstitial melt at near-solidus temperatures (Ti-in-zircon thermometry=768–864°C) in plagioclase-rich cumulates. U–Pb dates for Stillwater AN2 zircon were established by several different methods, including (1) conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) of grains that were air-abraded, (2) chemical abrasion-ID-TIMS or CA-TIMS where grains were annealed and acid-leached prior to dissolution, and (3) LA-ICP-MS. The U–Pb results by all methods for Stillwater AN2 zircon are concordant and yield indistinguishable concordia and weighted mean 207Pb/206Pb dates, with the latter ranging in relative precision from 2710.44±0.32Ma (n=9) by CA-TIMS to 2710.56±0.65Ma by ID-TIMS (n=4) to 2715±9Ma by LA-ICP-MS (n=58). The in situ dates are statistically identical both at a scale of tens to hundreds of microns within individual zircon grains and between grains, and there is only rare evidence for localized domains that have lost small amounts of Pb since crystallization. In contrast, baddeleyite from the AN2 leucogabbro has micron-scale overgrowths and internal replacement patches of secondary zircon that has lost Pb; the weighted mean 207Pb/206Pb date of 2709.73±0.48Ma for baddeleyite that was air-abraded to remove the secondary zircon rims is indistinguishable from the igneous zircon dates. Consequently, it is concluded that the Stillwater AN2 zircon grains represent a suitable geochronological reference material that can be used for a variety of U–Pb zircon analytical methods, especially for LA-ICP-MS dating of Archean zircon, and for comparison of U–Pb zircon dating results between different analytical laboratories worldwide.

Comment on “Zircon U–Th–Pb dating using LA-ICP-MS: Simultaneous U–Pb and U–Th dating on 0.1 Ma Toya Tephra, Japan” by Hisatoshi Ito

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of eight zircon reference materials and synthetic zircon-hafnon end-members indicate that corrections for abundance sensitivity and molecular zirconium sesquioxide ions (Zr2O3+) are critical for reliable determination of 230Th abundances in zircon. Other polyatomic interferences in the mass range 223–233amu are insignificant. When corrected for abundance sensitivity and interferences, activity ratios of (230Th)/(238U) for the zircon reference materials we used average 1.001±0.010 (1σ error; mean square of weighted deviates MSWD=1.45; n=8). This includes the 91500 and Plešovice zircons, which were deemed unsuitable for calibration of (230Th)/(238U) by Ito (2014). Uranium series zircon ages generated by LA-ICP-MS without mitigating (e.g., by high mass resolution) or correcting for abundance sensitivity and molecular interferences on 230Th such as those presented by Ito (2014) are potentially unreliable.

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

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.

A Critical Evaluation of U–Pb Calibration Schemes Used in SIMS Zircon Geochronology

Precision and accuracy in SIMS zircon geochronology strongly depend on the method of determination of the interelement ion ratios (e.g., 206Pb/238U) from the measured secondary ion ratios (206Pb+/238U+). Six possible U–Pb calibrations (Pb/U–UO2/U, Pb/U–UO/U, Pb/U–UO2/UO, Pb/UO–UO2/U, Pb/UO–UO/U, Pb/UO–UO2/UO) based on simple power law relationships, and Pb/UO2‐related one‐ and two‐ (a power law) dimensional ones were compared using data acquired on the 91500 zircon reference material from one hundred measurement sessions, to determine the most statistically reliable scheme. Taking advantage of U oxide species (UO and UO2) over atomic U, due to their similar energy distribution to Pb and higher intensities, the data calibrated with Pb/UO–UO2/UO showed the smallest mean uncertainties and dispersions, and provided the best‐fit calibration curve consistently. Although it was demonstrated with Temora 2 that the unknown zircon age was not changed significantly by different calibrations, its precision could be improved using the Pb/UO–UO2/UO calibration in zircon geochronology.

Chemical Abrasion Applied to LA-ICP-MS U–Pb Zircon Geochronology

Zircon (ZrSiO4) is the most commonly used mineral in U–Pb geochronology. Although it has proven to be a robust chronometer, it can suffer from Pb-loss or elevated common Pb, both of which impede precision and accuracy of age determinations. Chemical abrasion of zircon involves thermal annealing followed by relatively low temperature partial dissolution in HF acid. It was specifically developed to minimize or eliminate the effects of Pb-loss prior to analysis using Thermal Ionization Mass Spectrometry (TIMS). Here we test the application of chemical abrasion to Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) by analyzing zircons from both untreated and chemically abraded samples. Rates of ablation for high alpha-dose non-treated zircons are up to 25% faster than chemically abraded equivalents. Ablation of 91500 zircon reference material demonstrates a ca. 3% greater down-hole fractionation of 206Pb/238U for non-treated zircons. These disparities necessitate using chemical abrasion for both primary reference material and unknowns to avoid applying an incorrect laser induced fractionation correction. All treated samples display a marked increase in the degree of concordance and/or lowering of common Pb, thereby illustrating the effectiveness of chemical abrasion to LA-ICP-MS U–Pb zircon geochronology.

U[sbnd]Pb LA-(MC)-ICP-MS dating of rutile: New reference materials and applications to sedimentary provenance

In response to the general lack of sufficiently abundant and high quality rutile UPb reference materials for in situ geochronology, we have characterised two new potential rutile ~1.8Ga reference materials (Sugluk-4 and PCA-S207) from granulite facies belts of the Canadian Shield, namely the northern Cape Smith Belt of Quebec and the Snowbird Tectonic Zone (Sasatchewan). Characterisation includes ID-TIMS and LA-ICP-MS UPb dating, imaging, and trace element analysis. We compare these materials with existing rutiles used already (R19 and R10; Luvizotto et al., 2009; Zack et al., 2011) and show that the measured UPb compositions (i.e. including any common Pb) of our rutiles are considerably more homogeneous. This makes possible a UPb normalisation procedure (not reliant upon a common Pb correction) that results in a significant decrease in the uncertainty contribution from the common Pb correction and better reproducibility of reference materials and unknowns for provenance analysis and other applications. The reproducibility is 2–4% (2RSD) for 206Pb/238U and 207Pb/206Pb, only slightly greater than long-term data for zircon reference materials. We show in a rutile provenance study from young orogens (Bhutan Himalaya and Canadian Cordillera) that the sensitivity of our analytical set-up allows dating of ~90% of rutiles in a sediment using a 50μm laser ablation spotsize within samples containing rutile as young as 10–20Ma, and obviates the requirement for U concentration pre-screening, thus reducing or eliminating rutile selection bias. Unsuccessful analyses are due to poor quality rutiles with predominant common Pb, 207Pb signal below detection, or U content below ~1–2ppm. We have used the ‘207Pb-method’ (using the Tera-Wasserburg diagram) to correct for substantial common Pb in very young and/or very low-U rutiles, rather than developing an on-line correction. Since rutile ages reflect mainly the time of cooling, rutile is a sensitive recorder of metamorphic thermochronological information and therefore is an excellent complement to detrital zircon U207Pb data. The contrast between zircon and rutile signatures in Himalayan samples with rutile as young as 10Ma is shown to be very dramatic (most zircons from the same sample are >480Ma, with only a few grains or metamorphic rims reflecting Miocene metamorphism); as such rutile provides complementary information about the thermal events within the source regions of the grains. Rutile UPb dating is an underexploited provenance method with wide applicability to sedimentary provenance studies.

VizualAge: A Novel Approach to Laser Ablation ICP‐MS U‐Pb Geochronology Data Reduction

VizualAge, a new computer software tool for analysing U‐Pb data obtained by laser ablation‐inductively coupled plasma‐mass spectrometry, was developed. It consists of a data reduction scheme (DRS) for Iolite (a general mass spectrometry data analysis tool) as well as visualisation routines. In addition to the U/Pb and Th/Pb ages calculated by Iolite’s U‐Pb geochronology DRS, VizualAge also calculates 207Pb/206Pb ages and common Pb corrections for each time‐slice of raw data. Importantly, VizualAge allows one to display a live concordia diagram for visualising data on such a diagram as an integration interval is being adjusted. This provides instantaneous feedback regarding discordance, uncertainty, error correlation and common Pb. Several zircon data sets were used to illustrate how the live concordia could be used as a powerful inspection tool, revealing a single analysis to consist of zones of concordance, metamict areas, as well as inherited cores or younger overgrowths. VizualAge also constructs histograms, conventional and Tera‐Wasserburg type concordia diagrams, as well as 3D U‐Th‐Pb and total U‐Pb concordia diagrams. The precision and accuracy of data reduced with VizualAge are demonstrated with examples of the Plešovice, Temora‐2 and Penglai zircon reference materials. Data for zircon from the Long Lake Batholith (Wyoming craton) were used to illustrate how VizualAge calculated common Pb corrections and helped to expose as yet unexplained difficulties with accurately determining 204Pb.