The “laser ablation split stream” (LASS) technique is a powerful tool for mineral‐scale isotope analyses and in particular, for concurrent determination of age and Hf isotope composition of zircon. Because LASS utilizes two independent mass spectrometers, a large range of masses can be measured during a single ablation, and thus, the same sample volume can be analyzed for multiple geochemical systems. This paper describes a simple analytical setup using a laser ablation system coupled to a single‐collector (for U‐Pb age determination) and a multicollector (for Hf isotope analyses) inductively coupled plasma mass spectrometer (MC‐ICPMS). The ability of the LASS for concurrent Hf + age technique to extract meaningful Hf isotope compositions in isotopically zoned zircon is demonstrated using zircons from two Proterozoic gneisses from northern Idaho, USA. These samples illustrate the potential problems associated with inadvertently sampling multiple age and Hf components in zircons, as well as the potential of LASS to recover meaningful Hf isotope compositions. We suggest that such inadvertent sampling of differing age and Hf components can be a significant cause of excess scatter in Hf isotope analyses and demonstrate that the LASS approach offers a robust solution to these issues. The veracity of the approach is demonstrated by accurate analyses of 10 reference zircons with well‐characterized age and Hf isotopic composition, using laser spot diameters of 30 and 40 µm. In order to expand the database of high‐precision Lu‐Hf isotope analyses of reference zircons, we present 27 new isotope dilution‐MC‐ICPMS Lu‐Hf isotope measurements of five U‐Pb zircon standards: FC1, Temora, R33, QGNG, and 91500.

Results are presented for in situ simultaneous determination of U‐Pb and Sm‐Nd isotopes in monazite using the Laser Ablation Split‐Stream (LASS) method. This method uses a laser ablation system coupled to a magnetic‐sector inductively coupled plasma mass spectrometer (HR) (ICP‐MS) for measuring U‐Pb isotopes and a multicollector (MC) ICP‐MS for measuring Sm‐Nd isotopes. Ablated material is split using a Y‐connector and transported simultaneously to both mass spectrometers. In addition to Sm and Nd isotopes, the MC‐ICP‐MS is configured to also acquire Ce, Nd, Sm, Eu, and Gd elemental abundances. This approach provides age, tracer isotope, and trace element data in the same ablation volume, reducing sampling problems associated with fine‐scale zoning in accessory minerals and minimizing the material needed for ablation. Precision and accuracy of the U‐Pb method (and the precision of the Sm‐Nd method) is demonstrated with results from well‐characterized monazite reference materials. The LASS results agree within uncertainty with the isotope dilution thermal ionization mass spectrometry (ID‐TIMS) U‐Pb dates. The accuracy of the Sm‐Nd method is assessed by comparing the LA‐MC‐ICP‐MS results with ID‐TIMS determinations on a well‐characterized, in‐house monazite reference material. The LASS method is then applied to monazite from the Birch Creek Pluton in the White Mountains of California as a case study to illustrate the utility of this method for solving geologic problems. The U‐Pb ages and Sm‐Nd isotopic data from the LASS method support the conclusions drawn from previous results that monazite can record timing and information about the source region(s) of hydrothermal fluids.

Zircon U/Pb geochronology using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is rapidly being adopted in the earth sciences. Challenges in the accurate determination of ages and uncertainties, however, remain, and include finding the optimal ways to calculate and calibrate U/Pb and Pb/Pb measurements of unknowns. Of particular importance is the determination of U/Pb ratios that are complicated by laser-induced fractionation of U relative to Pb. In order to address these challenges and provide best practice recommendations, this study systematically explores and fully documents three algorithms for calculating U/Pb and Pb/Pb ratios (ratio of mean signals, mean of replicate ratios, and intercept of ratio vs. ablation time relationship), four different placements of calibration materials within a series of unknowns, and three different least-squares approaches for interpolation of drift correction factors (mean, linear, and quadratic). The lowest mean relative standard errors (2σ) for single uncalibrated laser analyses were obtained for the ratio of means and mean of ratios algorithms, and were ∼1% for ²⁰⁶Pb/ ²³⁸U and ²⁰⁷Pb/ ²⁰⁶Pb. The experimental uncertainty for the ratio of the mean and the mean of ratios algorithms was nearly three times higher for ²⁰⁶Pb/ ²³⁸U and 1.5 times higher for ²⁰⁷Pb/ ²⁰⁶Pb compared to that predicted from counting statistics. The experimental uncertainty for the intercept method, however, for ²⁰⁶Pb/ ²³⁸U was nearly six times higher than that predicted due to counting statistics. These results suggest that additional and unaccounted for sources of error are present. The best reproducibility (2σ) for both ²⁰⁶Pb/ ²³⁸U (∼2 to 3% 2RSD) and ²⁰⁷Pb/ ²⁰⁶Pb (∼1.5% 2RSD) was also obtained using the ratio of means and mean of ratios algorithms. The most precise fits to calibration materials placed in various positions within a ‘run’ of twenty data acquisitions were obtained using a quadratic fit to the data and when the calibration materials were closely spaced, as would be expected. There was no significant gain in accuracy, however, using a quadratic fit compared to other methods, suggesting that the more complex fits are simply following noise in the data. This clearly demonstrates that drift corrections based on mean or linear least squares are most appropriate for optimizing precision and accuracy for U/Pb dating of zircon using LA-ICP-MS. Close spacing of calibration materials between single or small groups of unknowns (6 or less) was shown to produce the most accurate results, demonstrating that long, continuous sequences of unknowns introduce significant deviation from the true age of the samples. The mean precision (2σ) of individual age analyses of the 1065 Ma 91500 zircon obtained in this study range from 16–38 Ma (1.5–3.5%, 2σ RSE), improving as more acquisitions of calibration materials are utilized. The accuracy was 1% or better.

The Swakane Gneiss, interpreted to represent sedimentary strata metamorphosed at 8–12 kbar, is the deepest exposed crustal levels within the exhumed North Cascades continental magmatic arc, yet the nature and age of its protolith and the mechanism by which it was transported to deep-crustal levels remains unclear. Zircons from 11 paragneiss and schist samples were analyzed for U-Pb age and Hf-isotope composition in order to investigate the tectonic history of the Swakane Gneiss from protolith deposition to metamorphism within the North Cascades arc. Zircons interpreted to have crystallized in situ during metamorphism and/or melt-crystallization within the Swakane Gneiss at depth have ca. 74–66 Ma ages. Detrital-zircon age and Hf-isotope characteristics demonstrate provenance shifts that correlate with maximum depositional ages of ca. 93–81 Ma. Samples deposited between ca. 93 and 88 Ma have dominantly Mesozoic age peaks with initial ε_Hf values between depleted mantle and chondritic uniform reservoir (CHUR), whereas ca. 86–81 Ma sample show the addition of distinct Proterozoic populations (ca. 1380 and 1800–1600 Ma) and Late Cretaceous zircons with unradiogenic Hf-isotope compositions. Similar detrital-zircon age and Hf-isotope patterns are observed in several Upper Cretaceous forearc and accretionary wedge units between southern California and Alaska along the North American continental margin. The connection between the Swakane Gneiss and these potential protoliths located outboard of Cordilleran arc systems indicate burial by either underplating of accretionary-wedge sediments or underthrusting of forearc sediments. Therefore, the protolith and incorporation history for the Swakane Gneiss is likely similar to those of deep crustal metasedimentary units elsewhere in the North Cascades (i.e., the Skagit Gneiss Complex) and to the south along the continental margin (i.e., the Pelona-Orocopia-Rand schists and Schist of Sierra de Salinas). These observations suggest that burial of sediment outboard of continental magmatic arc systems may be a major mechanism for the transfer of sediment to the deep levels of continental arcs.