Christopher M. Fisher

Accurate Hf isotope determinations of complex zircons using the “laser ablation split stream” method

[1]xa0The “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.

Simultaneous in situ determination of U‐Pb and Sm‐Nd isotopes in monazite by laser ablation ICP‐MS

Results are presented for the 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 both a magnetic-sector inductively coupled plasma mass spectrometer (HR) (ICP-MS) for measuring U-Pb isotopes and multicollector (MC) ICP-MS for measuring Sm-Nd isotopes. The ablated material is split using a glass Y-connector and transported simultaneously to both mass spectrometers. In addition to Sm and Nd isotopes, the MC-ICP-MS is configured also acquire Ce, Nd, Sm, Gd, and Eu elemental abundances. This approach provides age, tracer isotope, and trace element data in the same ablation volume, thus reducing but not eliminating sampling problems associated with fine-scale zoning in accessory minerals. The precision and accuracy of the U-Pb method (along with the precision of the Sm-Nd method) is demonstrated by analysis of six well-characterized monazite reference materials. The LASS results agree within uncertainty with previously determined isotope dilution thermal ionization mass spectrometry (ID-TIMS) ages. 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 (BCP) in the White Mountains of southeastern 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 determined using the LASS method support the conclusions drawn from previous results that monazite can record both timing and potential sources of hydrothermal fluids.

Data acquisition and calculation of U–Pb isotopic analyses using laser ablation (single collector) inductively coupled plasma mass spectrometry

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.

Combining Nd isotopes in monazite and Hf isotopes in zircon to understand complex open-system processes in granitic magmas

Mapping the age and trace element and Sm-Nd isotope compositions of monazite grains from a peraluminous Cretaceous granite using laser ablation–split stream analysis reveals a wide range in Nd isotope and rare earth element (REE) compositions within and between single grains. These data corroborate isotopic variability indicated by Hf isotope analysis of zircon in the same granite sample. The REE variations indicate that monazite grew during fractional crystallization. Hf and Nd isotopes indicate that the granitic magma was generated from at least two distinct Proterozoic sources of approximately the same age: one component that had highly radiogenic initial 176Hf/177Hf and 143Nd/144Nd and a second component that was notably less radiogenic. This study highlights the utility of in situ REE and Sm-Nd isotope data in monazite in magmatic systems. Further, it refines the zircon-based constraints on magmatic processes because of sensitivity of light REEs to fractional crystallization, lower probability of complications owing to inheritance, and smaller analytical volumes required.

Neoarchean and Paleoproterozoic crystalline basement rocks of north-central Idaho: Constraints on the formation of western Laurentia

Zircon U-Pb analyses of 18 orthogneisses from the Clearwater and Priest River complexes of northern Idaho have identified important exposures of Neoarchean and Paleoproterozoic basement rocks in northwest Laurentia. All samples have ages that fall into two tightly defined age ranges: in the Neoarchean from 2.67 to 2.65 Ga and in the Paleoproterozoic from 1.88 to 1.84 Ga. The Neoarchean orthogneisses show no evidence of older components, whereas some of the Paleoproterozoic orthogneisses have xenocrystic zircon cores with ages overlapping with those of the Neoarchean gneisses. Collectively, these two packages of rocks represent the two main periods of crust formation in northwest Laurentia, one wholly juvenile in the Neoarchean, and the other a mixture of juvenile Paleoproterozoic and inherited Neoarchean components. Based on existing data, the ages of orthogneisses in the Clearwater and Priest River complexes are identical and probably represent one continuous crustal block; we refer to this basement domain as the Clearwater block. The Paleoproterozoic 1.86 Ga magmatism described here is distributed throughout the northwest Laurentian margin and does not coincide with the proposed trend of the Great Falls tectonic zone. Therefore, a model of a single linear arc preserved within the Great Falls tectonic zone is inadequate in describing the majority of the known 1.86 Ga crust in the region.

Tracing crustal evolution by U-Th-Pb, Sm-Nd, and Lu-Hf isotopes in detrital monazite and zircon from modern rivers

Detrital zircon U-Pb age and Hf isotope studies are useful for identifying the chemical evolution of the continental crust. Zircon, however, is typically a magmatic mineral and thus often fails to document the timing of low-grade metamorphism, and its survival through multiple sedimentary cycles potentially biases the crustal evolution record toward older events. In contrast, monazite typically records metamorphic events and is less likely to survive sedimentary recycling processes, thus providing information not available by zircon. Here, we demonstrate that monazite apparently faithfully records the Sm-Nd isotope composition of the bulk rock and can therefore track the record of crustal evolution and growth, similar to Hf isotopes in zircon. We examine the utility of detrital zircon and monazite for studies of crustal evolution through a comparison of age and tracer isotope information using sediments from two large rivers draining the South China block (SCB). Monazite and zircon grains yield mostly Mesozoic and Paleozoic U-Pb ages and depleted mantle model age peaks at ca. 1900–1300 Ma, indicating that both minerals preserve similar, yet critical, information on the crustal evolution of the catchment area. In contrast, zircon yields abundant Neoproterozoic and older U-Pb ages with a very large spread of model ages, preserving a history strongly skewed to older ages. Based on the lack of known rocks of this age in the catchments, ancient zircon was likely sourced from sedimentary rocks within the catchment area. This combined data set presents a more complete history of crustal evolution and growth in the SCB and demonstrates the advantages of an integrated approach that includes both detrital monazite and zircon.

Elucidating the magmatic history of the Austurhorn silicic intrusive complex (southeast Iceland) using zircon elemental and isotopic geochemistry and geochronology

The Austurhorn intrusive complex (AIC) in southeast Iceland comprises large bodies of granophyre and gabbro, and a mafic–silicic composite zone (MSCZ) that exemplifies magmatic interactions common in Icelandic silicic systems. Despite being one of Iceland’s best-studied intrusions, few studies have included detailed analyses of zircon, a mineral widely recognized as a valuable tracer of the history and evolution of its parental magma(s). In this study, we employ high spatial resolution zircon elemental and isotopic geochemistry and U–Pb geochronology as tools for elucidating the complex construction and magmatic evolution of Austurhorn’s MSCZ. The trace element compositions of AIC zircon crystals form a broad but coherent array that partly overlaps with the geochemical signature for zircons from Icelandic silicic volcanic rocks. Typical of Icelandic zircons, Hf concentrations are relatively low (<10,000xa0ppm) and Ti concentrations range from 5 to 40xa0ppm (Ti-in-zircon model temperaturesxa0=xa0761–981xa0°C). Zircon δ18O values vary from +2.2 to +4.8xa0‰, consistent with magmatic zircons from other Icelandic silicic rocks, and preserve evidence for recycling of hydrothermally altered crust as a significant contribution to the generation of silicic magmas within the AIC. Zircon εHf values generally range from +11 to +15. This range overlaps with that of Icelandic basalts from off-rift settings as well as the least depleted rift basalts, suggesting that the AIC developed within a transitional rift environment. In situ zircon U–Pb ages yield a weighted mean of 6.52xa0±xa00.03xa0Ma for the entire complex, but span a range of ~320xa0kyr, from 6.35xa0±xa00.08 to 6.67xa0±xa00.06xa0Ma (2σ SE). Gabbros and the most silicic units make up the older part of this range, while granophyres and intermediate units make up the younger part of the complex, consistent with field relationships. We interpret the ~320xa0kyr range in zircon ages to represent the approximate timescale of magmatic construction of the MSCZ. These U–Pb data suggest that the complex was constructed by multiple short-lived magmatic intrusion events occurring closely spaced in time, allowing periodic re-melting and rejuvenation of mush-like material and a prolonged lifetime for the complex.

Evolution of the Jura-Cretaceous North American Cordilleran margin: Insights from detrital-zircon U-Pb and Hf isotopes of sedimentary units of the North Cascades Range, Washington

The U-Pb age and Hf-isotope composition of detrital zircons from Jurassic to Upper Cretaceous sedimentary rocks adjacent to the southern North Cascades–Coast Plutonic Complex continental magmatic arc document shifting provenance, the tectonic evolution of the arc system, and translation along the continental margin. Systematic changes in the detrital-zircon data provide insight that the western margin of North America evolved from: marginal basins adjacent to continent-fringing oceanic arcs (ca. 160–140 Ma); forearc basins adjacent to mid-Cretaceous (ca. 120–90 Ma) Andean-type continental arcs; and addition of a cratonic source to forearc and accretionary wedge units to Cordilleran arc systems in the mid-Late Cretaceous (ca. 85 Ma). Jurassic Methow terrane, Nooksack Formation, and western mélange belt units dominantly contain detrital zircons derived from accreted oceanic terranes, whereas Lower Cretaceous strata from the same units have age peaks that correspond to known pulses of magmatism in Cordilleran continental magmatic arc systems. The age peaks and Hf-isotope signature of the Jurassic and Lower Cretaceous strata are comparable to multiple sources exposed along the margin. In contrast, the Upper Cretaceous western mélange belt has distinct Precambrian zircon populations and unradiogenic Late Cretaceous zircons that are more similar to southwestern than northwestern Laurentian sources. Statistical comparisons confirm provenance similarities between rocks of the North Cascades and those 700–2000 km to the south and, thus, support marginparallel translation from as far as the latitude of southern California.

Hafnium, oxygen, neodymium, strontium, and lead isotopic constraints on magmatic evolution of the supereruptive southern Black Mountains volcanic center, Arizona, U.S.A.: A combined LASS zircon-whole-rock study

Abstract The >700 km3 Peach Spring Tuff (PST), erupted at 18.8 Ma from the Silver Creek caldera in the southern Black Mountains volcanic center (SBMVC) of western Arizona, is the only supereruption-scale ignimbrite in the northern Colorado River Extensional Corridor. The SBMVC contains pre- and post-caldera volcanic rocks and caldera-related intrusions (∼19–17 Ma) that provide a detailed petrologic record of ignimbrite antecedence and aftermath. Whole-rock Sr-Nd-Pb-Hf isotopic data combined with complementary zircon O and Hf isotopic data from a suite of pre- through post-PST samples provide robust constraints on (1) how the SBMVC evolved with respect to magmatic sources and processes throughout its ∼2 Ma history and (2) the petrogenetic relationships between the PST and slightly younger intracaldera plutons. Both pre- and post-PST units have isotopic ranges (εNd = −8.3 to −11.6, εHf = −8.2 to −14.0, 87Sr/86Sr = 0.709–0.712; 206Pb/204Pb = 18.19–18.49, 207Pb/204Pb = 15.60–15.62, 208Pb/204Pb = 38.95–39.29) that fall within the spectrum of Miocene Colorado River Extensional Corridor rocks and are consistent with mixing of substantial fractions of Proterozoic (Mojave) crust and juvenile material derived from regional enriched mantle. Compared to the PST, which has relatively uniform isotopic ratios (εΜ = −11.4 to −11.7, εκ = −13.8 to −14.3, 87Sr/86Sri =0.709–0.712; 206Pb/204Pb = 18.20–18.29, 207Pb/204Pb = 15.60–15.62, 208Pb/204Pb = 39.02–39.33), individual pre- and post-PST units are isotopically more variable and generally more primitive. Consistent with whole-rock isotopes, zircon εHf (−8 to −14) and oxygen δ18O (+4.5 to +7.2‰) for most pre- and post-PST units also have wider ranges and more mantle-like values than those of the PST (−12 to −15, +6.1 to +7.1‰). Moreover, zircon isotopic compositions decrease in post-PST samples. A few zircons from post-PST intrusions have δ18O values lower than mantle values (<+5‰), suggesting incorporation of hydrothermally altered rock. Whole-rock and zircon elemental and isotopic analyses indicate that (1) most pre- and post-PST units are less evolved and less homogenized than the PST itself; (2) intrusions in the Silver Creek caldera are petrogenetically distinct from the PST and therefore represent discrete magmatic pulses, not unerupted PST mush; (3) enriched mantle input increased in the SBMVC following the paroxysmal PST eruption; (4) post-PST history of the SBMVC was characterized by periodic influx of magmas with varying juvenile fractions into pre-existing mushy or solidified intrusions, resulting in variable and incomplete hybridization; and (5) melting and assimilation of hydrothermally altered crust played a relatively minor role in the generation and evolution of magmas in the SBMVC.

Provenance, U-Pb detrital zircon geochronology, Hf isotopic analyses, and Cr-spinel geochemistry of the northeast Yukon-Koyukuk Basin: Implications for interior basin development and sedimentation in Alaska

The Yukon-Koyukuk Basin is a large depression that covers ∼118,000 km2 in western interior Alaska and is divided into two subbasins by a volcanic arc assemblage. Interpretations of the depositional ...