McClure Mountain Research Articles

The McClure Mountain Syenite Apatite as a Potential Age Control Reference Material for LA‐ICP‐MS AFT and U‐Pb Double Dating

Apatite fission track (AFT) dating is now routinely performed using LA‐ICP‐MS, and U‐Pb age data are often acquired at the same time, allowing double dating of individual grains. Furthermore, additional geochemical data, such as REE mass fractions are readily acquired during the analysis. It is therefore of practical interest to identify a single material as a quality control on ages and composition. Here we present AFT and U‐Pb data for the McClure Mountain Syenite (MMS) apatite, already widely used as an age reference material by the U‐Pb community. We performed double dating on 238 grains, in thirteen measurement sessions over two years. The AFT data show remarkable reproducibility with an overall central age of 254.1 ± 5.1 Ma. The U‐Pb results agree with the reference age for individual sessions (and for all data together). The results show that that the McClure Mountain apatite is a valid quality control material for double dating, allowing the use of the same material for both methods. We also propose to extend the quality control using MMS to trace element measurements performed in conjunction AFT and U‐Pb dating.

Ediacaran‐Ordovician Magmatism and REE Mineralization in the Wet Mountains, Colorado, USA: Implications for Failed Continental Rifting

AbstractStructures associated with Ediacaran‐Ordovician alkaline magmatism and the timing of rare earth element (REE) mineralization in the Wet Mountains, CO, were analyzed using field, geophysical, and U‐Th‐Pb isotope methods to interpret their tectonic setting in the context of previously proposed rift models. The Wet Mountains are known for thorium and REE mineralization associated with failed rift‐related, Ediacaran‐Ordovician alkaline intrusions and veins. Structural field data indicate that alkaline dikes and mineralized veins are controlled by a system of northwest‐striking, high‐angle faults and tension fractures formed in a 040°‐directed extensional regime. Magnetic and surface expressions of Democrat Creek and McClure Mountain complexes show tectonic elongation toward ∼045°, consistent with NE‐directed extension. Magnetic data also suggest the existence of a fourth, previously unrecognized mafic‐ultramafic complex of inferred Cambrian age with a similar elongated orientation. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) 208Pb/232Th analysis of low‐uranium zircon from carbonatite dikes and in situ 206Pb/238U LA‐ICP‐MS analysis of monazite in mineralized dikes yielded 465 ± 18 Ma and 489 ± 33 Ma ages, respectively. These ages are consistent with the expected age based on slightly older, cross‐cut syenite dikes and the hypothesized Ordovician end to failed rift‐related magmatism. The Ediacaran‐Ordovician age of alkaline magmatic rocks and the associated northeast‐directed extension direction are similar to those of the along‐strike, Ediacaran‐Cambrian Southern Oklahoma Aulacogen. Therefore, the failed rift system in the Wet Mountains is interpreted to be a northwestern continuation of the Southern Oklahoma Aulacogen with carbonatite magmatism and thorium/REE mineralization representing late intrusive phases.

LA‐ICP‐MS/MS Single‐Spot Rb‐Sr Dating

Due to developments in reaction cell technologies, it is now possible to collect large 87Rb/86Sr and 87Sr/86Sr data sets using LA‐ICP‐MS/MS. Until now, LA‐ICP‐MS/MS Rb‐Sr ages were constrained using conventional isochrons on a contiguous set of Rb‐Sr data. In this contribution, we present a straightforward procedure on how to measure, calculate and validate Rb‐Sr ages from individual laser spots even from detrital grains devoid of context. This approach does not only allow provenance studies of detrital micas, but also furthermore opens the opportunity to produce age maps with unprecedented spatial resolution, both for zoned single crystals and targeting different textural domains. The main challenge for calculating single‐spot Rb‐Sr ages is the estimation of the initial 87Sr/86Sr composition, if it cannot be constrained through analysis of a paragenetic phase. In this case, we propose to use a range of geologically relevant initial 87Sr/86Sr compositions rather than one fixed value: (1) 0.703 ± 0.003 (for mantle derived magmatic rocks), (2) 0.715 ± 0.015 (for evolved magmatic rocks) and (3) 0.730 ± 0.030 (for crustal rocks). Modelling shows that the 87Sr/86Sr ratio of the targeted material delineates the accuracy and precision of single‐spot Rb‐Sr ages. If the measured Sr isotopic composition is sufficiently radiogenic [87Sr/86Sr of > 1.0 for scenario (1) to > 4.5 for scenario (3)], the calculated age and its uncertainty are not significantly influenced by the initial 87Sr/86Sr composition. Using Mica‐Mg as the primary reference material, single‐spot Rb‐Sr dating was tested on biotite (Mount Dromedary, La Posta, McClure Mountain), muscovite (Högsbo) and a nano‐powder tablet (Mica‐Fe) that we further propose using as secondary reference materials for Rb‐Sr geochronology.

Apatite U‐Pb Dating with Common Pb Correction Using LA‐ICP‐MS/MS

Apatite is a ubiquitous accessory mineral in various rocks (e.g., igneous, metamorphic and clastic sedimentary rocks). However, precise and accurate U‐Pb LA‐ICP‐MS dating of apatite is often compromised by high common Pb levels. Among the different common Pb correction methods, the main advantage of the 204Pb correction method is that it does not assume U/radiogenic Pb concordance. However, 204Pb is difficult to measure using ICP‐MS instruments because of the isobaric interference of 204Hg on 204Pb. We overcome this limitation by using a reaction cell sandwiched between two quadrupoles within an ICP‐MS, which can allow the online chemical separation of two different elements. Ammonia reacts efficiently (> 98%) with Hg while isotopes of Pb are not affected. The approach was tested on eight apatite reference materials (McClure Mountain, NW‐1, UWA‐1, Otter Lake, Slyudyanka, MK‐1, Durango and Fish Canyon Tuff) for which there are independent constraints on the U‐Pb crystallisation age, by comparing U‐Pb dating results employing different reaction gas mixes (NH3‐N2O and NH3 only) in two laboratories. Based on the U‐Pb data and SEM analyses on each sample, we can exclude apatite inter‐ and intra‐grain U‐Pb age heterogeneity, except for a c. 4% variability of ages in the Otter Lake sample. Our results show that accuracy and precision for U‐Pb dating are not measurably affected by different reaction gases, and we accurately reproduce ages of numerous independently characterised apatites within 4% of the reference ages, and the age reproducibility is typically better than 2%.

Detrital zircon geochronology and provenance of the Middle to Late Jurassic Paradox Basin and Central Colorado trough: Paleogeographic implications for southwestern Laurentia

AbstractMiddle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; southwestern United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposition with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527–519 Ma) grains at this provenance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local basement uplifts were largely buried by Middle to Late Jurassic time.This study aims to delineate spatial and temporal patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrography, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth elemental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in western Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53–1.3 Ga, 1.3–0.9 Ga, and 500–300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540–500 Ma zircon grains are anomalously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochemistry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540–500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identification of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wanakah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin suggests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT.

U-Pb systematics of uranium-rich apatite from Adirondacks: Inferences about regional geological and geochemical evolution, and evaluation of apatite reference materials for in situ dating

High‑uranium apatite samples from iron oxide-apatite (IOA) deposits of the eastern Adirondack Mountains, New York, are evaluated as potential standards for in situ U-Pb dating of high-U apatite. Age data for these minerals also have implications for better understanding the geological evolution in the region. Secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analyses of these samples show that they have high and variable uranium contents of 5–485 ppm. High-precision isotope-dilution thermal ionization mass spectrometry (ID-TIMS) dating yielded U–Pb errorchron ages of 907 ± 14 Ma (MSWD = 20) and 924 ± 13 Ma (MSWD = 547), showed disturbance in the U–Pb systems, and revealed radiogenic compositions of initial Pb (206Pb/204Pb ratios of 339 ± 48 and 162 ± 39). Nominal age calculations using assumed initial Pb isotopic compositions close to average terrestrial crustal Pb yield biased and older ages despite the radiogenic Pb isotopic composition of the apatites (206Pb/204Pb from 1300 to >4000). The apatite ages calculated using the measured initial Pb isotopic compositions are younger than the U–Pb ages of 1008.0 ± 3.2 Ma and 992.4 ± 7.7 Ma determined by SIMS for zircon cores and rims, respectively, separated from one of these ore samples. It indicates protracted fluid activity in the deposit that lasted for ~100 Ma after the ore formation, or later disturbance of the apatite. The time of U enrichment that we inferred from radiogenic composition of initial Pb is within uncertainties with primary ore crystallization, indicating that high 206Pb/204Pb ratios of initial Pb were caused by dissolution-reprecipitation of primary apatite.Despite the variability in U contents and disturbance of the U–Pb system in the apatites, our SIMS and LA-ICPMS analyses show that these samples can be used, with due caution, as reference materials (RMs) for dating apatite with similarly high-U concentration. They potentially provide more precise and accurate results compared to the usage of better behaved but lower-U apatite RMs due to improved counting statistics and reduced matrix effects, especially important for LA-ICPMS with less sensitive quadrupole mass analysers.The McClure Mountain apatite, a widely used reference for in situ dating, was used in the present study as a matrix-matched reference material for in situ LA-ICPMS analyses. We also analysed it by ID-TIMS and obtained more radiogenic Pb-isotopic data and more precise ages that are in agreement within uncertainties with the previously published result (Schoene and Bowring, 2006); combined data yielded a U-Pb errorchron age of 525.3 ± 1.7 Ma (MSWD = 70). The data corrected for initial Pb using the total U/Pb isochron yield the single Concordia age of 525.10 ± 0.75 Ma (MSWD = 1.1). Two independent ID-TIMS U–Pb studies support the use of this apatite as a primary RM for dating of low-U apatite with a normal composition of initial Pb. However, its small grain size and low U concentration (11–29 ppm) can cause elevated uncertainty in LA-ICPMS age determinations for unknowns. Better precision in our LA-ICPMS analyses was achieved using Madagascar (MAD1 and MAD2) apatite RM with U contents in the range of 28–58 ppm.

Protracted magmatism and magnetization around the McClure Mountain alkaline igneous complex

AbstractThe McClure Mountain–Iron Mountain igneous complex is an alkalic intrusive center in the northern Wet Mountains of southern Colorado. It was emplaced in early Cambrian time into gneissic/granitic 1.75–1.45 Ga Proterozoic host rocks. Numerous dikes are associated with the complex, primarily along the western side. Although the main intrusive nepheline-syenite body is well dated, the ages of the surrounding dikes are poorly known. Crosscutting relationships and poorly defined K-Ar dates suggest that the dikes are younger than the main intrusion. Paleomagnetic samples were collected from dikes associated with the McClure Mountain igneous complex. Geochronologic samples were also collected from two dikes sampled for their paleomagnetism. We obtained U-Pb zircon ages of 526 ± 8 Ma for a lamprophyric extracomplex dike and 483 ± 2 Ma for a trachytic extracomplex dike. These ages suggest either multistage or protracted dike intrusion around the ca. 524 Ma McClure Mountain complex. Our paleomagnetic data are consistent with previously published results. Dikes of the complex primarily exhibit southeast and shallow paleomagnetic directions, with variable declinations. Results from several baked contact tests indicate that the magnetizations are secondary. A steeply inclined magnetization is pervasive and was acquired over a protracted interval from late Laramide time to the present day.

Comment on ‘Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard’ by Weisberg, Metcalf, and Flowers

Comment on ‘Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard’ by Weisberg, Metcalf, and Flowers

Response to comment on “Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard”

Response to comment on “Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard”

Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard

New zircon, titanite, and apatite (U-Th)/He (ZHe, THe, AHe) data for four samples of the 524 Ma McClure Mountain syenite (MMS) in the Wet Mountains of Colorado constrain the <200 °C thermal history of this 40Ar/39Ar geochronology standard. ZHe dates vary from 420 to 567 Ma, do not correlate with their limited eU range (66–388 ppm), have dispersion attributable to eU zonation, and show no systematic variation between samples. THe dates are 488 to 515 Ma. AHe dates range from 70 to 211 Ma, with some inter-sample variability that could be due to modest post-70 Ma displacements along faults. Our dataset is consistent with recently published (U-Th)/He data for the MMS, although the published ZHe results extend to higher eU and define a negative date-eU correlation not captured by our lower eU zircons. Existing U-Pb and 40Ar/39Ar thermochronologic data, together with the THe and ZHe results, document rapid post-emplacement cooling of the MMS to ≤200 °C by ~500 Ma. A recent (U-Th)/He study inferred that the MMS subsequently underwent simple slow monotonic cooling until present-day. However, geologic relationships are incompatible with such a history, instead requiring that rocks in the vicinity of the MMS were at the surface at ~480 Ma, ~180 Ma, and ~60 Ma, with possible phases of burial and erosion of varying magnitude between these times. Inverse thermal history modeling can simultaneously satisfy the new and previously published ZHe and AHe data while honoring these geologic constraints. The outcomes of this work highlight that date-eU patterns can be generated by protracted cooling as well as by multiphase heating and cooling histories, and underscore the importance of integrating geologic data to help identify the most probable explanation for a (U-Th)/He dataset.

Empirical constraints on the effects of radiation damage on helium diffusion in zircon

In this study, we empirically evaluate the impact of radiation damage on zircon (U-Th)/He closure temperatures for a suite of zircon crystals from the slowly cooled McClure Mountain syenite of south-central Colorado, USA. We present new zircon, titanite, and apatite conventional (U-Th)/He dates, zircon laser ablation (U-Th)/He and U-Pb dates, and zircon Raman spectra for crystals from the syenite. Titanite and apatite (U-Th)/He dates range from 447 to 523Ma and 88.0 to 138.9Ma, respectively, and display no clear correlation between (U-Th)/He date and effective uranium concentration. Conventional zircon (U-Th)/He dates range from 230.3 to 474Ma, while laser ablation zircon (U-Th)/He dates show even greater dispersion, ranging from 5.31 to 520Ma. Dates from both zircon (U-Th)/He datasets decrease with increasing alpha dose, indicating that most of the dispersion can be attributed to radiation damage. Alpha dose values for the dated zircon crystals range from effectively zero to 2.15×1019α/g, spanning the complete damage spectrum.We use an independently constrained thermal model to empirically assign a closure temperature to each dated zircon grain. If we assume that this thermal model is robust, the zircon radiation damage accumulation and annealing model of Guenthner et al. (2013) does not accurately predict closure temperatures for many of the analyzed zircon crystals. Raman maps of the zircons dated by laser ablation document complex radiation damage zoning, sometimes revealing crystalline zones in grains with alpha dose values suggestive of amorphous material. Such zoning likely resulted in heterogeneous intra-crystalline helium diffusion and may help explain some of the discrepancies between our empirical findings and the Guenthner et al. (2013) model predictions. Because U-Th zoning is a common feature in zircon, radiation damage zoning is likely to be a concern for most ancient, slowly cooled zircon (U-Th)/He datasets. Whenever possible, multiple mineral-isotopic systems should be employed to add additional, independent constraints to a sample’s thermal history.

(LA,Q)-ICPMS trace-element analyses of Durango and McClure Mountain apatite and implications for making natural LA-ICPMS mineral standards

Apatite, the most abundant phosphate mineral in the Earth's crust and uppermost mantle, is able to accept a wide variety of trace elements into its crystal structure. Many of these trace element substitutions are below the detection limit of Electron Microprobe Analysis, but can be determined by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). LA-ICPMS elemental abundance determinations typically employ sample-standard bracketing using either standard glasses or an appropriate matrix-matched reference material. In this study we have undertaken laser ablation (>3000 analyses) and low-blank solution Q(quadrupole)-ICPMS trace-element analyses on crushed 150–300μm aliquots of Durango and McClure Mountain apatite to assess the accuracy of apatite elemental abundance determinations when using NIST 612 standard glass as the primary LA-ICPMS trace element standard. An accuracy (relative to the solution data) and precision of <5% can be obtained for most trace elements (Y, the REE, Sr, Mn, V, Th and U) in LA-ICPMS analyses of crushed Durango separates; the McClure Mountain data are similarly accurate for most trace elements but yield larger intra-crystal variability. Durango raster and image mapping experiments demonstrate some Durango crystals are more homogenous than others; the raster experiments also show that Durango typically exhibits less zoning parallel to the C-axis compared to perpendicular to the C-axis. A protocol for developing a homogenous Durango apatite trace-element reference material is suggested, and involves slicing the interior portions of several Durango crystals parallel to the C-axis, undertaking rapid LA-ICPMS raster experiments to characterize trace-element zoning, crushing the most homogenous crystal to 150–300μm and determining its trace-element contents by low-blank solution ICPMS. This generic approach can easily be modified and applied to characterize other natural LA-ICPMS mineral standards.

Middle Jurassic landscape evolution of southwest Laurentia using detrital zircon geochronology

The Middle Jurassic Wanakah Formation in western Colorado is a poorly understood unit in terms of depositional environment and absolute age of deposition; however, a refined interpretation of the provenance has important implications for understanding the landscape evolution of southwestern Laurentia during Mesozoic rifting of the supercontinent Pangea and the opening of the Gulf of Mexico. This study presents the first U/Pb age dating of detrital zircons from the Middle to Late Jurassic Entrada Sandstone, Wanakah Formation, and Tidwell and Salt Wash Members of the Morrison Formation. Detrital zircon geochronology results show a marked increase in ca. 523 Ma grains (compared to most Mesozoic sediments on the Colorado Plateau) that begins abruptly in the Wanakah Formation and continues into the basal Marker Bed A of the Tidwell Member of the Morrison Formation. U/Pb ages and petrography suggest that the Wanakah Formation was sourced, in large part, from the McClure Mountain syenite on the southwestern flank of the Ancestral Front Range. This abrupt change in provenance occurred due to stream capture and drainage reorganization that input a large amount of water into the basin and caused a shift in depositional environment from the eolian Entrada Sandstone to the hypersaline lake environments of the Wanakah Formation and the Tidwell Member. Additionally, stratigraphic, petrological, and detrital zircon analyses suggest that the contact between the Wanakah Formation and the Tidwell Member of the Morrison Formation is conformable, and the previously interpreted J-5 unconformity is likely not present in western Colorado. The stream capture and drainage reorganization that created the lake system recorded in the Wanakah Formation and the Tidwell Member likely evolved into the major fluvial system that deposited the Salt Wash Member of the Morrison Formation. The evolution of paleodrainages and provenance are important to understand because they help to constrain landscape evolution across southwestern Laurentia, and these insights can help to illuminate the influence of tectonic and sediment controls on depositional environment.

Sr and Nd isotopic compositions of apatite reference materials used in U–Th–Pb geochronology

Apatite is an important common U- and Th-bearing accessory mineral in igneous, metamorphic and clastic sedimentary rocks. The advent of in situ U–Th–Pb apatite geochronology by the SIMS and LA-(MC)-ICP-MS methods has demonstrated the importance of having uniform and homogeneous reference materials. Recently, it has been shown that Sr and Nd isotopic data combined with U–Pb age and trace element concentration data can provide important constraints on apatite paragenesis because this phase usually exhibits high Sr and REE concentrations but has low Rb/Sr ratios which result in negligible corrections for the ingrowth of radiogenic Sr. However, as apatite can potentially have complex internal structures resulting from multiple thermal events, such as inherited cores and metamorphic overgrowths, requires that the Sr and Nd isotopic data should be measured with high spatial resolution. However isobaric interferences hamper the precise determination of Sr or Nd isotopic compositions in LA-MC-ICP-MS analysis. In this work we undertook in situ measurements of Sr and Nd isotopic compositions of eleven apatite reference materials (AP1, AP2, Durango, MAD, Otter Lake, NW-1, Slyudyanka, UWA-1, Mud Tank, McClure Mountain and SDG) commonly used in U–Th–Pb geochronology. Our obtained Sr and Sm–Nd isotopic compositions for these apatite samples are consistent with those values obtained by solution-based methods (isotope dilution and ion chromatography) using MC-ICP-MS or TIMS, which demonstrates the reliability and robustness of our analytical protocol.

U–Pb LA–ICPMS dating using accessory mineral standards with variable common Pb

Precise and accurate U–Pb LA–ICPMS dating of many U-bearing accessory minerals (e.g. apatite, allanite, titanite and rutile) is often compromised by common Pb. LA–ICPMS dating of these U-bearing accessory phases typically requires a matrix-matched standard, and data reduction is often complicated by variable incorporation of common Pb not only into the unknowns but also particularly into the reference material. We present here a general approach to common Pb correction in U–Pb LA–ICP–MS dating using a modified version of the VizualAge U–Pb data reduction package for Iolite (VizualAge_UcomPbine). The key feature of the method is that it can correct for variable amounts of common Pb in any U–Pb accessory mineral standard as long as the standard is concordant in the U/Pb (and Th/Pb) systems following common Pb correction. Common Pb correction of the age standard can be undertaken using either the 204Pb, 207Pb or 208Pb(no Th) methods, and the approach can be applied to raw data files from all widely used modern multi-collector and single-collector ICPMS instruments.VizualAge_UcomPbine first applies a common Pb correction to the user-selected age standard integrations and then fits session-wide “model” U–Pb fractionation curves to the time-resolved U–Pb standard data. This downhole fractionation model is applied to the unknowns and sample-standard bracketing (using a user-specified interpolation method) is used to calculate final isotopic ratios and ages. 204Pb- and 208Pb(no Th)-corrected concordia diagrams and 204Pb-, 207Pb- and 208Pb(no Th)-corrected age channels can be calculated for user-specified initial Pb ratio(s). All other conventional common Pb correction methods (e.g. intercept or isochron methods on co-genetic analyses) can be performed offline.The approach was tested on apatite and titanite age standards (for which there are independent constraints on the U–Pb crystallization age) using a Thermo Scientific iCAP-Qc (Q–ICP–MS) coupled to a Photon Machines Analyte Excite 193nm ArF Excimer laser. Madagascar apatite, OLT1 titanite and R10 rutile were used as primary standards and were corrected for variable common Pb using the new VizualAge_UcomPbine DRS. The secondary Durango (31.44±0.18Ma) apatite standard yielded a U–Pb TW concordia intercept age of 31.97±0.59Ma (MSWD=1.09; primary standard corrected by the 207Pb-method) and a U–Pb concordia age of 31.82±0.40Ma (MSWD=1.4; primary standard corrected by the 204Pb-method). McClure Mountain (523.51±1.47Ma) yielded a U–Pb TW concordia intercept age of 524.5±3.7Ma (MSWD=0.72) while the Fish Canyon Tuff (28.201±0.046Ma) and Khan (522.2±2.2Ma) titanite standards yielded U–Pb TW concordia intercept ages of 28.78±0.41Ma (MSWD=1.4) and 520.9±3.9Ma (MSWD=4.2) respectively. The suitability of the 208Pb(no Th)-correction is demonstrated by the agreement between a U–Pb TW concordia intercept age of 452.6±4.7Ma (MSWD=0.89) and a 208Pb(no Th)-corrected TW concordia age of 448.6±4.5Ma (MSWD=1.4) on a c. 450Ma rutile which exhibits variable incorporation of common Pb.A range of LA–ICPMS U–Pb dating applications are presented and include U–Pb dating of apatite from >3.8Ga gneisses from Akilia, SW Greenland. These apatites host 13C-depleted graphite inclusions that are interpreted as biogenic in origin and representing the oldest indications of life on Earth. The U–Pb age profiles on single apatite grains presented here are characteristic of Pb loss by volume diffusion with core–rim age differences of up to 300Ma. These data explain the scatter and poor precision of earlier U–Pb apatite age determinations on Akilia apatite. Other LA–ICPMS dating applications include U–Pb apatite dating as a rapid method for determining the age of mafic intrusions, U–Pb titanite and apatite dating of ash fall tuffs, determining temperature–time histories using multiple U–Pb thermochronometers and improving concordance in LA–ICPMS primary zircon standard datasets by analysing young, common Pb-bearing primary zircon standards that have not accumulated significant radiation damage.

Routine low‐damage apatite U‐Pb dating using laser ablation–multicollector–ICPMS

Apatite is a common U‐bearing accessory mineral with a U‐Pb closure temperature of ∼500°C, making U‐Pb dating of apatite a potentially valuable thermochronometer. However, its low U concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods. We overcome previous limitations by using a Nu Plasma multicollector ICPMS with an attached short‐pulse excimer laser, and by identifying two new matrix‐matched reference apatites to correct for elemental fractionation: gem‐quality 485 Myr old apatite from Madagascar which we independently characterized by ID‐TIMS analysis, and 523.5 Ma apatite from the McClure Mountain syenite (source of the40Ar/39Ar reference MMhb). Common Pb is corrected using measured 204Pb isobarically corrected for Hg interference and a five‐step iterative process using Stacey and Kramers' common Pb model. We accurately reproduce ages of numerous independently characterized apatites, regularly achieving precision of &lt;2% (2σ) by pooling as few as five 30 μm spot analyses. Data quality in apatite with low U concentrations, low 206Pb/204Pb values (&lt;∼30) and young ages (&lt;∼75 Ma) is compromised by the goal of avoiding significant grain damage. Such limitations can be overcome by using spot sizes 65 μm or greater, but at the expense of substantial grain damage. For single detrital apatite grains with ages of ∼500 Ma, precision of &lt;4% (2σ) was achieved by pooling 2 to 3 spots per grain. The accuracy of our detrital results is supported by a good age match with similar closure temperature 40Ar/39Ar detrital hornblende ages from the same sediment.

U–Pb systematics of the McClure Mountain syenite: thermochronological constraints on the age of the 40Ar/39Ar standard MMhb

Recent advances in U-Pb geochronology allow unprecedented levels of precision in the determination of geological ages. However, increased precision has also illuminated the importance of understanding subtle sources of open-system behavior such as Pb-loss, inheritance, intermediate daughter product disequilibria, and the accuracy of the model assumptions for initial Pb. Deconvolution of these effects allows a much richer understanding of the power and limitations of U-Pb geochronology and thermochronology. In this study, we report high-precision ID-TIMS U-Pb data from zircon, baddelleyite, titanite and apatite from the McClure Mountain syenite, from which the 40 Ar/ 39 Ar hornblende standard MMhb is derived. We find that excess 206 Pb in zircon due to inclusions of high-Th minerals and ele- vated Th/U in titanite and apatite jeopardize the utility of the 238 U- 206 Pb system in this rock. Strongly air- abraded zircons give dates that are younger than chemical-abraded zircons, which yield a statistically robust 207 Pb/ 235 U date of 523.98±0.12 Ma that is interpreted as the crystallization age. We explore the best method of Pbc correction in titanite and apatite by analyzing the U-Pb isotopes of K-feldspar and using 2-D and 3-D regression methods—the latter of which yields the best results in each case. However, the calcu- lated compositions of Pbc for titanite, apatite and K- feldspar are different, implying that using a single Pbc correction for multiple U-Pb thermochronometers may be inaccurate. The U-Pb thermochronological results are used to predict a closure time for Ar in hornblende of 522.98±1.00 Ma. Widely cited K-Ar and 40 Ar/ 39 Ar dates overlap with the U-Pb date, and relatively large errors make it impossible to verify whether U-Pb dates are systematically £ 1% older than K-Ar and 40 Ar/ 39 Ar dates.

Intercalibration of standards, absolute ages and uncertainties in 40Ar/ 39Ar dating

The 40Ar/ 39Ar dating method depends on accurate intercalibration between samples, neutron fluence monitors, and primary 40Ar/ 40K (or other external) standards. The 40Ar/ 39Ar age equation may be expressed in terms of intercalibration factors that are simple functions of the relative ages of standards, or equivalently are equal to the ratio of radiogenic to nucleogenic K-derived argon ( 40Ar/ 39Ar K) values for one standard or unknown relative to another. Intercalibration factors for McClure Mountain hornblende (MMhb-1), GHC-305 biotite, GA-1550 biotite, Taylor Creek sanidine (TCs) and Alder Creek sanidine (ACs), relative to Fish Canyon sanidine (FCs), were derived from 797 analyses involving 11 separate irradiations with well-constrained neutronfluence variations. Values of the intercalibration factors are R FCs MMhb-1 = 21.4876 ± 0.0079; R FCs GA-1550 = 3.5957 ± 0.0038; R FCs TCs = 1.0112 ± 0.0010; R FCs ACs = 0.04229 ± 0.00006, based on the mean and standard error of the mean resulting from four or more spatially distinct co-irradiations of FCs with the other standars. Analysis of 35 grains of GHC-305 irradiated in a single irradiation yields R FCs GH C-305 = 3.8367 ± 0.0143. Results at these levels of precision essentially eliminate intercalibration as a significant source of error in 40Ar/ 39Ar dating. Data for GA-1550 (76 analyses, 5 fluence values), TCs (54 analyses, 4 fluence values), FCs (380 analyses, 40 fluence values) and ACs (86 analyses, 11 fluence values) yield MSWD values showing that the between-grain dispersion of 40Ar ∗/ 39Ar K values is consistent with analytical errors alone, whereas MMhb-1 (167 analyses, 4 irradiations) and GHC-305 (34 analyses, 1 fluence value) are heterogeneous and therefore unsuitable as standards for small sample analysis. New K measurements by isotope dilution for two primary standards, GA-1550 biotite (8 analyses averaging 7.626 ± 0.016 wt%) and intralaboratory standard GHC-305 (10 analyses averaging 7.570 ± 0.011 wt%), yield values slightly lower and more consistent than previous data obtained by flame photometry, with resulting 40Ar/ 40K ages of 98.79 ± 0.96 Ma and 105.6 ± 0.3 Ma for GA-1550 and GHC-305, respectively. Combining these data with the intercalibration approach described herein and using GA-1550 as the primary standard (1.343 × 10 −9 mol/g of 40Ar ∗; [McDougall, I., Roksandic, Z., 1974. Total fusion 40Ar/ 39Ar ages using HIFAR reactor. J. Geol. Soc. Aust. 21, 81–89.]) yields ages of 523.1 ± 4.6 Ma for MMhb-1, 105.2 ± 1.1 Ma for GHC-305, 98.79 ± 0.96 Ma for GA-1550, 28.34 ± 0.28 Ma for TCs, 28.02 ± 0.28 for FCs, and 1.194 ± 0.012 Ma for ACs (errors are full external errors, including uncertainty in decay constants). Neglecting error in the decay constants, these ages and uncertainties are: 523.1 ± 2.6 Ma for MMhb-1, 105.2 ± 0.7 Ma for GHC-305, 98.79 ± 0.54 for GA-1550, 28.34 ± 0.16 Ma for TCs, 28.02 ± 0.16 Ma for FCs, and 1.194 ± 0.007 Ma for ACs. Using GHC-305 as the primary standard (1.428 ± 0.004 × 10 −9 mol/g of 40Ar ∗), ages are 525.1 ± 2.3 Ma for MMhb-1, 105.6 ± 0.3 Ma for GHC-305, 99.17 ± 0.48 Ma for GA-1550, 28.46 ± 0.15 Ma for TCs, 28.15 ± 0.14 Ma for FCs, and 1.199 ± 0.007 Ma for ACs, neglecting decay constant uncertainties. The approach described herein facilitates error propagation that allows for straightforward inclusion of uncertainties in the ages of primary standards and decay constants, without which comparison of 40Ar/ 39Ar dates with data from independent geochronometers is invalid. Re-examination of 40K decay constants would be fruitful for improved accuracy.

Paleomagnetism of the Cambro-Ordovician McClure Mountain alkalic complex, Colorado

The Middle to Late Cambrian loop in the North American apparent polar wander path (APWP) has been variously attributed to tectonic rotations, remagnetizations and primary magnetizations. Although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated, the temporally self-consistent nature of the loop has been used as an argument for primary magnetizations. We have studied 535 ± 5Ma nepheline syenites and syenites of the McClure Mountain alkalic complex, as well as 495 ± 10Ma red trachyte dikes which intruded the complex, in an effort to find a primary TRM. Because Zijderveld analysis yielded consistent results for only one trachyte dike, remagnetization great-circle analysis was employed, giving a pole for the trachyte dikes at the tip of the loop (43°N, 114°E), while the syenites and nepheline synenites gave a pole at the base of the loop (18°N, 142°E). The magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration. K Ar whole rock dating of the trachytes suggests a Pennsylvanian age for the alteration, and thus a late Paleozoic remagnetization of the trachytes. Thus, the low-latitude Cambrian pole is confirmed, but we find no evidence in this study to support the primary nature of the Cambrian APWP.

Genetic implications of minor-element and Sr-isotope geochemistry of alkaline rock complexes in the Wet Mountains area, Fremont and Custer counties, Colorado

Concentrations of Rb, Sr, and REE (rare earth elements), and Sr-isotopic ratios in rocks of the Cambrian alkaline complexes in the Wet Mountains area, Colorado, show that rocks formed as end-products of a variety of magmas generated from different source materials. The complexes generally contain a bimodal suite of cumulus mafic-ultramafic rocks and younger leucocratic rocks that include nepheline syenite and hornblende-biotite syenite in the McClure Mountain Complex, nepheline syenite pegmatite in the Gem Park Complex, and quartz syenite in the complex at Democrat Creek. The nepheline syenite and hornblende-biotite syenite at McClure Mountain (535±5m.y.) are older than the syenitic rocks at Democrat Creek (511±8m.y.). REE concentrations indicate that the nepheline syenite at McClure Mountain cannot be derived from the hornblende-biotite syenite, which it intrudes, or from the associated mafic-ultramafic rocks. REE also indicate that mafic-ultramafic rocks at McClure Mountain have a source distinct from that of the mafic-ultramafic rocks at Democrat Creek.