Initial Pb Composition Research Articles

U-Pb isotopic dating of cassiterite: Development of reference materials and in situ applications by LA-SF-ICP-MS

Cassiterite, the economically most important tin mineral, typically has moderate U and variable common Pb contents, making it amenable for UPb dating. Cassiterite has extremely low Th/U ratios (Th/U < 0.01) and its 208Pb is dominantly common Pb. This is particularly helpful as there is significant interference of tungsten oxides on 202Hg and 204Pb. The feasibility of the 208Pb correction procedure is discussed in detail. The 208Pb corrected LA-SF-ICP-MS data are in good agreement with intercept ages in the Tera-Wasserburg diagram and 207Pb corrected ages.Twelve cassiterite samples were investigated using the ID-TIMS and LA-SF-ICP-MS methods. The ID-TIMS results of Pit-AB, Rond-A, RG-114, BB#7 and 19GX cassiterite are reported for the first time in this study. RG-114, BB#7 and 19GX cassiterite have very low common Pb contents and are recommended for use as primary reference materials for in situ cassiterite. Pit-AB, Rond-A and Yankee cassiterite contain a small amount of common Pb, produce reliable and consistent ages and are suitable as primary reference materials. The remaining five cassiterite samples (Kard, Zinnwald, Els, XBD-W and Y724) were only investigated using the LA-SF-ICP-MS method and produce ages consistent with published age data from the host rocks associated with the tin deposits and with published UPb ages of cassiterite from the same deposits. We present an ID-TIMS UPb of 154.3 ± 0.7 Ma for the commonly used cassiterite reference material AY-4. This age differs from previously reported ID-TIMS ages. This age discrepancy is caused by different initial common Pb compositions rather than age heterogeneity.

U-Pb ID-TIMS reference ages and initial Pb isotope compositions for Durango and Wilberforce apatites

We present new Isotope Dilution Thermal Ionisation Mass Spectrometry (ID-TIMS) U-Pb reference ages for apatites from type locations Durango, Cerro de Mercado, Mexico and Wilberforce, Ontario, Canada. A weighted mean 206Pb/238U obtained from 14 Durango apatite fragments yields an age of 32.716 ± 0.061/0.063/0.072 Ma (2σ analytical uncertainty; analytical and tracer uncertainty combined; analytical, tracer, and decay constant uncertainties combined; MSWD = 0.81). Correction of excess 206Pb from Th-disequilibrium in Durango apatite, using a Th/Umelt of 3.5 ± 1, yields a weighted mean 206Pb/238U age of 32.29 ± 0.12/0.12/0.12 Ma. A concordia-constrained linear 3-D isochron yields an age of 32.683 ± 0.050 Ma for Durango apatites (ratio + error (%) + correlation coefficients; MSWD = 7.6; no propagation of decay constant and Pbc composition uncertainty, isoplot v. 4.15). A weighted mean 206Pb/238U obtained from 9 Wilberforce apatite fragments yield an age of 930.9 ± 8.0/8.0/8.1 Ma (MSWD = 0.65). Excess 206Pb correction for Wilberforce is negligible (<100kyr) relative to the uncertainty. A concordia-constrained linear 3-D isochron yields an age of 923 ± 14 Ma (ratio + error (%) + correlation coefficients; MSWD = 367; no propagation of decay constant and Pbc composition uncertainty, isoplot v. 4.15) for Wilberforce apatites. Initial Pb isotope compositions for these apatites were obtained by regression of the U-Pb data (3-D linear regression) and yielded a 206Pb/204Pb of 18.19 ± 0.12 and 207Pb/204Pb of 15.67 ± 0.10 for the Durango apatite reference material, and a 206Pb/204Pb of 17.06 ± 0.37 and 207Pb/204Pb of 15.33 ± 0.29 for the Wilberforce apatite reference material. Varying the assumed initial Pb composition can have significant effects on the calculated U-Pb age of apatites, which is highlighted with synthetic apatite data based on the composition of the Durango apatite reference material. For Durango apatites, the frequently used estimates of common Pb from two-stage Pb evolution models can artificially shift the U-Pb data and yield inaccurate dates, relative to the use of 3-D isochrons constructed using data obtained from Durango apatite. The use of 3-D isochrons to extract initial Pb isotope data should be restricted to samples/material with single U-Pb date population (closed system) and where estimates from Pb-bearing-U-free co-genetic mineral phases is unavailable and evaluated on a case-by-case basis.

Apatite U-Pb Thermochronology: A Review

The temperature sensitivity of the U-Pb apatite system (350–570 °C) makes it a powerful tool to study thermal histories in the deeper crust. Recent studies have exploited diffusive Pb loss from apatite crystals to generate t-T paths between ~350–570 °C, by comparing apatite U-Pb ID-TIMS (isotope dilution-thermal ionisation mass spectrometry) dates with grain size or by LA-MC-ICP-MS (laser ablation-multicollector-inductively coupled plasma-mass spectrometry) age depth profiling/traverses of apatite crystals, and assuming the effective diffusion domain is the entire crystal. The key assumptions of apatite U-Pb thermochronology are discussed including (i) that Pb has been lost by Fickian diffusion, (ii) can experimental apatite Pb diffusion parameters be extrapolated down temperature to geological settings and (iii) are apatite grain boundaries open (i.e., is Pb lost to an infinite reservoir). Particular emphasis is placed on detecting fluid-mediated remobilisation of Pb, which invalidates assumption (i). The highly diverse and rock-type specific nature of apatite trace-element chemistry is very useful in this regard—metasomatic and low-grade metamorphic apatite can be easily distinguished from sub-categories of igneous rocks and high-grade metamorphic apatite. This enables reprecipitated domains to be identified geochemically and linked with petrographic observations. Other challenges in apatite U-Pb thermochronology are also discussed. An appropriate choice of initial Pb composition is critical, while U zoning remains an issue for inverse modelling of single crystal ID-TIMS dates, and LA-ICP-MS age traverses need to be integrated with U zoning information. A recommended apatite U-Pb thermochronology protocol for LA-MC-ICP-MS age depth profiling/traverses of apatite crystals and linked to petrographic and trace element information is presented.

Reconsidering initial Pb in titanite in the context of in situ dating

Abstract In situ U-Pb dating of titanite, which can preserve trace-element records of various petrologic processes but also incorporates significant initial Pb, has proliferated in recent years. The widespread use of titanite data to construct tectonic P-T-t paths warrants careful assessment of the available dating techniques, as well as attention to the assumptions that underpin the U-Pb data analysis. This contribution provides the first direct comparison of the two major analytical methods [SHRIMP (SIMS) and LA-ICP-MS] for in situ U-Pb titanite dating. A set of well-characterized titanite grains from Harrisville, New York, in the Adirondack Mountains were analyzed for U-Th-Pb isotopes along the same cross-grain traverses by Sensitive High Resolution Ion Microprobe (SHRIMP) and LA-ICP-MS. Both LA-ICP-MS and SHRIMP data sets define approximately linear arrays on the Tera-Wasserburg Concordia (semi-total Pb/U) diagram and would commonly be interpreted as representing a single date population with minor scatter. However, previous studies have suggested that Adirondack titanite actually records two regionally well-defined thermal events, ~50–100 m.y. apart. When titanite data arrays are treated in detail, attempts to determine concordia-intercept ages by robust three-dimensional linear regression produce large uncertainties and/or poor fit statistics that suggest that the data are not, in fact, isochronous. Grain-by-grain analysis of U-Pb titanite data shows that different subsets of titanite (determined by additional geochemical and microstructural data) show different patterns of U-Pb data. By comparing predictions for Pb-ingrowth evolution paths in Tera-Wasserburg diagrams with observed data, it is possible to recognize both a change in initial Pb composition and Pb loss in the Adirondack titanite U-Pb data set. This study provides an example of how greater geochronologic detail can be extracted from large in situ U-Pb titanite data sets. Even when precise dates are not recovered, geological processes and events that cause data scatter can be recognized through analysis of U-Pb data patterns using the Tera-Wasserburg diagram.

U–Pb geochronology of apatite and zircon from the Brent impact structure, Canada: a Late Ordovician Sandbian–Katian boundary event associated with L-Chondrite parent body disruption

In situ LA-ICP-MS U–Pb geochronology has been performed on apatite and zircon within thermally recrystallized clast-laden and clast-poor impact melt rocks from the Brent impact structure. A total of 377 laser analyses on 120 impact melt-grown (n = 9) and impact-recrystallized zircon grains (n = 111) were obtained, from which a concordia age of 452.8 ± 2.7 Ma (MSWD 0.57, n = 11), and a weighted average mean 206Pb/238U age of 453.2 ± 2.9 Ma (MSWD 0.60) (n = 11) are calculated. A total of 300 laser analyses from 100 relict apatite grains were obtained, with an unanchored regression through all data yielding a lower intercept age of 453.2 ± 6.0 Ma (MSWD 5.8, n = 300), that overlaps within error of zircon. 207Pb/206Pb ratios obtained from feldspar clasts within clast-laden impact melt retain the same initial Pb composition as the target rocks from which they are derived, while feldspars that crystallized from impact melt have 207Pb/206Pb ratios indicative of isotopic re-equilibration between basement lithologies of two different ages. A similar variability in 207Pb/206Pb is recorded by apatite. This provides evidence for the involvement of Neoproterozoic Lake Nipissing alkaline suite, as well as Mesoproterozoic Grenville gneisses in the production of impact melt at Brent. Recrystallized apatite grains exhibit enrichments in light rare earth elements (LREEs) along neoblast grain boundaries, indicative of trace element substitution and phase precipitation during impact-induced recrystallization. An age of 452.8 ± 2.7 Ma from zircon and 453.2 ± 6.0 Ma from apatite places the impact event in the Late Ordovician, at or near the Sandbian–Katian boundary, confirming Brent’s involvement in the Middle to Late Ordovician crater cluster event—a period of enhanced impactor flux to Earth related to the L-Chondrite parent body disruption.

Insights into the chemical diversity of the martian mantle from the Pb isotope systematics of shergottite Northwest Africa 8159

Shergottite Northwest Africa (NWA) 8159 is a basaltic rock derived from a mantle source with chemical characteristics that are unique in the martian meteorite suite. To further investigate this source reservoir, the Pb isotope compositions of plagioclase/maskelynite, pyroxene, phosphates, and shock melt-glass in NWA 8159 have been measured in situ by Secondary Ion Mass Spectrometry (SIMS). Due to the limited spread in Pb isotope data, these Pb isotope compositions have been used to calculate an imprecise PbPb isochron age of 3.4 ± 2.1 Ga (2σ), which is broadly consistent with the crystallization age of 2.37 ± 0.25 Ga determined previously by 147Sm143Nd. The lack of radiogenic in-growth within individual minerals since 2.4 Ga means that this sample is depleted in U, which is in agreement with NWA 8159's positive initial ε143Nd. An initial Pb composition was calculated using an x-y weighted average of the least radiogenic Pb isotope population measured in the sample. This initial Pb composition is not consistent with the model for Pb growth in the shergottite mantle at 2.4 Ga. This composition is, however, consistent with the model for the Nakhla-Chassigny mantle. Using the latter model, a source μ (238U/204Pb) of 2.6 ± 0.6 has been calculated. This μ-value is in contrast with the other depleted shergottites (1.4-1.5) and falls significantly off the array of source ε143Nd vs. μ defined by the rest of the martian meteorite suite and thus, necessitates a differentiation history distinct from the other martian meteorites. Sequestering Pb in sulphides during differentiation is the only mechanism to fractionate U from Pb and create a low-μ reservoir. Consequently, the relatively high μ-value of the source of NWA 8159 is in contrast with the positive initial ε143Nd and indicates that its mantle source region likely lacked significant sulphur. This is consistent with the lack of sulphides in the sample itself and could have played a role in its complicated oxidation history.

Trace elements and U-Pb ages in petrified wood as indicators of paleo-hydrologic events

Subsurface fluid systems are important for chemical weathering, ore formation and thermal evolution of the crust. Changes in the dynamics and distribution of subsurface fluid flow systems are controlled by changes in global and regional terrestrial climate, tectonics, and elevation. This paper concerns the dating of changes in ancient subsurface hydrologic systems. However, direct dating of water-rock interaction is challenging because of the lack of appropriate materials to date and the more open and complex nature of subsurface flow regimes. Here, we explore the prospects of using U-Pb dating of petrified (silicified) wood as a means of quantifying continental paleo-hydrology. Oxidizing fluids, often of meteoric origin, tend to leach and mobilize U from the country rock, but when such waters contact organic-rich material, U can become reduced and immobilized, resulting in U-rich silicified wood. We present in situ laser ablation ICPMS analyses of U-Pb isotopes and trace elements in petrified wood from the Upper Triassic Chinle Formation (225–208 Myr) in the Petrified Forest National Park in Arizona (USA), allowing us to establish a generalized workflow for making meaningful paleo-hydrologic interpretations of the U-Pb systematics of silicified wood. Wood characterized by brownish colors and preservation of cellular structure have low Fe contents and positive Ce anomalies, indicating silicification in reducing environments and isolation in relatively reduced conditions after silicification, resulting in closed system behavior of U and Pb. Wood characterized by vivid colors (orange, red, etc.) and little to no preservation of cellular structure are much higher in Fe and exhibit negative Ce anomalies, indicating influence by more oxidized fluids. The brownish samples yield U-Pb ages clustered between 250 and 200 Ma with a peak coinciding with the time of deposition (~220 Ma), which indicates that fossilization largely took place almost immediately after deposition and that U-Pb in quartz faithfully retains the time of such fossilization. In contrast, the orangish-reddish-whitish samples yield younger U-Pb ages, defining distinct errorchron ages, which reflect subsequent generations of quartz crystallization. Scatter associated with errorchrons are likely due to local (mm- to cm-scale) U or Pb mobility or variable initial Pb composition. Distinct younger age peaks appear to correlate with the timing of regional unconformities associated with tectonic or epeirogenic uplift. We suggest that uplift and exhumation may initiate the onset of oxidizing fluid systems, resulting in leaching and transport of U from the surroundings, followed by subsequent generations of quartz precipitation. In summary, U-Pb dating of petrified wood or silicified organic material, has high potential for dating paleo-hydrologic events. However, due to complexities in terrestrial hydrologic systems, interpretations of U-Pb systematics must be informed by accompanying geochemical and textural observations.

Pb evolution in the Martian mantle

The initial Pb compositions of one enriched shergottite, one intermediate shergottite, two depleted shergottites, and Nakhla have been measured by Secondary Ion Mass Spectrometry (SIMS). These values, in addition to data from previous studies using an identical analytical method performed on three enriched shergottites, ALH 84001, and Chassigny, are used to construct a unified and internally consistent model for the differentiation history of the Martian mantle and crystallization ages for Martian meteorites. The differentiation history of the shergottites and Nakhla/Chassigny are fundamentally different, which is in agreement with short-lived radiogenic isotope systematics. The initial Pb compositions of Nakhla/Chassigny are best explained by the late addition of a Pb-enriched component with a primitive, non-radiogenic composition. In contrast, the Pb isotopic compositions of the shergottite group indicate a relatively simple evolutionary history of the Martian mantle that can be modeled based on recent results from the Sm–Nd system. The shergottites have been linked to a single mantle differentiation event at 4504 Ma. Thus, the shergottite Pb isotopic model here reflects a two-stage history 1) pre-silicate differentiation (4504 Ma) and 2) post-silicate differentiation to the age of eruption (as determined by concordant radiogenic isochron ages). The μ-values (238U/204Pb) obtained for these two different stages of Pb growth are μ1 of 1.8 and a range of μ2 from 1.4–4.7, respectively. The μ1-value of 1.8 is in broad agreement with enstatite and ordinary chondrites and that proposed for proto Earth, suggesting this is the initial μ-value for inner Solar System bodies. When plotted against other source radiogenic isotopic variables (Sri, γ187Os, ε143Nd, and ε176Hf), the second stage mantle evolution range in observed mantle μ-values display excellent linear correlations (r2>0.85) and represent a spectrum of Martian mantle mixing-end members (depleted, intermediate, enriched).

Pb-Pb ages of feldspathic clasts in two Apollo 14 breccia samples

Pb-Pb isochron ages of ca. 3.92Ga for three K-feldspar-rich clasts from Apollo 14 breccias 14303 and 14083 were determined using Secondary Ion Mass Spectrometry (SIMS). These ages are interpreted to represent the resetting of the U-Pb system in the clasts as a result of brecciation during the Imbrium impact. One of the clasts contains zircon grains that record a significantly older crystallization age (ca. 4.33–4.35Ga) for the rock represented by that clast. Initial Pb compositions determined for the clasts, combined with the previously measured Pb isotopic compositions of K-feldspar grains from several Apollo 14 breccia samples, constrain a range of initial Pb compositions in the ca. 3.9Ga Fra Mauro formation at the Apollo 14 landing site. This range in initial Pb compositions indicates that the rocks represented by these clasts, or the sources of those rocks, evolved with a high 238U/204Pb (μ-value) for substantial periods of time, although the precise crystallization ages of the rocks represented by at least two of the clasts investigated here are unknown.

Pb isotopes in the impact melt breccia 66095: Association with the Imbrium basin and the isotopic composition of lithologies at the Apollo 16 landing site

Recent in situ Secondary Ion Mass Spectrometry (SIMS) Pb isotope analyses of lunar basalts have provided precise crystallisation ages and initial Pb isotopic compositions for these samples. In this study, the same approach has been tested in the Apollo 16 impact melt breccia 66095, referred to as the “Rusty Rock” due to its enrichments in volatile elements, including Pb. Based on these analyses of the breccia, a Pb-Pb isochron age of 3909±17Ma (at the 95% confidence level) and an initial Pb composition for 66095 have been determined. This age is interpreted as representing the time of breccia formation that, when combined with recent studies of lunar breccias, can be linked to the Imbrium basin forming impact. The directly measured initial Pb composition of the breccia from this work is similar a modelled compositions presented previously, and likely reflects an average value for the lithologies present at the Apollo 16 landing site at the time that the Imbrium ejecta was emplaced. The 66095 initial Pb isotopic composition is compared with the compositions in other lunar samples and the nature of the endmember lithologies in this mixture has been discussed within the framework of a multiple stage model of Pb isotope evolution on the Moon. This study demonstrates the effectiveness of this technique beyond its application in crystalline basalts, opening up the possibility of obtaining precise geochronological and Pb isotopic compositions from a broader sample set than was previously recognised.

Theory of a methodology for initial lead determination, and a procedure to resolve it to some of its stages

A method for determining the initial Pb of a terrain, on the basis of the measured isotopic compositions of its rocks, is put forward in this report. The method was inspired by the premise that the initial Pb inherited by the rocks from a reservoir from which they were extracted, is immutable and inerasable, irrespective of the multitude of disturbances that may have subsequently been superimposed on the terrain. This is because while these disturbances may have altered the isotopic composition of some or all the rocks, they lacked the vehemence to re-melt the entire terrain or at least a very large portion of it, which is a pre-requisite for altering the isotopic composition of initial Pb. If this rational is valid, then a large Pb isotope database (including data on mineral separates with low affinities for U and Th) that is representative of a terrain, when plotted on any Pb isotope correlation diagram (e.g., the conventional Pb/Pb plot), may define a dispersion field that tapers toward a single spot. That single spot (once unambiguously determined) is the initial Pb isotopic composition.Furthermore, from the equations of radioactive decay as applied in geochronology, the author provides evidence for the potential existence within a Pb isotope dispersion field, of three classes of lines that converge in different types of Pb isotope correlations, to always meet in a point that yields the composition of initial Pb.These lines are: (1) isochron: defined by samples that remained as closed systems since crystallization, (2) transposichrons: each made up of samples that experienced in a disturbance episode, fractionation by the same constant factor F=(238U/204Pb)pc/(238U/204Pb)cr, and/or the same constant factor K=(232Th/238U)pc/(232Th/238U)cr, where the subscripts stand for post-crystallization and crystallization, and (3) Heterochrons: each defined by samples produced by different evolution scenarios (including different multiple stages with different values for the geochemical parameters F and K), which happen to accidentally have the same average F and/or the same average K.As demonstrated by synthetic examples, heterochrons occur because the production of a Pb isotopic ratio by radioactive decay is controlled by multiple Independent variables. This circumstance allows for various combinations of the parameters, to accidentally produce same radiogenic isotopic ratio. An application to a terrestrial terrain (out of four, discussed in companion paper # 2), further illustrates in this report, the validity of the rational and the practicality of the method.The determination of the initial Pb composition is a necessary step toward elucidation of the early evolution history of a planet, but alone, such a step falls short of reaching the goal. Full chronometric elucidation of a planet's history, requires complete resolution of initial Pb into all the multiple stages that led to its final composition. As this is not readily tenable, a procedure (termed ‘Congruently Associated Profiles’, CAPs) in which initial Pb is partially resolved (into two or three stages) is introduced as an alternative.However, now that initial Pb is shown to be routinely determinable, could resolution of its multi-stages, someday become possible? To some, including the author, even the apparent ‘impossibility’ of an objective neither over rides hope nor stops prospecting for its realization.

U–Pb systematics in carbonates of the Postmasburg Group, Transvaal Supergroup, South Africa: Primary versus metasomatic controls

Petrography, geochemistry and U–Pb geochronology of a section through the Mooidraai carbonates which overly the Hotazel iron formation in the Postmasburg Group of the Transvaal Supergroup, and comparison with previous work, demonstrate different degrees of localized metasomatic overprints causing chemical changes and affecting the U–Pb isotopic features of the rocks. The lower parts of the section in drillhole OLP-2, at the transition from iron formation to carbonates, exhibit extensive formation of riebeckite in concordant seams and layers, probably crystallized in response to addition of Na from early pore water or brines. Mooidraai limestone samples throughout the section are characterized by the extensive development of stylolites, which are enriched in U with respect to intervening massive carbonate laminae. The latter preserve Pb–Pb systematics close to those documented previously in dolomitized Mooidraai Formation, whereas the stylolites have strongly discordant U–Pb systems. The U–Pb data define an upper intercept age of 2392±23Ma, which confirms the previous Pb–Pb age of the formation, and a lower intercept age of 588±31Ma, which is interpreted to reflect a metasomatic overprint, also recorded elsewhere in the wider region by Ar–Ar dating of hydrothermal minerals. The combined available geochronological record indicates the occurrence of several extensive episodes of fluid-controlled metasomatism, apparently representing the far-field response to distal orogenic processes. The initial Pb composition of the Mooidraai carbonates is much less radiogenic than that in carbonates associated with the stratigraphically lower Makganyene diamictite Formation, reflecting a provenance of the Pb from erosion of older Archean crust rather than a typical sea-water signal as recorded in the Mooidraai carbonates.

Geochemistry and Sr–Nd–Pb–Hf isotopes of the Mesozoic Dadian alkaline intrusive complex in the Sulu orogenic belt, eastern China: Implications for crust–mantle interaction

The Dadian alkaline intrusive complex is located within the Sulu orogenic belt, and includes hornblende syenite, syenite, quartz syenite and syenite porphyry. All the rocks from the complex show high SiO 2, K 2O + Na 2O and LREE and low CaO, FeO T, MgO and HFSE (Nb, Ta, P, Ti) concentrations. The Sr, Nd, and Hf isotopic compositions (( 87Sr/ 86Sr) i = 0.708505–0.70876, ε Nd( t) = − 16.5 to − 17.9, ε Hf( t) = − 21.5 to − 16.4) of the rocks fall within the compositional field of mafic dykes occurring nearby, suggesting that the Dadian alkaline complex was mainly derived from an enriched lithospheric mantle. The initial Pb compositions (( 206Pb/ 204Pb) i = 16.706–16.779, ( 207Pb/ 204Pb) i = 15.427–15.450, ( 208Pb/ 204Pb) i = 37.266–37.352) of the Dadian rocks are lower than those of the Yangtze lithospheric mantle, albeit similar to that reported from beneath the North China Craton, implying that the mantle source of the Dadian alkaline complex most likely belong to the North China Craton. The relatively high SiO 2 and HREE contents and low Nb/Ta ratios as well as strongly negative Eu and Sr anomalies of some of the rocks from the Dadian complex suggest that crustal material was also involved in the formation of this complex and that the crustal component was probably derived through partial melting at low pressure. The high crystallization temperatures revealed by zircon saturation thermometry and low pressures suggested by Al-in-hornblende barometry indicate that the Dadian alkaline complex crystallized at high temperature and intruded into shallow crustal level. This intrusive complex does not show any relationship with post-orogenic process in the region, and its petrogenesis is inferred to be associated with the Paleo-Pacific subduction tectonics.

U–Pb and Th–Pb dating of apatite by LA-ICPMS

Apatite is a common U- and Th-bearing accessory mineral in igneous and metamorphic rocks, and a minor but widespread detrital component in clastic sedimentary rocks. U–Pb and Th–Pb dating of apatite has potential application in sedimentary provenance studies, as it likely represents first cycle detritus compared to the polycyclic behavior of zircon. However, low U, Th and radiogenic Pb concentrations, elevated common Pb and the lack of a U–Th–Pb apatite standard remain significant challenges in dating apatite by LA-ICPMS, and consequently in developing the chronometer as a provenance tool. This study has determined U–Pb and Th–Pb ages for seven well known apatite occurrences (Durango, Emerald Lake, Kovdor, Mineville, Mud Tank, Otter Lake and Slyudyanka) by LA-ICPMS. Analytical procedures involved rastering a 10 μm spot over a 40 × 40 μm square to a depth of 10 μm using a Geolas 193 nm ArF excimer laser coupled to a Thermo ElementXR single-collector ICPMS. These raster conditions minimized laser-induced inter-element fractionation, which was corrected for using the back-calculated intercept of the time-resolved signal. A Tl–U–Bi–Np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias. External standards (Plešovice and 91500 zircon, NIST SRM 610 and 612 silicate glasses and STDP5 phosphate glass) along with Kovdor apatite were analyzed to monitor U–Pb, Th–Pb, U–Th and Pb–Pb ratios Common Pb correction employed the 207Pb method, and also a 208Pb correction method for samples with low Th/U. The 207Pb and 208Pb corrections employed either the initial Pb isotopic composition or the Stacey and Kramers model and propagated conservative uncertainties in the initial Pb isotopic composition. Common Pb correction using the Stacey and Kramers (1975) model employed an initial Pb isotopic composition calculated from either the estimated U–Pb age of the sample or an iterative approach. The age difference between these two methods is typically less than 2%, suggesting that the iterative approach works well for samples where there are no constraints on the initial Pb composition, such as a detrital sample. No 204Pb correction was undertaken because of low 204Pb counts on single collector instruments and 204Pb interference by 204Hg in the argon gas supply. Age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common Pb-corrected ages, a Tera–Wasserburg Concordia intercept age and a Tera–Wasserburg Concordia intercept age anchored through common Pb. The samples in general yield ages consistent (at the 2σ level) with independent estimates of the U–Pb apatite age, which demonstrates the suitability of the analytical protocol employed. Weighted mean age uncertainties are as low as 1–2% for U- and/or Th-rich Palaeozoic–Neoproterozoic samples; the uncertainty on the youngest sample, the Cenozoic (31.44 Ma) Durango apatite, ranges from 3.7–7.6% according to the common Pb correction method employed. The accurate and relatively precise common Pb-corrected ages demonstrate the U–Pb and Th–Pb apatite chronometers are suitable as sedimentary provenance tools. The Kovdor carbonatite apatite is recommended as a potential U–Pb and Th–Pb apatite standard as it yields precise and reproducible 207Pb-corrected, 232Th– 208Pb, and common Pb-anchored Tera–Wasserburg Concordia intercept ages.

U–Pb dating of speleothems from Spannagel Cave, Austrian Alps: A high resolution comparison with U–series ages

U–Pb dating is increasingly used to date speleothems that are too old for precise U–Th disequilibrium dating; however there is little data that can independently validate its application to such material. This study presents U–Pb ages for speleothems from the Spannagel Cave in the Austrian Alps including a detailed comparison with U–Th ages from an unusually U–rich sample that yields precise ages by both methods. Sample SPA4 is a flowstone with three growth phases separated by distinct hiatuses. For the youngest growth phase the U–Pb and U–Th ages are 267 ± 1 ka and 267 ± 5 ka respectively; the middle growth phase is 291 ± 1 versus 295 ± 11 ka while for the oldest growth phase a single sub-sample, assuming the same initial Pb composition as for the younger phases, yields an age of 340 ± 2 ka compared to 353 ± 9 ka by U–Th. Correlation of these ages with the marine isotope stages confirms that these speleothems grew during glacial stages as suggested by previous work on the same sample. Sample SPA 15 has U–Th isotopic compositions indistinguishable from secular equilibrium; the U–Pb data on the main growth phase of this sample give an age of 551 ± 10 ka, whereas a single analysis from the oldest phase suggests it may be on the order of 40 ka older. This detailed comparison of U–Pb and U–Th ages provides important support for the potential validity of the U–Pb method in older samples beyond the range of U–Th.

Perovskite U–Pb ages and the Pb isotopic composition of alkaline volcanism initiating the Permo-Carboniferous Oslo Rift

The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period of some 50 million years through a process characterized by moderate extension and widespread magmatism. The overall tectonic situation places the Oslo Rift in a post-collisional, dextral transtensional setting related to the convergence between Baltica, Laurentia, Gondwana and Siberia during assembly of Pangea, the location probably reflecting the control by pre-existing lithospheric structures. Although a detailed understanding of these factors and processes relies strongly on having a good age control, the dating of mafic to ultramafic alkalic volcanic units formed during initial rifting has been a very challenging task. In this study we have successfully employed perovskite from melilitic and nephelinitic volcanic rocks, together with magmatic titanite in a more evolved ignimbrite, to obtain ID-TIMS high-precision U–Pb ages. Three samples from various levels of the Brunlanes succession, in the southernmost exposures of the Oslo Graben, yield ages of 300.2 ± 0.9, 300.4 ± 0.7 and 299.9 ± 0.9 Ma. A melililitic tuff at the base of the Skien succession further to the northwest yields a slightly younger age of 298.9 ± 0.7 Ma. The initial Pb compositions derived mainly from coexisting pyroxene, apatite and hornblende are characterized by extremely radiogenic initial 206Pb/ 204Pb ratios (up to 21.3) that confirm a provenance of these early alkaline basalts from HIMU-type sources. The U–Pb ages coincide with the Gzhelian age inferred from fossils in the upper part of the basal rift sedimentary fill of the Asker Group, and post-date the underlying basal sedimentary sequences by some 10 million years, pointing to a relatively rapid initiation of the rifting process.

The origin of geochemical diversity of lunar mantle sources inferred from the combined U–Pb, Rb–Sr, and Sm–Nd isotope systematics of mare basalt 10017

The results of our combined U–Pb, Rb–Sr, and Sm–Nd isotope study of mare basalt 10017 contribute to the understanding of the petrogenetic processes involved in the origin of geochemical diversity in lunar mare basalt sources, as well as the U–Pb isotope systematics of the Moon. The Rb–Sr, Sm–Nd, and 238U– 206Pb isotope systems yield concordant crystallization ages of 3.633 ± 0.057 Ga, 3.678 ± 0.069 Ga, and 3.616 ± 0.098 Ga, respectively. The 235U– 207Pb isochron yields an older, though still concordant, age of 3.80 ± 0.12 Ga. Neither the 206Pb– 207Pb system nor U–Pb concordia system yields an age for 10017 that is concordant with the age determined from the Sm–Nd, Rb–Sr, and 238U– 206Pb systems. The initial 87Sr/ 86Sr of 10017 is 0.69941 ± 7 and the initial ε Nd is +3.2 ± 0.4. Initial Pb isotopic compositions, determined from the U–Pb isochrons, are 206Pb/ 204Pb i = 31 ± 11 and 207Pb/ 204Pb i = 34 ± 15. Together, these initial Pb compositions constrain the μ value of the 10017 source to be 70 ± 30, assuming a single-stage Pb growth model. This is considerably lower than μ values typically estimated for mare basalt sources (∼100–600). Regardless, the μ values calculated for the sources of mare basalts, as well as other lunar samples, show a range that is larger than can be explained by fractionation of U from Pb solely by crystallization of silicate phases and ilmenite during magma ocean solidification and formation of lunar mantle sources. The U–Pb isotope systematics may reflect late-stage formation of a sulfide phase, which strongly fractionates Pb from U but has minimal effect on Rb/Sr or Sm/Nd compositions, during crystallization of the lunar magma ocean.

Pb, Sr and Nd isotope systematics of uranium mineralised stromatolitic dolomites from the proterozoic Cuddapah Supergroup, south India: constraints on age and provenance

The Pb, Sr and Nd isotopic compositions were determined on uranium mineralised and barren stromatolitic dolomite samples from the Vempalle and Tadpatri Formations of the Cuddapah Supergroup in southern India. Subsamples from individual handspecimens were analyzed to minimize the potential of heterogeneous initial Pb compositions. All of the U-mineralised samples gave Pb–Pb ages close to 1800 Ma but with high MSWD values, presumably due to local redistribution of U and Pb in the samples. The best Pb–Pb age of 1756±29 Ma is interpreted as the time of U-mineralisation and as a minimum age for carbonate sedimentation and dolomitisation within the Cuddapah Supergroup. The Sr isotopic compositions of the samples vary with the mineralised samples being more radiogenic. Even the lowest 87 Sr/ 86 Sr of the barren sample exceeds the seawater 87 Sr/ 86 Sr about 1800 Ma ago. On the other hand, the Nd systematics of the samples does not appear to have been disturbed by processes subsequent to carbonate sedimentation. The calculated initial Nd isotopic compositions suggest a dominant continental crustal source that was at least 600 Ma old at the time of deposition of the carbonate sediment.

Radiogenic isotope systematics of the Herefoss granite, South Norway: an indicator of Sveconorwegian (Grenvillian) crustal evolution in the Baltic Shield

A new PbPb mineral isochron dates the crystallization of the Herefoss granite (South Norway) to 926 ± 8 Ma, confirming earlier RbSr evidence of a late Sveconorwegian (Grenvillian) age. Initial Sr and Nd isotopic compositions recalculated to ε Sr(926 Ma) ≤ 40, ε Nd(926 Ma) = −3.2 to −0.8 indicate that the magma has originated from a mixture of two distinct source components: (1) A crustal component, which has resided in a moderately LILE-enriched reservoir ( f Rb = +7 to +8, 238 U 204 Pb = 17–22 ) since 1.6–1.9 Ga. This crustal component is clearly distinct from the extremely LILE-enriched metasedimentary country rocks of the granite ( f Rb > 21, 238 U 204 Pb > 24 ). (2): A mantle-derived component ( ε Nd = +4.6 to +5.9, ε Sr = −12 to −9, 238 U 204 Pb = 7.96 ) younger than 1.5 Ga. The ranges of initial Sr, Nd and Pb composition observed in the intrusion indicate that the magma was isotopically heterogeneous at the time of emplacement, and that the crustal contribution amounted to ∼ 35–55%. By the end of the Sveconorwegian orogeny, the southwestern part of the Baltic Shield had acquired a compositionally layered structure, where an “uppermost” zone of the crust with extreme LILE enrichment was underlain by a heterogeneous layer consisting of rocks with “normal upper crustal” degree of LILE enrichment and mafic intrusions younger than 1.5 Ga. There is no evidence of involvement of LILE-depleted “deep continental crust”-source material in the petrogenesis of the Herefoss granite. Rejuvenation of the lower crust by injection of mantle-derived magmas (“underplating”) has been an important process in the evolution of the southwestern part of the Baltic Shield. The absence of a young mantle-derived component from ∼ 1.12-Ga charnockitic intrusions in the Bamble sector and from the 989-Ma Grimstad granite suggests that this process may have been contemporaneous with late Sveconorwegian mafic magmatism in the Rogaland-Vest Agder sector further to the west.

Pb isotopic composition of late Archaean granites and the extent of recycling early Archaean crust in the Slave Province, northwest Canada

Initial Pb isotopic compositions have been determined for potassium feldspar from ca. 2.58 to 2.62 Ga plutonic rocks in the southern and central Slave Province of northwestern Canada to evaluate the extent of recycling of ancient crust within the province. Large differences in initial Pb compositions were measured which correlate with geographical areas of the province. Plutons in the east-central part of the province have initial compositions only slightly more radiogenic than estimated mantle values ( 207Pb/ 204Pb 14.8–14.9), and were dominantly deruved from juvenile crustal sources. In contrast, plutons in the Point Lake and western Contwoyto Lake areas of the western Slave Province have radiogenic compositions ( 207Pb/ 204Pb 15.1–15.2), and indicate significant recycling of pre-3.5 Ga crust. The Pb data support previous interpretations, based on Nd isotopes, for a major isotopic boundary in the central part of the province. Granites from the southern part of the province, near Yellowknife, have intermediate compositions which indicate: (1) the age of the protolith to the granitoids in the Yellowknife area is younger than at Point Lake, but older than in the eastern Slave; or (2) the granitoids in the Yellowknife area contain a mixture of an older Point Lake-type component and younger crust. The absence of pre-3.2 Ga crust in the Yellowknife area and lack of evidence for pre-2.8 Ga inherited zircons in the Yellowknife granitoids favour the former possibility. Evidence for recycling of ancient crustal sources, such as the Acasta Gneiss, is limited to a relatively small area of the west-central part of the province, suggesting that Acasta aged, or derived, crust is not widespread in the province. The marked regionality of isotopic composition may reflect a basement in the western part of the province which is itself a collage of crust of different age, being younger (ca 3.2-2.9) in the south, relative to the Point Lake region (3.9-3.2 Ga).