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The Bristol Channel Basin is a Mesozoic continental rift basin. The basin is an important analogue for offshore reservoirs. Relative cross-cutting relationships and correlation with adjacent sedimentary basins have previously been used to constrain the timing of basin development. In situ U–Pb carbonate geochronology has been used to date calcite slickenfibre development in the cores of normal, thrust and strike-slip faults in the East Quantoxhead and Kilve region of Somerset for the first time. Protracted north–south extension from c. 150 to 120 Ma formed normal faults. Subsequent north–south shortening from c. 50 to 20 Ma was accommodated by (1) mutually cross-cutting strike-slip faults, (2) minor east–west-striking thrust faults and (3) the reactivation of pre-existing normal faults. Throughout Cenozoic contraction, σ 2 and σ 3 remained similar in magnitude and periodically flipped to become vertical; this was probably controlled by local stress permutations and changes in fluid pressure. The timing of inversion is contemporaneous with dominant Pyrenean and later Alpine orogenic events, as well as the opening of the Mid-Atlantic Rift. Early inversion of the Bristol Channel Basin was probably driven by far-field Pyrenean deformation, with later contraction caused by Alpine forces. Ridge push from the Mid-Atlantic Rift exacerbated the reactivation of the basin. [ABSTRACT FROM AUTHOR]

A key aim of modern metamorphic geochronology is to constrain precise and accurate rates and timescales of tectonic processes. One promising approach in amphibolite and granulite-facies rocks links the geochronological information recorded in zoned accessory phases such as monazite to the pressure–temperature information recorded in zoned major rock-forming minerals such as garnet. Both phases incorporate rare earth elements (REE) as they crystallize and their equilibrium partitioning behaviour potentially provides a useful way of linking time to temperature. We report REE data from sub-solidus amphibolite-facies metapelites from Bhutan, where overlapping ages, inclusion relationships and Gd/Lu ratios suggest that garnet and monazite co-crystallized. The garnet–monazite REE relationships in these samples show a steeper pattern across the heavy (H)REE than previously reported. The difference between our dataset and the previously reported data may be due to a temperature-dependence on the partition coefficients, disequilibrium in either dataset, differences in monazite chemistry or the presence or absence of a third phase that competed for the available REE during growth. We urge caution against using empirically-derived partition coefficients from natural samples as evidence for, or against, equilibrium of REE-bearing phases until monazite–garnet partitioning behaviour is better constrained.

An improved method of U-Pb dating of vein calcite formed during deformation is used to determine the age and cause of folding along the south coast of England. Fractures arising from folding of Late Cretaceous Chalk of southern England occurred 34.7 ± 1.7 myr ago. Underlying Jurassic strata have veins within fractures with ages of 55, 48-42, 39-37, 34-31 and 25 Ma, with 34-31 Ma being the tectonic culmination. Folding was slightly younger than the age of the youngest strata in the overlying Solent Group, suggesting that folding terminated basin sedimentation. This age of north-south shortening is inconsistent with attribution to intraplate forces from the mainly younger Alps, but is plausibly a result of the Pyrenean Orogen, which evolved from 50 to 28 myr ago with a late Eocene culmination. A modified method for carbonate laser ablation inductively coupled plasma mass spectrometry U-Pb dating is presented that uses measured 232Th and 208Pb as a better monitor of common Pb, and it has distinct advantages over existing methods. Initial common lead determined on samples conforms closely to model compositions calculated by earlier workers, with free regressions giving the most robust dates. The modified method is applicable to structural geology, carbonate diagenesis and to dating of carbonate relevant to palaeo-environmental and archaeological studies. [ABSTRACT FROM AUTHOR]

±Lithotectonic mapping, metamorphic observations and U-Pb zircon ages underpin a substantial revision of central Bhutan geology, notably a more extensive and continuous outcrop of the Tethyan Sedimentary Series (TSS) than previously mapped. Metamorphic grade in the TSS increases downward towards a basal north-vergent tectonic contact with the underlying Greater Himalayan Series (GHS), interpreted as a southward continuation of the South Tibetan Detachment (STD). Miocene (c. 17-20 Ma) leucogranite sheets are associated with the STD in this region but appear to diminish southwards. Two leucogranite dykes that cross-cut TSS structures yield ages of 17.8 ± 0.2 and 17.9 ± 0.5 Ma. A 500 ± 4 Ma (U-Pb zircon) metamorphosed ash bed in the Pele La Group within the psammite-dominated lower TSS yields the first direct isotopic age for the TSS in the eastern Himalaya, confirming existing age constraints from detrital zircon and fossil studies. A continuation of the Paro metasedimentary unit underlying the GHS was mapped near Wangdue Phodrang. Our observations, notably the exposure of a wholly ductile STD so far south and the significance of large nappe-like structures in the TSS, prompt a major revision to the geological map of the Bhutan Himalaya and require a reassessment of tectonic interpretations of the Bhutan Himalaya. [ABSTRACT FROM AUTHOR]

This Update is part of a series of annual reviews which cover various aspects of analytical atomic spectrometry. The series of mass spectrometry reviews was initiated in 1988 under the auspices of Allan Ure, who made a major contribution to the development of the ASU reviews and their predecessor, the Annual Reports in Analytical Atomic Spectrometry. After relinquishing the helm of this MS review, Allan continued to oversee its development by acting as an assessor until shortly before his death in December 2005. Details of Allan’s life and his contributions to atomic spectroscopy can be found in his obituary.1Allan will be missed by many in the analytical atomic spectrometry community.We welcome Randy Parrish to the writing team and value his in-depth knowledge of high-precision isotope ratio analysis. This year’s review follows the same format as last year’s.2Although an attempt is made to consider all relevant refereed papers, conference abstracts, reports, book chapters and patents for inclusion, the content of the review is highly selective. The selection of papers is based on criteria applied to focus sharply on the most significant developments in instrumentation and methodology or improved understanding of the fundamental phenomena involved in the MS process. The increasing importance of speciation and the blurring of boundaries between atomic and molecular MS require a high degree of judgement to be made in considering papers for inclusion. The main ruling criterion for all speciation papers is that the work should involve or be intended for the study of natural systems. For example, the study of synthetic metal clusters is generally not included whereas the determination of organometallic compounds in environmental samples is.Applications of atomic MS are not covered in this Update and readers are referred to the Updates on Industrial Analysis: Metals, Chemicals and Advanced Materials,3Environmental Analysis4and Clinical and Biological Materials, Food and Beverages.5Throughout this review, the term molecular ion will be restricted to denote only the positive or negative radical ion formed by removal or capture, respectively, of an electron. In contrast, addition of a proton or cation to a neutral molecule gives molecular adduct ions. Deprotonated molecules are considered as fragments.Although reproducibility or precision is a key figure of merit in MS, there is no agreed format for quoting it. The reader can assume that values of precision given in this Update as a percentage correspond to the RSD unless otherwise specified. For isotope ratios, however, values of precision are generally given as the SD of a permil value.It is a widespread phenomenon that analytical techniques in general and MS in particular spawn a large number of abbreviations and acronyms. A glossary of all abbreviations used in this Update appears at the end of the review. Most abbreviations are not defined in the text but those which are unlikely to be commonly known are defined in the text when used first and again in the glossary.The developments highlighted in this review indicate the very wide area of applications for which atomic MS can be used. From the measurement of very low isotopic abundances to the measurement of element concentrations at any scale, from bulk analysis to depth profiling and localized analysis, there is likely to be an MS procedure which can provide analytical performance competitive with any other technique. Two substantial reviews highlighted the important role of MS in areas of considerable importance. The excellent review by Becker6on recent developments in isotope analysis focused on methodological and instrumental developments, novel approaches and applications using different MS techniques. The list of techniques reviewed by Hou et al.7for application in nuclear forensics was dominated by MS methods. The two reviews emphasised the dominant role that ICP-MS now plays in these areas of research.

Two approaches to correct for mass discrimination effects associated with Cu and Zn isotopic measurements on two different MC-ICP-MS instruments a Micromass IsoProbe and a VG Axiom have been compared and assessed in detail: 1 sample-standard bracketing SSB, and 2 the ‘empirical external normalisation’ EEN whereby a second element is used to simultaneously correct for mass discrimination. This has provided new insights into the mass discrimination behaviours of Cu and Zn under varying instrumental set-ups, and has allowed improvements to be made to the existing correction procedures. With the SSB approach, mass bias stability is a prerequisite, and matrix components must be removed from the analyte to avoid matrix-related mass discrimination effects. By comparison, the EEN approach requires a degree of mass bias instability, and automatically corrects for matrix-related mass discrimination effects. The EEN correction may therefore appear more robust. However, while the EEN correction yields high-precision 65Cu63Cu and 66Zn64Zn data, an as yet unidentified source of systematic drift in the 67Zn and 68Zn signals through time hinders analyses of ratios incorporating these isotopes. Using the EEN correction where analyte and spike ratios were measured sequentially within a peak-switching protocol led to a three-fold deterioration in precision relative to static measurements. This is consistent with mass bias drift on the scale of a single five-second-measurement integration. For relative 65Cu63Cu and 66Zn64Zn ratio measurements, the SSB and EEN corrections give long-term reproducibilities of less then ±0.07‰ 2SD for pure Cu and Zn reagents. This is sufficient for resolving mass-dependent isotopic variability in natural and anthropogenic materials.

Provides information on mid-triassic integrated uranium and lead geochonology and ammonoid biochronology from the Balaton Highland in Hungary. Geological setting and localities in the Balaton Highlands; Ammonoid biochronological subdivision and correlation; Calibration points for the Triassic time scale.

This paper proposes a new approach to the estimation and correction of mass bias based on modelling the underlying instrument response function. Conventional definitions of mass bias are shown to be flawed and it is proposed that this quantity be recognised as merely the consequence of the instrument response function whose constants have a more fundamental importance. More accurate prediction of the bias in isotope ratio determinations is necessary and possible because of the improved precision afforded by multi-collector ICP-MS instrumentation. Isotope ratio measurements of Cd and Sn were used to study the variation of the mass bias with time, absolute mass and mass difference. No statistically significant variations were seen over a 20 min period, after which the data deviated significantly from the original measurement. After inclusion of the uncertainties in the natural abundances used to calculate the mass bias, no significant variation with increasing average isotope mass was observed. The reproducibility of the pattern of the points about the mean value suggested spectral interference and/or inaccurate values for the true isotope ratios. This was illustrative of the danger of using locally determined parameters to predict the mass bias. The variation of bias with mass difference showed a linear relationship, the implications of this for modelling are discussed. The common mass bias correction models are shown to be directly derivable from assumptions about the nature of the instrument response function. When the true instrument response function was investigated using a multi-element solution, a second order polynomial was found to provide the best fit to the data. The mass bias correction expression derived from such a model was used to calculate corrected Cd isotope ratios that were closer to the natural values than those obtained from the commonly used correction expressions. Increasing the concentration of a matrix element (bismuth or calcium) was found to significantly affect the value of Cd and Mg isotope ratios measured by multi-collector ICP-MS. The direction and magnitude of the effect was dependent on the position on the multi-collector array in which the isotopes were collected, with the heavier isotopes suffering higher levels of suppression. Measurements using an instrument with different multi-collector hardware did not show the same behaviour. A method of semi-quantitative analysis was developed that used the bias of 16 isotope ratios across the mass range to define the parameters in a quadratic instrument response function. This function was then applied to calculate the concentration of 24 analyte elements based on knowledge of ionisation efficiencies and the concentration of a single internal standard. This approach gave errors in the calculated concentrations that were comparable to the results obtained by using 6 internal standards, and did not require separate measurement of a standard solution to predetermine the instrument response.

LA-MC-ICP-MS is shown to be a rapid, precise and accurate method for determination of U–Pb ages of accessory minerals. For the protocol described, total analysis time is <3 min with a main acquisition sequence of only 30 s. Using a raster ablation protocol, within-run inter-element fractionation can be effectively eliminated and an external ablation standard used to quantify an overall error for the analysis. Reproducibilities of 206Pb238U ca.3 and 207Pb206Pb <1 2 σ are achieved, with the resulting age accurate to within 1 as determined using in-house samples previously characterised by TIMS. A key control on the PbPb reproducibility is shown to be the size of the 207Pb peak and an error propagation curve is determined for the accurate representation of this data. Propagation of these errors allows each individual sample analysis to be considered a stand-alone result, removing the need for statistical averaging of multiple data points. Simultaneous collection of flat-topped peaks enables precise measurement and correction of isobaric interference from 204Hg and a procedure for the consistent correction of common-Pb using 204Pb is described. Determination and correction of the common-Pb component is shown to be critical to the reliable interpretation of the data for certain minerals including those phases where a correction is often deemed unnecessary. Combined with time-resolved analysis of the data, this allows the Pb-loss history and nature of discordance within individual crystal domains to be ascertained. Successful analyses of zircons using a non-matrix matched monazite standard are also demonstrated suggesting that particle size distribution, ionisation efficiency and plasma loading, are more important issues in controlling inter-element fractionation in the plasma than absolute matrix matching.

LA-MC-ICP-MS is shown to be a rapid, precise and accurate method for determination of U–Pb ages of accessory minerals. For the protocol described, total analysis time is &lt;3 min with a main acquisition sequence of only 30 s. Using a raster ablation protocol, within-run inter-element fractionation can be effectively eliminated and an external ablation standard used to quantify an overall error for the analysis. Reproducibilities of 206Pb/238U = ca. 3% and 207Pb/206Pb = &lt;1% (2 σ) are achieved, with the resulting age accurate to within 1% as determined using in-house samples previously characterised by TIMS. A key control on the Pb/Pb reproducibility is shown to be the size of the 207Pb peak and an error propagation curve is determined for the accurate representation of this data. Propagation of these errors allows each individual sample analysis to be considered a stand-alone result, removing the need for statistical averaging of multiple data points. Simultaneous collection of flat-topped peaks enables precise measurement and correction of isobaric interference from 204Hg and a procedure for the consistent correction of common-Pb using 204Pb is described. Determination and correction of the common-Pb component is shown to be critical to the reliable interpretation of the data for certain minerals including those phases where a correction is often deemed unnecessary. Combined with time-resolved analysis of the data, this allows the Pb-loss history and nature of discordance within individual crystal domains to be ascertained. Successful analyses of zircons using a non-matrix matched (monazite) standard are also demonstrated suggesting that particle size distribution, ionisation efficiency and plasma loading, are more important issues in controlling inter-element fractionation in the plasma than absolute matrix matching.

The Tioga ash bed, bed B of the Zeigler pit, Union County, Pennsylvania, lies in the Polygnathus costatus costatus Zone of the Eifelian Stage of the Middle Devonian Series. The 27.5-cm-thick bed contains igneous zircon and monazite crystals that have been dated by precise U-Pb techniques. Zircons are characterized by inherited Pb that occurs as barely perceptible interior regions enriched in inclusions. The zircons are discordant and do not reveal the igneous age of the bed. In contrast, high quality monazite crystals lack any evidence of inheritance or loss of Pb, and multiple analyses have overlapping $$^{207}Pb/^{235}U$$ages of $$390.0 \pm 0.5 Ma$$. The monazite analyses are concordant or very slightly reversely discordant. The consistency of the monazite data provides a new precise estimate of age for the lower Eifelian.

The simplest interpretation of uranium–lead mineral ages in igneous rocks embodies the assumption that melting of pre-existing rocks leads to homogenization of the lead isotopes, thus resetting the radiometric 'clock'. Zircons often violate this assumption by only partially dissolving and preserving a component of lead 'inherited' from before the melting episode, but until now such behaviour has not been observed in monazite. Here we report U–Pb data for zircons and monazites from a Tertiary granite from the Himalayas, in which zircons and monazites contain inherited radio-genic lead. An analysis of a single entirely igneous zircon crystal plots on concordia at 19.5±0.4 Myr. Eight monazite fractions lie on a line from an upper intercept of 471±10 Myr to the youngest fraction, which plots slightly above concordia at 21 Myr. The presence of excess 206Pb in the youngest monazite fractions indi-cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component, rather than a resetting of 471 Myr monazites at ∼20 Myr. These data suggest a closure temperature for Pb in monazite significantly higher than previously published estimates.

The Early Jurassic is the most poorly constrained part of the geochronologic scale within the Mesozoic, owing to the scarcity of calibration points. We report a new U&z.sbnd;Pb zircon age of 181.4 ± 1.2 Ma (2σ) from an ash layer within the type section of Crassicosta Zone, a North American regional standard ammonite zone in the Middle Toarcian. For time scale calibration, the isotopically dated level needs to be tied to the Jurassic chronostratigraphic framework established in northwest European successions. Three major contributing factors (difference between local observed and true ranges, uncertainties of correlation, and taxonomic noise) are assessed to determine the maximum biochronologic dating error. The local ammonite distribution data are robust in that 95% confidence level extensions of observed ranges of common ammonite taxa would not significantly modify the zonal assignment. The computer-assisted unitary association (UA) method is used for world-wide biochronologic correlation. Representative North and South American, western Tethyan, and northwest European sections were selected to establish the global maximum ranges for 103 Middle Toarcian ammonite taxa and a sequence of 40 UA. Maximum permissible correlatives of the Crassicosta Zone lie between the northwest European standard Semipolitum Subzone (late Bifrons Zone) and Bingmanni Subzone (early Thouarsense Zone). In particular, the dated tuff layer cannot be older than the Semipolitum Subzone or younger than the Illustris Subzone (Variabilis Zone). Taxonomic inconsistencies are present in the source data but their effect in corrupting UA-based correlation is shown to be negligible. The precise U&z.sbnd;Pb age (2 σ error < ± 1%) that is constrained at the (sub)zonal level furnishes a useful new time scale calibration point. It warrants adjustments to previous Toarcian stage boundary age estimates and allows a significant reduction of their associated uncertainties.

Fission track results determined from granitic rocks exposed within the Coast Ranges of British Columbia indicate that significant variations in apatite chemistry may occur in small plutonic rock samples. To test ages determined earlier by Parrish using the population method of fission track dating, new apparent apatite ages were completed using the external detector method so that single-grain ages could be generated. In most cases, the new apatite ages were similar to the earlier ones (within ±2σ). However, in many cases, it was found that individual samples contained significant variations in single-grain apatite ages. Microprobe analyses showed the spread in ages were directly related to variations in apatite chemistry, such that chlorine-rich apatites retained older ages than fluorine-rich grains. Examination of the single-grain age data suggested that the spread in single-grain ages provided information which constrained the thermal history of the sample and that distributions of single-grain ages reflected the temperatures at which they resided during cooling.