Inter-laboratory comparison of fission track confined length and etch figure measurements in apatite

Apatite fission-track length and etch figure data are powerful tools for obtaining thermal history information, but both require human analysts making manual measurements and reproducibility is not assured. We report the results of an inter-laboratory study designed to clarify areas of congruence and divergence for these measurements and provide a basis for evaluating best practices to enhance intercompatibility of data sets. Four samples of megacrystic apatite from Durango, Mexico, with induced tracks, one unnannealed and three thermally annealed by varying amounts, were distributed internationally. In all, 55 analysts in 30 laboratory groups participated in the experiment. Relative mean track lengths among the samples were consistent across all analysts, but measurements for each sample showed scatter among labs and analysts considerably in excess of statistical expectation. Normalizing measurements of annealed samples using the unannealed sample improved consistency, as did normalizing for track angle using c-axis projection. Etch figure data also showed variability beyond statistical expectation, and consistency was improved by normalizing. Based on these data we recommend rigorous analyst training for length and etch figure measurement that includes measurement of standards, and that each analyst’s data on unknowns be normalized by that analyst’s own measurements on standards when using thermal history inverse modeling as part of the interpretation process

The Political Economy of Unfairness in U.S. Health Policy

Oberlander discusses the political economy of unfairness in US health policy by first highlighting the moral issues raised by the US\u27s system of financing medical care and then by analyzing the political dynamics that sustain that system

Improved modeling of fission-track annealing in apatite

Two abiding issues impact the reliability of apatite fission-track analysis and thermal history inversion in particular: reproducibility of track-length measurements and variability of annealing kinetics. In the companion to this paper, we addressed the first by demonstrating that using c-axis projection to normalize track lengths for crystallographic angle improves reproducibility among measurements acquired by Barbarand et al. (2003a, 2003b). We continue here by looking at the effect of c-axis projection on extrapolation of laboratory measurements to geological time scales. We find that c-axis projection improves agreement between predictions of empirical annealing models fit to measurements by Barbarand et al. (2003b) analysts 1 and 3, further corroborating its effectiveness in ameliorating observer bias. Furthermore, these annealing models closely match predictions from the Carlson et al. (1999) annealing data set, which was obtained by a different analyst using a different etching protocol. Normalizing for angle thus allows us to create a single annealing model that encompasses both data sets and etching techniques. By combining the different compositional varieties of apatite in the two data sets, the resulting model provides an improved basis for estimating apatite kinetic properties in unknowns using proxies such as composition, etch-figure dimension, and unit-cell parameters. Predictions from a fanning curvilinear model form accord with conventional expectations of annealing at high and low temperatures, and its use for thermal history inversion will not introduce spurious late cooling events

Accounting for long alpha-particle stopping distances in (U-Th-Sm)/He geochronology: Refinement of the baseline case

This contribution proposes a number of methodological refinements for accounting for ejection of energetic particles, most particularly in the case of long alpha-particle stopping distance effects in (U-Th-Sm)/He thermochronometry. Recent stopping-distance estimates for minerals commonly used for thermochronometry are up to 1.5 μm shorter than previously thought, which directly affects the magnitude of the ejection correction parameter FT. The revision in stopping distances necessitates re-fitting the empirical polynomials that have been used to calculate FT corrections for various crystal forms. Reformulation and re-parameterization of these polynomials enables them to account for present and any future change in stopping distances without needing to be recalibrated. The library of shapes described with these polynomials to accommodate habits is expanded for all crystal systems and also ellipsoids, thus covering all minerals currently used and under development for (U-Th-Sm)/He thermochronometry. In addition, the inaccuracy caused by characterizing a set of alpha particles from a decay chain with a single mean stopping distance is examined, and a simple method for compensation is recommended. A new method of using FT parameters to calculate corrected ages by applying them to parents instead of daughters or age is presented, which maximizes accuracy for old specimens such as meteorites. Altogether, the refinements proposed will affect (U-Th-Sm)/He ages by ∼1-5%, depending on size and age. Finally, the concept of an FT-equivalent sphere is introduced as a sphere with the same effective FT value as a given mineral grain. Testing indicates that the FT-equivalent sphere is of comparable or superior accuracy to a sphere of the same surface to volume ratio for diffusion modeling. This finding greatly facilitates utilization of (U-Th-Sm)/He data from standard data tables for forward and inverse modeling

Improved modelling of fission-track annealing in apatite

Apatite fission-track length data are used extensively for thermal history inversion. However, several studies have documented instances of poor reproducibility of length data. We address this problem by using c-axis projection to normalize track lengths for crystallographic angle in the extensive laboratory annealing data set acquired by Barbarand et al. (2003a, 2003b). A new simplification reduces the c-axis projection model from six to four fitted parameters. Normalizing for track angle using c-axis projection improves every aspect of length measurement reproducibility examined. It accelerates convergence of mean length in single analyses; increases consistency among replicate measurements by a single analyst; enhances consistency of measurements of the same mounts by different analysts; and improves the match between analyses conducted with and without Cf-irradiation. C-axis projection is also shown to enhance the thermal sensitivity of length data. Based on these results, we assert that c-axis projection is a good means of compensating for observer bias, although it does not overcome differences caused by experimental error

Accounting for long alpha-particle stopping distances in (U–Th–Sm)/He geochronology: 3D modeling of diffusion, zoning, implantation, and abrasion

In apatite (U–Th)/He thermochronology the helium distribution in a crystal is a function of the simultaneous processes of radiogenic production, thermally activated volume diffusion and the ejection of He caused by long alpha stopping distances. These processes are further complicated by zonation of U, Th and Sm within the grain and implantation of 4He from neighboring U–Th–Sm bearing minerals. We use a refined version of the 3D Monte Carlo diffusion code of Gautheron and Tassan-Got (2010) to simulate the interplay between ejection and diffusion with or without zonation, ejection and abrasion for a suite of thermal histories. We examine the phenomenon of over-correction produced by the alpha ejection correction parameter (FT or FZAC for homogeneous or heterogeneous eU repartition) by comparing the raw (measured) and FT- or FZAC-corrected ages for a number of scenarios to the ejection-free age (AEF), which we define as the age that would be obtained if alpha ejection had not occurred, or equivalently if the stopping distance was zero. We show that the use of FT- or FZAC-corrected ages generally reproduces the ejection-free age to within typical (U–Th)/He uncertainties (±8%), even for zoned apatites. We then quantify the effect of alpha implantation on (U–Th)/He ages, showing that implantation from a single external source with modest relative U or Th enrichment can generate as much as 50% excess He. For more extreme cases where an apatite is surrounded by multiple external sources the measured age can be >300% of that determined from an isolated crystal. While abrasion of the outer 20–25 μm can significantly reduce the age dispersion for rapidly cooled samples, slowly cooled samples can still retain 10–30% excess He. The removal of the rim of the crystal reduces the thermal information from very low temperatures (<40 °C), and introduces additional technical complications and biases, and should therefore be used with caution. Overall we demonstrate that although zonation and implantation may not be routinely determined, we now have the 3D modeling capability to fully investigate and constrain the causes of age dispersion within a sample, leading to significant improvement in our ability to interpret (U–Th)/He data

High-resolution 3D analyses of the shape and internal constituents of small volcanic ash particles: The contribution of SEM micro-computed tomography (SEM micro-CT)

The morphology of small volcanic ash particles is fundamental to our understanding of magma fragmentation, and in transport modeling of volcanic plumes and clouds. Until recently, the analysis of 3D features in small objects ( 20 μm³ (~ 3.5 μm in diameter) can be successfully reconstructed and quantified. In addition, new functionalities of the Blob3D software were developed to allow the particle shape factors frequently used as input parameters in ash transport and dispersion models to be calculated. This study indicates that SEM micro-CT is very well suited to quantify the various aspects of shape in fine volcanic ash, and potentially also to investigate the 3D morphology and internal structure of any object < 0.1 mm³