Richard A. Ketcham

Acquisition, optimization and interpretation of X-ray computed tomographic imagery: applications to the geosciences

High-resolution X-ray computed tomography (CT) is a novel technology ideally suited to a wide range of geological investigations. It is a quick and nondestructive method to produce images that correspond closely to serial sections through an object. Sequential contiguous images are compiled to create three-dimensional representations that can be manipulated digitally to perform efficiently a large array of measurement and visualization tasks. Optimal data acquisition and interpretation require proper selection of scanning configuration, use of suitable X-ray sources and detectors, careful calibration, and attention to origins and modes of artifact suppression. Visualization of CT data typically profits from the ability to view arbitrarily oriented sections through the three-dimensional volume represented by the data, and from the capability to extract features of interest selectively and display perspective views of them using methods of isocontouring or volume rendering. Geological applications include interior examination of one-of-a-kind fossils or meteorites; textural analysis of igneous and metamorphic rocks; geometric description and quantification of porosity and permeability in rocks and soils; and any other application demanding three-dimensional data that formerly required physical serial sectioning.

VARIABILITY OF APATITE FISSION-TRACK ANNEALING KINETICS : III. EXTRAPOLATION TO GEOLOGICAL TIME SCALES

Abstract A new model for examining fission-track data from natural specimens has been developed on the basis of new laboratory data describing fission-track annealing in a wide variety of apatites and the empirical correction for fission-track length anisotropy presented in earlier papers. Using revised and simplified statistical methods, we examine how well various empirical equations are able to fit the laboratory data and reproduce expected behavior on geological time scales. Based on the latter criterion, we find that so-called “fanning Arrhenius” models of mean track length are not the bestsuited for our data. Instead, we find that fitting c-axis projected lengths with a model that incorporates some curvature on an Arrhenius plot produces results that are in better agreement with the available geological benchmarks. In examining the relative annealing behavior of apatites with different resistance to annealing, we find that the laboratory-time-scale behavior of any two apatites can be reproduced well by a simple one- or two-parameter equation. This function is used to convert the reduced fission-track length of one apatite that has undergone a certain time-temperature history into the length that would be measured in a second, less-resistant apatite that has undergone the same history. Using this conversion, we create a single model that encompasses the annealing behavior of all of the apatites we studied. The predictions made by this model match closely those made by fits to data for individual apatites. We therefore infer that, although the conversion equation is imperfect, it presents an excellent practical solution to characterizing the range of kinetic variability for annealing of fission tracks in apatite.

Variability of apatite fission-track annealing kinetics: I. Experimental results

Abstract Annealing rates for fission tracks in apatite vary markedly as a complex function of composition, based on an experimental study of 15 well-characterized, compositionally diverse apatites. Extensive annealing data were obtained in 69 experiments (durations of 1, 10, 100, and 1000 h at temperatures from 75 to 400 °C) on each of four apatites, three with near end-member occupancy of the halogen site by F, Cl, and OH, plus the well-known apatite from Durango, Mexico. These results were supplemented by less-comprehensive annealing data from 12 experiments over the same range of time and temperature on each of the remaining 11 apatites. Measurements of initial fission-track length, a parameter of considerable importance to the derivation of time-temperature paths from fission-track data, reveal substantial variations from one apatite to another; initial lengths are best predicted from etch figures. Interlaboratory comparisons of data on annealing kinetics highlight discrepancies that appear to result largely from differences in the precision and accuracy of experimental temperatures. None of the factors previously proposed as the dominant compositional controls on annealing rates can account completely for annealing behavior over the full range of compositions studied. Nevertheless, relative rates of annealing among all apatites are highly systematic, which allows this data set to be used in its entirety to constrain multikinetic annealing models that predict fission-track lengths as a function of time and temperature.

Definitive fossil evidence for the extant avian radiation in the Cretaceous

Long-standing controversy surrounds the question of whether living bird lineages emerged after non-avian dinosaur extinction at the Cretaceous/Tertiary (K/T) boundary or whether these lineages coexisted with other dinosaurs and passed through this mass extinction event. Inferences from biogeography and molecular sequence data (but see ref. 10) project major avian lineages deep into the Cretaceous period, implying their ‘mass survival’ at the K/T boundary. By contrast, it has been argued that the fossil record refutes this hypothesis, placing a ‘big bang’ of avian radiation only after the end of the Cretaceous. However, other fossil data—fragmentary bones referred to extant bird lineages—have been considered inconclusive. These data have never been subjected to phylogenetic analysis. Here we identify a rare, partial skeleton from the Maastrichtian of Antarctica as the first Cretaceous fossil definitively placed within the extant bird radiation. Several phylogenetic analyses supported by independent histological data indicate that a new species, Vegavis iaai, is a part of Anseriformes (waterfowl) and is most closely related to Anatidae, which includes true ducks. A minimum of five divergences within Aves before the K/T boundary are inferred from the placement of Vegavis; at least duck, chicken and ratite bird relatives were coextant with non-avian dinosaurs.

Computational methods for quantitative analysis of three-dimensional features in geological specimens

Volumetric imaging techniques, such as high-resolution X-ray computed tomography, allow three-dimensional measurements of discrete objects inside solid samples. This paper introduces a new computer program called BLOB3D that is designed to allow efficient measurement of up to thousands of features in a single sample, such as porphyroblasts, sediment grains, clasts, and voids. BLOB3D implements an original suite of software methods, divided into three modules, which respectively enable the tasks of segmenting those regions in the data volume that correspond to the material of interest; separating touching or impinging objects; and extracting measurements from the interpreted volume. Program functions are demonstrated and verified with a set of phantoms used to test determinations of particle size distributions and particle-particle contact orientations.

Helium diffusion in natural zircon: Radiation damage, anisotropy, and the interpretation of zircon (U-Th)/He thermochronology

Accurate thermochronologic interpretation of zircon (U-Th)/He dates requires a realistic and practically useful understanding of He diffusion kinetics in natural zircon, ideally across the range of variation that characterize typically dated specimens. Here we present a series of date and diffusion measurements that document the importance of alpha dose, which we interpret to be correlated with accumulated radiation damage, on He diffusivity. This effect is manifest in both date-effective uranium (eU) correlations among zircon grains from single hand samples and in diffusion experiments on pairs of crystallographically oriented slabs of zircon with alpha doses ranging from ∼1016 to 1019 α/g. We interpret these results as due to two contrasting effects of radiation damage in zircon, both of which have much larger effects on He diffusivity and thermal sensitivity of the zircon (U-Th)/He system than crystallographic anisotropy. Between 1.2 × 1016 α/g and 1.4 × 1018 α/g, the frequency factor, D0, measured in the c-axis parallel direction decreases by roughly four orders of magnitude, causing He diffusivity to decrease dramatically (for example by three orders of magnitude at temperatures between 140 and 220 °C). Above ∼2 × 1018 α/g, however, activation energy decreases by a factor of roughly two, and diffusivity increases by about nine orders of magnitude by 8.2 × 1018 α/g. We interpret these two trends with a model that describes the increasing tortuosity of diffusion pathways with progressive damage accumulation, which in turn causes decreases in He diffusivity at low damage. At high damage, increasing diffusivity results from damage zone interconnection and consequential shrinking of the effective diffusion domain size. Our model predicts that the bulk zircon (U-Th)/He closure temperature (Tc) increases from about 140 to 220 °C between alpha doses of 1016 to 1018 α/g, followed by a dramatic decrease in Tc above this dose. Linking this parameterization to one describing damage annealing as a function of time and temperature, we can model the coevolution of damage, He diffusivity, and (U-Th)/He date of zircon. This model generates positive or negative date-eU correlations depending on the extent of damage in each grain and the date-eU samples time-temperature history.

Improved measurement of fission-track annealing in apatite using c-axis projection

Abstract 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.

The avian nature of the brain and inner ear of Archaeopteryx

Archaeopteryx, the earliest known flying bird (avialan) from the Late Jurassic period, exhibits many shared primitive characters with more basal coelurosaurian dinosaurs (the clade including all theropods more bird-like than Allosaurus), such as teeth, a long bony tail and pinnate feathers. However, Archaeopteryx possessed asymmetrical flight feathers on its wings and tail, together with a wing feather arrangement shared with modern birds. This suggests some degree of powered flight capability but, until now, little was understood about the extent to which its brain and special senses were adapted for flight. We investigated this problem by computed tomography scanning and three-dimensional reconstruction of the braincase of the London specimen of Archaeopteryx. Here we show the reconstruction of the braincase from which we derived endocasts of the brain and inner ear. These suggest that Archaeopteryx closely resembled modern birds in the dominance of the sense of vision and in the possession of expanded auditory and spatial sensory perception in the ear. We conclude that Archaeopteryx had acquired the derived neurological and structural adaptations necessary for flight. An enlarged forebrain suggests that it had also developed enhanced somatosensory integration with these special senses demanded by a lifestyle involving flying ability.

CALSOYASUCHUS VALLICEPS, A NEW CROCODYLIFORM FROM THE EARLY JURASSIC KAYENTA FORMATION OF ARIZONA

Abstract We describe a new fossil crocodyliform archosaur from the Early Jurassic Kayenta Formation of the Navajo Nation that is surprisingly derived for so ancient a specimen. High-resolution X-ray CT analysis reveals that its long snout houses an extensive system of pneumatic paranasal cavities. These are among the most distinctive features of modern crocodylians, yet the evolutionary history of this unique system has been obscured by the inaccessibility of internal structures in most fossil crania. Preliminary phylogenetic analysis indicates that the new species is the oldest known member of a monophyletic Goniopholididae, and within this lineage to be the sister taxon of Eutretauranosuchus, from the Late Jurassic Morrison formation of Colorado. Goniopholididae became extinct at the end of the Cretaceous, but it is more closely related to living crocodylians than are several lineages known only from Cretaceous and younger fossils. The new taxon nearly doubles the known length of goniopholid history and implies a deep, as yet undiscovered, Mesozoic history for several crocodyliform lineages that were once thought to have relatively complete fossil records.

Migration of Cenozoic deformation in the Eastern Cordillera of Colombia interpreted from fission track results and structural relationships: Implications for petroleum systems

Previously unreleased fission-track results and regional structural relationships are used to interpret the migration of deformation during Cenozoic orogenesis in the Eastern Cordillera (Cordillera Oriental) of the Colombian Andes. Low-temperature thermochronological results are based on apatite and zircon fission-track analyses of 41 samples collected along vertical and horizontal transects across the Eastern Cordillera at 4–7N latitude. Inverse modeling of fission-track results helps delimit the most probable cooling histories caused by exhumation linked to upper-crustal deformation. These inverse models are constrained by known structural geometries, chronostratigraphy, biostratigraphy, and vitrinite reflectance data. Fission-track data and modeling results indicate a close correspondence in the timing and style of deformation along the western and eastern flanks of the Eastern Cordillera. East-directed fold-thrust deformation along the eastern boundary with the Llanos foreland basin was underway by the late Oligocene and early Miocene. Similarly, west-directed fold-thrust structures along the western boundary with the intermontane middle Magdalena Valley Basin became active at approximately the same time. Less well known is the time of initial shortening within the axial segment of the Eastern Cordillera; although fission-track results suggest active exhumation by the early Miocene, shortening may have commenced much earlier during the late Eocene. Timing relationships for the Eastern Cordillera have important implications for the generation, migration, and accumulation of petroleum in the middle Magdalena Valley intermontane basin and the Llanos foreland basin. Our study provides a regional context to assess the timing of structural trap development and improve exploration and development of new and existing reservoirs in Colombia and analogous fold-thrust systems elsewhere.