Geoscience Applications of Synchrotron X-ray Computed Microtomography
Computed microtomography is the extension to micron spatial resolution of the CAT scanning technique developed for medical imaging. Synchrotron sources are ideal for the method, since they provide a monochromatic, parallel beam with high intensity. High energy storage rings such as the Advanced Photon Source at Argonne National Laboratory produce x-rays with high energy, high brilliance, and high coherence. All of these factors combine to produce an extremely powerful imaging tool for earth science research. Techniques that have been developed include: - Absorption and phase contrast computed tomography with spatial resolution approaching one micron - Differential contrast computed tomography, imaging above and below the absorption edge of a particular element - High-pressure tomography, imaging inside a pressure cell at pressures above 10GPa - High speed radiography, with 100 microsecond temporal resolution - Fluorescence tomography, imaging the 3-D distribution of elements present at ppm concentrations. - Radiographic strain measurements during deformation at high confining pressure, combined with precise x- ray diffraction measurements to determine stress. These techniques have been applied to important problems in earth and environmental sciences, including: - The 3-D distribution of aqueous and organic liquids in porous media, with applications in contaminated groundwater and petroleum recovery. - The kinetics of bubble formation in magma chambers, which control explosive volcanism. - Accurate crystal size distributions in volcanic systems, important for understanding the evolution of magma chambers. - The equation-of-state of amorphous materials at high pressure using both direct measurements of volume as a function of pressure and also by measuring the change x-ray absorption coefficient as a function of pressure. - The formation of frost flowers on Arctic sea-ice, which is important in controlling the atmospheric chemistry of mercury. - The distribution of cracks in rocks at potential nuclear waste repositories. - The location and chemical speciation of toxic elements such as arsenic and nickel in soils and in plant tissues in contaminated Superfund sites. - The strength of earth materials under the pressure and temperature conditions of the Earth's mantle, providing insights into plate tectonics and the generation of earthquakes.
Synchrotron X-ray Fluorescence Microtomography in Geo-, Cosmo-, and Bio- chemistry
Synchrotron-based X-ray fluorescence computed microtomography (xrfCMT) is a unique method for imaging major and trace element distributions within natural materials nondestructively and with high spatial resolution. The technique is particularly useful in imaging and quantifying elemental abundance in small objects that may be too precious or too difficult to section, or in the analysis of materials in which sectioning may potentially alter elemental distributions. This presentation will highlight how this technique is being applied at beamlines X26A and X27A at the National Synchrotron Light Source (Brookhaven National Laboratory) and at 13-ID at the Advanced Photon Source (Argonne National Laboratory). These instruments utilize 1-10 μm diameter focused, monochromatic X-ray beams to non- destructively measure x-ray fluorescence from a sample as it is translated and rotated within the beam. The resultant fluorescence intensities are then reconstructed as either two-dimensional cross sectional or three- dimensional elemental distribution using a fast fourier transform based computational reconstruction algorithm. Reconstruction of multi-elemental distributions at concentrations down to approximately 1 μg g-1 (element dependent) can be obtained. By collecting and storing full energy dispersive spectra from a multi-channel analyzer for every pixel (rather than regions of interest), it is possible to evaluate a reconstructed spectrum within the object for more robust elemental analysis. For high density matrices in particular, corrections are necessary to account for x-ray absorption by the object of both incoming X-rays and outgoing fluorescent X-rays. These effects limit the size of objects and elements that can be imaged; however reasonable corrections can be made if an estimate of linear absorption coefficient through the material is made. It is also possible to couple fluorescence tomography with microbeam x-ray absorption and diffraction analysis. When coupled to absorption spectroscopy, the xrfCMT analysis is conducted at multiple incident x-ray energies that preferentially target a given oxidation state of an element. This allows for three-dimensional visualization of an element's speciation. Coupled with simultaneous x-ray scattering studies utilizing a CCD area detector, individual mineral reflections can be reconstructed in three dimensions simultaneously with the x-ray fluorescence data. Examples of materials analyzed by this technique at X26A, X27A and 13-ID include interplanetary dust particles, fluid inclusions, plant materials, and heavy metals sorbed to mineral grains.
Rapid segregation of core-forming melts; X-ray tomographic imaging of melt geometry
Core formation is one of the most profound events in the early evolution of a planet. Although there is much evidence that favors a very hot and deep magma ocean for the major core formation event on Earth, smaller planetesimals likely never became hot enough to generate the wide-scale melting required for such a scenario. Furthermore, there is evidence that planetesimal cores formed rapidly (within 3My). An inefficient percolative flow mechanism has been suggested to be viable for systems that have a metallic melt fraction in excess of the percolation threshold (approximately 5 vol%), provided that the permeability of these connected melts is high enough to remove the majority of the core liquid from the silicate matrix in such a relatively short time span. More accurate knowledge of the permeability of core forming melts requires a detailed understanding of how the melt is connected in three dimensions, and the complex relationships between melt volume, connectedness and permeability. We have calculated the permeability of core forming metallic liquids (FeS and Fe67S33) in an olivine matrix by lattice-Boltzmann simulations of flow through digital volumes generated from synchrotron based x-ray 3-dimensional tomographic images of experimental run samples. Mixtures of San Carlos olivine and sulfide liquids were synthesized in a piston-cylinder apparatus at conditions relevant to core segregation in planetesimals (1300°C and 1GPa for 24 hours). The recovered samples were imaged at ~1micron resolution using the dedicated tomography beam-line (8.3.2) at the Advanced Light Source (Lawrence Berkeley National Laboratory). The percolation threshold of these samples was determined to be close to earlier measurements (3-6 vol%) through both experimental methods, but the calculated permeability is substantially lower than previously estimated. As a consequence, although percolation still appears viable for some planetesimal sized objects, it may be a secondary mechanism acting in conjunction with flow induced through other processes such as deformation. Preliminary results of in-situ, x- ray tomography on similar samples that have undergone shear deformation at high pressure and temperature will also be discussed. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
A Closer Look at Pumice and Scoria Textures by Using the Third Dimension
The textures of volcanic rocks have long proved to provide important constraints on processes occurring in magma chambers, volcanic conduits, and beyond the craters during magma emplacement on the volcano flanks. One approach to investigating such textures is to acquire 2-D images of volcanic samples via optical and/or scanning electron microscopy on areal sections (i.e. thin sections). An advantage of this procedure is that it offers a fast, quantitative inspection of volcanic textures in 2-D, which can be very useful in the short time usually involved with eruption monitoring and civil protection planning. However, because it provides no direct information in the third dimension, this approach cannot be used to investigate the internal structure of volcanic materials, limiting the information that can be provided on vesiculation, degassing and crystallization processes, as well as on the overall eruption dynamics. Recently, the application of X-ray computed microtomography to geological specimens has opened the opportunity to visualize the internal structure of porous materials, such as volcanic scoria and pumice clasts, in 3-D. Here we will first demonstrate how we reconstructed and quantitatively processed the 3-D vesicle textures in volcanic products from explosive activity of several different active and hazardous volcanic areas in Italy (i.e Stromboli, Etna, Campi Flegrei). We will then compare the 3-D and the 2-D results and describe how we used the 3-D dataset to constrain the dynamics of vesiculation and degassing in basaltic and trachytic magmas, and, ultimately, the implications of these results for the eruptive styles of volcanoes.
X-Ray Tomography: The Ultimate Petrographic Tool for Studying Pumice
Over the last several years, we have been studying pumice from large-volume tuffs using synchrotron-based x- ray computed microtomography. Our goal is to document in detail and quantitatively crystal and bubble textures in pumice to retrieve information on the evolution of giant magma bodies over time, particularly for the stages leading to supereruptions. X-ray tomography yields 3D maps of x-ray linear attenuation coefficients, allowing documentation and visualization of textures in 3D, with resolution down to micrometer scale or better. X-ray tomography is ideal for the study of pumice, where crystals are sparse and mostly separated from each other by low-density vesicular glass. This is particularly fortunate because (a) pumice is poorly suited to being studied using thin-sections due to the low abundance of crystals, and (b) stereological corrections are unnecessary. Imaging is non-destructive, can be performed quickly (minutes to hours), and with little or no sample preparation. X-ray tomography is the perfect complement to physical separation methods (e.g. crushing, sieving and winnowing), which do allow extraction of individual crystals for detailed study, but also cause crystal breakage and yield little to no information on the vesicle populations. The application of x-ray tomography to pumice from the Bishop Tuff (California) and Peach Spring Tuff (Nevada-Arizona-California) reveals that, at the energies of interest, contrasts in attenuation are such that resulting tomograms enable the distinction of at least 5-6 of the most abundant phases present (i.e. void space, glass, quartz, feldspar, 1-2 mafic minerals, oxides). Using differential absorption x-ray tomography, we can map the distribution of key chemical elements in 3D, notably Zr and Ce, allowing the unambiguous identification and quantification of the sizes and spatial distribution of important accessory minerals such as zircon, titanite, and allanite (± chevkinite), for which textural information is almost completely lacking. Study of pumice from the Bishop Tuff shows that: (1) crystal fragmentation is an important magmatic process, with fragment size distributions following power laws typical of fractal processes; (2) quartz + feldspar crystal size distributions reveal two distinct crystal populations, one formed by crystallization under low supersaturation over millennial timescales, and another formed under high supersaturation (possibly reflecting decompression crystallization) within the final years to months before eruption; (3) a large vesicle with >50 magnetite crystals attached to its wall likely represents the first textural evidence for the presence of pre-eruptive bubbles in Bishop magma; (4) it may be possible to retrieve information on the pre-and syn-eruptive bubble populations from the study of vesicle size distributions. Study of pumice from the Peach Spring Tuff using Zr and Ce maps reveals that, surprisingly, most of the zircon crystals are included in large titanite crystals (also attached to allanite), while only a small number of zircon crystals occur isolated within the glassy matrix. This suggests that different zircon crystals may record different aspects of the crystallization history of pumice. Our ongoing studies of pumice from the Bishop and Peach Spring Tuff show that, especially when combined to crystal chemistry information, x-ray tomography provides invaluable information on the evolution of magmatic systems, with unprecedented level of detail.
Use of High-resolution X-ray Computed Tomography for Unsaturated Fine Granular Materials
While many unsaturated soil mechanics principles are based on fundamental concepts and theories, often one or more simplifying assumptions have to be made due to the lack of pore-level details of one or more of the following: granular material packing; pore size/shape distribution, pore network structure; and fluid distribution. Recent advances in high-resolution X-ray computed tomography now allow for non-invasive imaging of porous media systems under a variety of conditions. This technique provides micron-scale images that, when combined with quantitative analysis programs, provide details that allow for the advancement of the principles that govern unsaturated systems. In this work, a series of sand columns at varying degrees of water saturation were imaged at the Advanced Photon Source GSECARS 13-BMD tomography beamline. Once the three phases (sand, water, and air) were segmented, a suite of image analysis programs was used to determine the grain characteristics and packing structure; pore size distribution, pore network structure; and fluid phase characteristics, distribution and correlation to the pore network structure. Here, we will present the results of this analysis and provide some examples of how this level of detail allow for advancements in our ability to measure, understand and model unsaturated fine granular materials.
Synchrotron X-ray Microtomography of Chondritic Meteorites: Methods and Applications
The three dimensional (3D) imaging technique synchrotron x-ray computed microtomography offers the ability to reconstruct the spatial relationship of materials with differing x-ray attenuation properties in small (cm-sized) samples at resolutions down to the single micrometer scale in samples that are free of potential sample preparation artifacts inherent to thin sectioning. When combined with advanced 3D object analysis software, quantitative volumetric and spatial information (such as orientation and spatial distributions) and can be extracted. We have applied such combinations of techniques to chondritic meteorites to learn about the physical evolution of asteroids. In one application, to examine the role of impacts in the evolution of asteroids, we performed a quantitative 3D study of metal grains in a suite of increasingly shocked L chondrites. Our results demonstrate that collective degrees of metal grain preferred orientation increase with greater degrees of impact-related compaction and shock loading. The ductile metal grains in L chondrites begin to show foliation at peak shock pressures of less than 5 GPa, pressures that are great enough to compact and indurate loosely bound chondritic material, and our results constitute evidence for an impact origin for the foliation seen in a majority of chondritic meteorites. In another application, we used x-ray microtomography to document the size distributions and locations of voids present within five porous equilibrated ordinary chondrites. When used in conjunction with Helium pycnometry, a detailed size distribution of the porosity can be obtained. Such data is necessary for modeling impact-related processes such as shock wave propagation and heat flow within primordial chondritic parent bodies.
Non-destructive 3D Imaging of Extraterrestrial Materials by Synchrotron X-ray Micro- tomography (XR-CMT) and Laser Confocal Scanning Microscopy (LCSM): Beyond Pretty Pictures
We report scientific results made possible only by the use these two non-destructive 3D imaging techniques.
XR-CMT provides 3D image reconstructions at spatial resolutions of 1 to 17 micron/voxel edge. We use XR-
CMT to locate potential melt-inclusion-bearing phenocrysts in batches of 100-200 micron lunar fire-fountain
spherules; to locate and visualize the morphology of 1-2mm size, irregular, unmelted Ca-, Al-rich inclusions
(CAIs) and to quantify chondrule/matrix ratios and chondrule size distributions in 6x6x20mm chunks of
carbonaceous chondrites; to quantify the modal abundance of opaque phases in similar sized Martian
meteorite fragments, and in individual 1-2mm diameter chondrules from chondrites. LCSM provides 3D image
stacks at resolutions < 100 nm/pixel. We are the only group creating deconvolved image stacks of 100 to
over 1000 micron long comet particle tracks in aerogel keystones from the Stardust mission. We present
measurements of track morphology in 3D, and locate high-value particles using complementary synchrotron x-
ray fluorescence (XRF) examination. We show that bench-top LCSM extracts maximum information about
tracks and particles rapidly and cheaply prior to destructive disassembly. Using XR-CMT we quantify, for the
first time, the volumetric abundances of metal grains in 1-2 mm diameter CR chondrite chondrules. Metal
abundances vary from 1 to 37 vol.% between 8 chondrules (and more by inspection), in a meteorite with solar
(chondritic) Fe/Si ratio, indicating that chondrules formed and accreted locally from bulk solar composition
material. They are 'complementary' to each other in Fe/Si ratios. Void
spaces in chondritic CAIs and chondrules are shown to be a primary feature, not due to plucking during
sectioning. CAI morphology in 3D reveals pre-accretionary impact features, and various types of mineralogical
layering, seen in 3D, reveal the formation history of these building blocks of planets and asteroids. We also
quantify the x-ray attenuation parameters that limit discrimination between phases (e.g., glass/phenocrysts) in
XR-CMT work. Results are posted at: http://research.amnh.org/users/debel/meteorites/index.html