Michael J. Pellin

Barium isotopic composition of mainstream silicon carbides from Murchison: Constraints for s-process nucleosynthesis in asymptotic giant branch stars

We present barium, carbon, and silicon isotopic compositions of 38 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing barium contamination. Strong depletions in δ( 138 Ba/ 136 Ba) values are found, down to −400‰, which can only be modeled with a flatter 13 C profile within the 13 C pocket than is normally used. The dependence of δ( 138 Ba/ 136 Ba) predictions on the distribution of 13 C within the pocket in asymptotic giant branch (AGB) models allows us to probe the 13 C profile within the 13 C pocket and the pocket mass in AGB stars. In addition, we provide constraints on the 22 Ne(α, n) 25 Mg rate in the stellar temperature regime relevant to AGB stars, based on δ( 134 Ba/ 136 Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ( 134 Ba/ 136 Ba) values that cannot be explained by any of the current AGB model calculations. Instead, such negative values are consistent with the intermediate neutron capture process (i process), which is activated by the very late thermal pulse during the post-AGB phase and characterized by a neutron density much higher than the s process. These two grains may have condensed around post-AGB stars. Finally, we report abundances of two p-process isotopes, 130 Ba and 132 Ba, in single SiC grains. These isotopes are destroyed in the s process in AGB stars. By comparing their abundances with respect to that of 135 Ba, we conclude that there is no measurable decay of 135 Cs (t1/2 = 2.3 Ma) to 135 Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before 135 Cs decayed.

Potassic, high-silica Hadean crust

Significance To understand early Earth’s habitability, we need to know when the continental crust first formed. However, due to the combined actions of plate tectonics and erosion, most of the evidence of the early crust has been destroyed. To shed light on this debate, we analyzed the strontium isotopic composition of apatite inclusions in zircons from Nuvvuagittuq, Canada, where independent evidence suggests a crust-forming event prior to 4.2 Ga, possibly as early as 4.4 Ga. Our results show that this early crust had a high Rb/Sr ratio and therefore a high silica content. This suggests that the early Earth was capable of forming continental crust within <350 million y of solar system formation. Understanding Hadean (>4 Ga) Earth requires knowledge of its crust. The composition of the crust and volatiles migrating through it directly influence the makeup of the atmosphere, the composition of seawater, and nutrient availability. Despite its importance, there is little known and less agreed upon regarding the nature of the Hadean crust. By analyzing the 87Sr/86Sr ratio of apatite inclusions in Archean zircons from Nuvvuagittuq, Canada, we show that its protolith had formed a high (>1) Rb/Sr ratio reservoir by at least 4.2 Ga. This result implies that the early crust had a broad range of igneous rocks, extending from mafic to highly silicic compositions.

Strontium and Barium isotopes in Type X presolar Silicon Carbide grains analyzed with Chili-Two types of Supernova grains

Introduction: Baddeleyite (monoclinc-ZrO2) is a widely occurring accessory phase reported from an array of terrestrial mafic and ultra-mafic rocks [e.g. 1] as well as within shergottites [2,3], Lunar meteorites and Apollo samples [e.g. 4,5], asteroidal achondrites [6] and ordinary chondrites [7]. As an established U-Pb geochronometer [8], baddeleyite has the potential to resolve the timing of Solar System crystallization events for a number of low-Si lithologies where zircon (ZrSiO4) is absent. However, the exposure of these grains to shock metamorphism induces partial to complete loss of lead, resetting the U-Pb chronometer and complicating their interpretation. Recent work has focused on coupling isotopic analysis with microstructural observations, linking the extent of amorphisation and recrystallisation to the severity of lead-loss for the first time [i.e. 2, 9]. This approach allows for the targeting of pristine or deformed crystals in an attempt to differentiate the timings of igneous and impact events. However, given a discrepancy between the severity of lead diffusion within natural [2, 9] and experimental [10] shock conditions, our fundamental understanding of U-Pb age resetting in this potentially key planetary chronometer is poorly constrained. The application of atom probe tomography (APT) to zircon has proven exceptionally useful in distinguishing the response of Pb to both post-crystallization annealing [11] and deformation [12]. However, this approach has never been applied to heavily shock loaded material. Here we present the first insights into the atomic-scale shock response of lead cations within baddeleyite, coupling these observations with detailed EBSD analysis to produce a first order insight into the mechanisms of U-Pb age resetting in baddeleyite.

Atom-Probe Tomography of Meteoritic Nanodiamonds.

1 Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, IL, USA. E-mail: prheck@fieldmuseum.org 2 Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL, USA. 3 Northwestern University Center for Atom-Probe Tomography, Department of Materials Science & Engineering, Northwestern University, Evanston, IL, USA. 4 Materials Science Division, Argonne National Laboratory, Argonne, IL, USA. 5 Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA. 6 Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA. 7 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL, USA. 8 Department of Materials Science and Engineering and Department of Bioengineering, University of Texas-Dallas, Richardson, TX, USA. 9 Energy Systems Division, Argonne National Laboratory, Argonne, IL, USA. 10 CAMECA Instruments, Inc., Madison, WI, USA. (* E-mail: prheck@fieldmuseum.org)

Ion microscopy with resonant ionization mass spectrometry: time-of-flight depth profiling with improved isotopic precision.

There are four generally mutually exclusive requirements that plague many mass spectrometric measurements of trace constituents: (1) the small size (limited by the depth probed) of many interesting materials requires high useful yields to simply detect some trace elements, (2) the low concentrations of interesting elements require efficient discrimination from isobaric interferences, (3) it is often necessary to measure the depth distribution of elements with high surface and low bulk contributions, and (4) many applications require precise isotopic analysis. Resonant ionization mass spectrometry has made dramatic progress in addressing these difficulties over the past five years.

CHILI, a Nanobeam Secondary Neutral Mass Spectrometer with Extraordinary Spatial Resolution, Sensitivity, and Selectivity: First Results

CHILI, the Chicago Instrument for Laser Ionization, is a nanobeam secondary neutral mass spectrometer using laser resonance ionization that is designed for a lateral resolution as small as 10 nm, a useful yield (atoms detected per atom desorbed) of 30–50%, and nearly complete suppression of isobaric interferences from both monatomic and molecular ions [1 and references therein]. It is equipped with an Orsay Physics Cobra liquid metal ion gun with a lateral resolution of 2.5 nm for desorbing from small spots and rastered areas (less than a few μm) and a 351 nm laser for desorbing from spots or rastered areas from the μm scale upwards. An optical microscope is built into CHILI and is used to image the sample and introduce the ablation laser. An Orsay Physics eCLIPSE-plus field emission electron gun with a resolution of 4 nm is also used for imaging samples. CHILI is equipped with six tunable Ti:sapphire lasers that we designed and built, pumped by three Photonics DM-40 40 W Nd:YLF lasers (527 nm) for simultaneous isotopic measurement of two or three elements. The Ti:sapphire lasers are tunable from ~700–1000 nm, but can be frequency doubled (350–500 nm), tripled (233–333 nm) or quadrupled (205–250 nm) with nonlinear optical crystals. We anticipate a lateral resolution of ~10 nm for resonance ionization using the Ga primary beam, as this is the approximate analytical volume for sputtering; for laser ablation, the minimum spot size will be ~1 μm. CHILI is housed within a dedicated laboratory with controlled temperature and humidity. The analytical sequence is the following: release atoms from the surface by sputtering; eject secondary ions by applying high accelerating voltage to the cloud; apply the normal accelerating voltage; fire the photoionization lasers; and mass analyze the photoions. CHILI’s removable sample holder is capable of mounting a variety of common types of samples, including 1” diameter polished sections, 1⁄2” and 1 cm diameter SEM stubs, and TEM samples.

Molecular Desorption by Laser–Driven Acoustic Waves: Analytical Applications and Physical Mechanisms

Analytical mass-spectrometry (MS) is a powerful, widely-used tool for materials analysis, helping to make progress in materials and environmental sciences, chemistry, biology, astrophysics, etc (Dass 2007). Often the sample to be studied (analyte) is a solid requiring: a) volatilization/desorption of the analyte atoms/molecules and b) their consequent conversion to the charged particles (ionization) prior to mass analysis. The last two decades have seen revolutionary advances in these techniques (Dass 2007) and the use of direct laser irradiation to achieve volatilization is one of the wide-spread methods (Lubman 1990) These pulsed laser-based techniques for the desorption/emission of the atoms, molecules and ions from the surface of solids has benefitted from fundamental study of the process beginning with the invention of the lasers (Honig and Woolston 1963) . A short laser pulse hitting a solid absorbing surface delivers high energy in a small volume inducing a variety of state changes. One consequence is the evaporation/desorption of surface atoms and molecules could be used for further analysis by MS technique. However, the increasing use of MS methods in analytical chemistry of organic and biomolecules revealed that this direct desorption process had significant drawbacks for the analysis of molecular solids. Most importantly, the high energy density produced during irradiation results in not only surface heating but also in excitation of internal vibrational and electronic states of desorbed molecules leading to their partial or even complete fragmentation (Lubman 1990). This difficulty was overcome for many samples by the development of Matrix Assisted Laser Desorption and Ionization (MALDI), which by imbedding the analyte in a specialized UV absorbing molecular solid (the “matrix”) allows UV lasers to both desorb and ionize large organic and biomolecules without significant fragmentation (Cole 2010). Because MALDI combines both of the needed initial processes (desorption and ionization) it very quickly following the pioneering publication (Karas, Bachmann et al. 1985) became a key analytical tool. MALDI is now one of principle research tools in proteomics (Cole 2010) and its discovery was recognized with the Nobel Prize in chemistry in 2002. Despite the success of the MALDI technique current active areas of research include quantification and analysis in the low mass region. Application of MALDI to analyte quantification while possible requires careful attention to matrix/analyte sample

Waste Volume Reduction Using Surface Characterization and Decontamination By Laser Ablation

The U.S. Department of Energys nuclear complex, a nation-wide system of facilities for research and production of nuclear materials and weapons, contains large amounts of radioactively contaminated concrete[1]. This material must be disposed of prior to the decommissioning of the various sites. Often the radioactive contaminants in concrete occupy only the surface and near-surface ({approx}3-6 mm deep) regions of the material. Since many of the structures such as walls and floors are 30 cm or more thick, it makes environmental and economic sense to try to remove and store only the thin contaminated layer rather than to treat the entire structure as waste. Current mechanical removal methods, known as scabbling, are slow and labor intensive, suffer from dust control problems, and expose workers to radiation fields. Improved removal methods are thus in demand[2-5]. Prior to decontamination, the surface must be characterized to determine the types and amounts of contaminants present i n order to decide on an appropriate cleaning strategy. Contamination occurs via exposure to air and water-borne radionuclides and by neutron activation. The radionuclides of greatest concern are (in order of abundance) [1]: 137Cs & 134Cs, 238U, 60Co, and 90Sr, followed by 3H, radioactive iodine, and a variety of Eu isotopes and transuranics. A system capable of on- line analysis is valuable since operators can determine the type of contaminants in real time and make more efficient use of costly sampling and characterization techniques. Likewise, the removed waste itself must be analyzed to insure that proper storage and monitoring techniques are used. The chemical speciation of radionuclides in concrete is largely unknown. Concrete is a complex material comprising many distinct chemical and physical phases on a variety of size scales[6-8]. Most studies of radionuclides in cements and concrete are for the most part restricted to phenomenological treatments of diffusion of ion s, particularly Cs, in and out of model waste forms and engineered barriers[9-21]. Few studies exist on the chemical speciation of the contaminants themselves in concrete [22-25]. For example, the extent to which various contaminants react with the cement and various aggregate particles is currently unknown, as is the role of the high pH of the cement pore water on ion partitioning and chemical speciation. DOE has designated understanding the chemical nature of the contaminants as important in the rational design of characterization, decontamination, and waste handling strategies[26, 27]. We have investigated laser ablation as a means of concrete surface removal[28-31]. Lasers are attractive since the power can be delivered remotely via articulated mirrors or fiber optic cables and the ablation head can be manipulated by robots, thus avoiding exposing workers and the laser system to the radiation field. In addition, lasers can be instrumented with spectrometers or effluent sampling devices to provide for on-line analysis. In contrast to mechanical scabbling systems, laser beams can penetrate cracks or follow very rough or irregularly shaped surfaces. Finally, a laser ablation system produces the smallest possible waste stream since no cleaning agents such as detergents or grit (from grit blasting systems) are mixed with the effluent.