Jeremy N. Mitchell

High-Al gabbros in the Laramie Anorthosite Complex, Wyoming: implications for the composition of melts parental to Proterozoic anorthosite

High-Al gabbro represents one of the latest phases of magmatism in the 1.43 Ga Laramie anorthosite complex (LAC) in southeastern Wyoming. This lithology, which is mineralogically and geochemically the most primitive in the LAC, forms dikes and small intrusions that cross cut monzonitic and anorthositic rocks. High-Al gabbro is characterized by high Al2O3 (15–19 wt%), REE patterns with positive europium anomalies (Eu/Eu*=1.2–3.8), and the lowest initial 87Sr/86Sr (as low as 0.7033) and highest initial ɛNd (up to +2) in the LAC. Their Sr and Nd isotopic characteristics indicate a mantle origin followed by crustal assimilation during ascent. Intermediate plagioclase (An50–60) and mafic silicate (Fo54–63) compositions suggest that they are not primary mantle melts and that they differentiated prior to final emplacement. High-Al gabbros of the LAC are similar compositionally to gabbros from several other Proterozoic anorthosite complexes, including rocks from the Harp Lake complex and the Hettasch intrusion in Labrador and the Adirondack Mountains of New York. These gabbros are considered to be parental to their associated anorthositic rocks, a theory that is supported by recent experimental work. We interpret LAC high-Al gabbros to represent mantle-derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber. Continued evolution of this magma eventually resulted in the formation of plagioclase-rich diapirs which ascended to mid-crustal levels and formed the anorthositic rocks of the LAC. Because these gabbros intrude the anorthositic rocks, they do not represent directly the magma from which anorthosite crystallized and instead are younger samples of magma formed by identical processes.

Residual-liquid origin for a monzonitic intrusion in a mid-Proterozoic anorthosite complex: The Sybille intrusion, Laramie anorthosite complex, Wyoming

The Sybille intrusion (≈100 km 2) is one of three large monzonitic intrusions in the 1.43 Ga Laramie anorthosite complex of southeastern Wyoming. The petrographic, geochemical, isotopic, and geophysical characteristics of Sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates (ferrodiorites) and contamination by Archean wall rocks. The exposed part of the intrusion is composed mainly of coarse-grained monzosyenites with abundant alkali feldspar phenocrysts. The monzosyenites preserve mineralogical evidence for high crystallization temperatures (>1000 °C), mid-crustal emplacement pressures (≈3 kbar), relatively reduced crystallization conditions (2 log units below the fayalite + magnetite + quartz [FMQ] oxygen buffer), and they crystallized in the presence of a CO2-rich fluid phase (Fuhrman et al., 1988; Frost and Touret, 1989). The eastern monzosyenites, those adjacent to contemporaneous anorthosite, are distinguished by an anhydrous mineral assemblage (Fo16-Fo8 olivine, high-Ca pyroxene) lacking modal quartz, silica contents of 60 wt%, and smaller Eu anomalies (Eu/Eu* = 1.2 to 1.3). Abundant xenoliths of Archean wall rocks and anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping emplacement mechanism near the roof of the chamber. We propose that the monzosyenites represent a relatively thin, 0.5-1.0-km-thick, roof to a magma chamber dominated by dense ferrodioritic cumulates at depth. Extensive, open-system fractionation of a ferrodioritic parent magma, residual after crystallization of anorthosite, produced Fe-enriched monzodioritic and/or monzonitic magma in the upper part of the chamber and complementary Fe- and Ti-rich cumulates in the lower levels. We have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance equilibrium-crystallization experiments. The presence of dense ferrodioritic cumulates at depth is consistent with the prominent positive gravity anomaly associated with the Sybille intrusion (Hodge et al., 1973). In the upper parts of the chamber, the fractionated monzodioritic and/or monzonitic magmas eventually became saturated in alkali feldspar. Owing to density contrasts, the alkali feldspar phenocrysts floated to the roof of the chamber, thus producing the exposed porphyritic monzosyenites. In addition, the roof of the chamber was the site of significant melting of Archean gneiss and, locally, metapelite. The Sr and Nd isotopic compositions of the monzosyenites, with Sr isotopic ratios becoming increasingly radiogenic from east ( I Sr = 0.7059 and initial ϵNd = −2.5) to west ( I Sr = 0.7092 and initial ϵNd = −2.6), are consistent with a 5% to 15% addition of Archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics similar to adjacent anorthositic rocks. The stratigraphic and compositional similarity of the Sybille monzosyenites to mangerites in the Bjerkreim-Sokndal intrusion of the Rogaland anorthosite complex, southern Norway, indicates that similar open-system magmatic processes are capable of having produced high-temperature, K-rich monzonitic rocks in other Proterozoic anorthosite complexes.

Multiconfigurational nature of 5f orbitals in uranium and plutonium intermetallics

Uranium and plutonium’s 5f electrons are tenuously poised between strongly bonding with ligand spd-states and residing close to the nucleus. The unusual properties of these elements and their compounds (e.g., the six different allotropes of elemental plutonium) are widely believed to depend on the related attributes of f-orbital occupancy and delocalization for which a quantitative measure is lacking. By employing resonant X-ray emission spectroscopy (RXES) and X-ray absorption near-edge structure (XANES) spectroscopy and making comparisons to specific heat measurements, we demonstrate the presence of multiconfigurational f-orbital states in the actinide elements U and Pu and in a wide range of uranium and plutonium intermetallic compounds. These results provide a robust experimental basis for a new framework toward understanding the strongly-correlated behavior of actinide materials.

The Valence-Fluctuating Ground State of Plutonium

Determination of plutonium’s fluctuating electronic and magnetic ground state resolves long-standing “missing” magnetism puzzle. A central issue in material science is to obtain understanding of the electronic correlations that control complex materials. Such electronic correlations frequently arise because of the competition of localized and itinerant electronic degrees of freedom. Although the respective limits of well-localized or entirely itinerant ground states are well understood, the intermediate regime that controls the functional properties of complex materials continues to challenge theoretical understanding. We have used neutron spectroscopy to investigate plutonium, which is a prototypical material at the brink between bonding and nonbonding configurations. Our study reveals that the ground state of plutonium is governed by valence fluctuations, that is, a quantum mechanical superposition of localized and itinerant electronic configurations as recently predicted by dynamical mean field theory. Our results not only resolve the long-standing controversy between experiment and theory on plutonium’s magnetism but also suggest an improved understanding of the effects of such electronic dichotomy in complex materials.

In situ MeV ion beam analysis of ceramic surfaces modified by 100-400 keV ion irradiation

Abstract This paper describes the use of an in situ ion beam analysis facility developed at Los Alamos National Laboratory for the study of irradiation effects in ceramic materials. In this facility, an analytical beamline of 3 MV tandem accelerator and an irradiation beamline of 200 kV ion implanter are connected at 60° to a common target chamber. This facility provides a fast, efficient, and quantitative measurement tool to monitor changes of composition and crystallinity of materials irradiated by 100–400 keV ions through sequential measurement of backscattering events of MeV ions combined with ion channeling techniques. We will describe the details of the in situ ion beam analysis and ion irradiation and discuss some of the important issues and their solutions associated with the in situ experiment. These issues include (1) the selection of an axial ion channeling direction for the measurement of radiation damage; (2) sample surface charging and charge collection for data acquisition; (3) surface sputtering during ion irradiation; (4) the effects of MeV analytical beam on the materials; and (5) the sample heating effect on ion beam analysis.

Alpha-plutonium's polycrystalline elastic moduli over its full temperature range.

alpha-plutoniums volume-corrected polycrystal elastic moduli were measured between 18 K and the upper limit of its occurrence, near 400 K. The two independent moduli for a polycrystal-bulk and shear-behave smoothly, indicating no phase transition. Both moduli show the same 50% increase on cooling, an order of magnitude larger than in other metals. The Debye temperature obtained from low-temperature elastic moduli, 207 K, significantly exceeds most previous estimates. The Gruneisen parameter gamma=5.3, obtained from the temperature dependence of the bulk modulus, is intermediate among previous estimates using other approaches, alpha-plutoniums Poisson ratio nu is low: 0.18, nearly temperature independent, and its small decrease on warming opposes usual behavior. The high gamma, large but equal bulk modulus and shear modulus fractional stiffening on cooling, and near-temperature-invariant nu are attributed to a single mechanism: 5-f electron localization-delocalization.

Self-irradiation damage to the local structure of plutonium and plutonium intermetallics

The effect of self-irradiation damage on the local structure of δ-Pu, PuAl2, PuGa3, and other Pu intermetallics has been determined for samples stored at room temperature using the extended x-ray absorption fine-structure (EXAFS) technique. These measurements indicate that the intermetallic samples damage at a similar rate as indicated in previous studies of PuCoGa5. In contrast, δ-Pu data indicate a much slower damage accumulation rate. To explore the effect of storage temperature and possible room temperature annealing effects, we also collected EXAFS data on a δ-Pu sample that was held at less than 32 K for a two month period. This sample damaged much more quickly. In addition, the measurable damage was annealed out at above only 135 K. Data from samples of δ-Pu with different Ga concentrations and results on all samples collected from different absorption edges are also reported. These results are discussed in terms of the vibrational properties of the materials and the role of Ga in δ-Pu as a network...

Avoided valence transition in a plutonium superconductor

Significance One way to search for new superconductors is to find a magnetic metal and then suppress the magnetism using chemical doping or pressure. Heavy-fermion superconductors are the archetypal family of magnetic superconductors, but PuCoGa5—the heavy fermion with the highest Tc (18.5 K)—has no static magnetism. What other mechanism, then, is driving superconductivity in PuCoGa5? We measured the elastic constants of PuCoGa5 and found that the bulk modulus softens dramatically before Tc—evidence for fluctuations of the plutonium valence as opposed to magnetic fluctuations associated with the suppression of magnetic order. Valence fluctuations resolve the missing magnetism conundrum in PuCoGa5 by providing an alternative mechanism for the high-temperature superconductivity. The d and f electrons in correlated metals are often neither fully localized around their host nuclei nor fully itinerant. This localized/itinerant duality underlies the correlated electronic states of the high-Tc cuprate superconductors and the heavy-fermion intermetallics and is nowhere more apparent than in the 5f valence electrons of plutonium. Here, we report the full set of symmetry-resolved elastic moduli of PuCoGa5—the highest Tc superconductor of the heavy fermions (Tc = 18.5 K)—and find that the bulk modulus softens anomalously over a wide range in temperature above Tc. The elastic symmetry channel in which this softening occurs is characteristic of a valence instability—therefore, we identify the elastic softening with fluctuations of the plutonium 5f mixed-valence state. These valence fluctuations disappear when the superconducting gap opens at Tc, suggesting that electrons near the Fermi surface play an essential role in the mixed-valence physics of this system and that PuCoGa5 avoids a valence transition by entering the superconducting state. The lack of magnetism in PuCoGa5 has made it difficult to reconcile with most other heavy-fermion superconductors, where superconductivity is generally believed to be mediated by magnetic fluctuations. Our observations suggest that valence fluctuations play a critical role in the unusually high Tc of PuCoGa5.

Initial electron backscattered diffraction observations of a plutonium alloy

Abstract In this work, the first electron backscattered diffraction patterns (EBSPs) were captured for a plutonium–gallium (Pu–Ga) alloy. The experimental techniques used for EBSP acquisition are described in detail. This demonstrated sample preparation and characterization technique is expected to be a powerful means to further understand phase transformation behavior, orientation relationships, and texture in the complicated Pu and Pu-alloy systems.

Single crystal growth of plutonium compounds from molten metal fluxes

The synthesis of single crystals of plutonium intermetallic compounds by the molten-metal flux growth technique is reviewed. The growth conditions, crystal structure, and physical properties including magnetic susceptibility, specific heat, and electrical resistivity of the binary and ternary Pu-based single crystals synthesized at Los Alamos National Laboratory are discussed.