Exotic Compositions Research Articles

A Chondritic Solar Neighborhood

A persistent question in exoplanet demographics is whether exoplanetary systems form from similar compositional building blocks to our own. Polluted white dwarf stars offer a unique way to address this question, as they provide measurements of the bulk compositions of exoplanetary material. We present a statistical analysis of the rocks polluting oxygen-bearing white dwarfs and compare their compositions to rocks in the solar system. We find that the majority of the extrasolar rocks are consistent with the composition of typical chondrites. Measurement uncertainties prevent distinguishing between chondrites and bulk Earth but do permit detecting the differences between chondritic compositions and basaltic or continental crust. We find no evidence of crust among the polluted white dwarfs. We show that the chondritic nature of extrasolar rocks is also supported by the compositions of local stars. While galactic chemical evolution results in variations in the relative abundances of rock-forming elements spatially and temporally on galaxy-wide scales, the current sample of polluted white dwarfs are sufficiently young and close to Earth that they are not affected by this process. We conclude that exotic compositions are not required to explain the majority of observed rock types around polluted white dwarfs and that variations between exoplanetary compositions in the stellar neighborhood are generally not due to significant differences in the initial composition of protoplanetary disks. Nonetheless, there is evidence from stellar observations that planets formed in the first several billion years in the Galaxy have lower metal core fractions compared with Earth on average.

Simulating sedimentary burial cycles – Part 2: Elemental-based multikinetic apatite fission-track interpretation and modelling techniques illustrated using examples from northern Yukon

Abstract. Compositionally dependent apatite fission track (AFT) annealing is a common but underappreciated cause for AFT age dispersion in sedimentary samples. We present an interpretation and modelling strategy for samples with variable apatite composition that exploits multikinetic AFT annealing to obtain thermal histories that can provide more detail and better resolution compared to conventional methods. We illustrate our method using a Permian and a Devonian sample from northern Yukon, Canada, both with complicated geological histories and long residence times in the AFT partial annealing zone. Effective Cl values (eCl; converted from rmr0 values) derived from detailed apatite elemental data are used to define AFT statistical kinetic populations with significantly different total annealing temperatures (∼110–185 ∘C) and ages that agree closely with the results of age mixture modelling. These AFT populations are well resolved using eCl values but exhibit significant overlap with respect to the conventional parameters of Cl content or Dpar. Elemental analyses and measured Dpar for Phanerozoic samples from Yukon and the Northwest Territories confirm that Dpar has low precision and that Cl content alone cannot account for the compositional and associated kinetic variability observed in natural samples. An inverse multikinetic AFT model, AFTINV, is used to obtain thermal-history information by simultaneously modelling multiple kinetic populations as distinct thermochronometers with different temperature sensitivities. A nondirected Monte Carlo scheme generates a set of statistically acceptable solutions at the 0.05 significance level and then these solutions are updated to the 0.5 level using a controlled random search (CRS) learning algorithm. The smoother, closer-fitting CRS solutions allow for a more consistent assessment of the eCl values and thermal-history styles that are needed to satisfy the AFT data. The high-quality Devonian sample (39 single-grain ages and 202 track lengths) has two kinetic populations that require three cycles of heating and cooling (each subsequent event of lower intensity) to obtain close-fitting solutions. The younger and more westerly Permian sample with three kinetic populations only records the latter two heating events. These results are compatible with known stratigraphic and thermal maturity constraints, and the QTQt software produces similar results. Model results for these and other samples suggest that elemental-derived eCl values are accurate within the range 0–0.25 apfu (atoms per formula unit, with rmr0 values of 0.73–0.84), which encompasses most of the data from annealing experiments. Outside of this range, eCl values for more exotic compositions may require adjustment relative to better-constrained apatite compositions when trying to fit multiple kinetic populations. Our results for natural and synthetic samples suggest that an element-based multikinetic approach has great potential to dramatically increase the temperature range and resolution of thermal histories relative to conventional AFT thermochronology.

Fabricating effective heterojunction in metal-organic framework-derived self-cleanable and dark/visible-light dual mode antimicrobial CuO/AgX (X = Cl, Br, or I) nanocomposites

Owing to the alarming consequences of antimicrobial resistance, the requirements for effective antimicrobial materials working under dark and visible-light conditions are on the high rise. On a different note, heterojunctions between semiconductors are highly desirable to enhance the photocatalytic activity of the individual components in a synergistic way. In this work, we address these two aspects in a single material composition through a rationally designed synthetic strategy. Among several candidates, silver and copper are known to exhibit high antimicrobial characteristics in the dark. Besides, CuO and silver halides are known for their visible-light photocatalytic activity. Hence, a copper-based porous metal organic framework, Cu-BTC, has been deposited with a silver halide followed by calcination to yield CuO/AgX (X = Cl, Br, or I) nanocomposites. The initial deposition of the silver halide over the MOF is envisaged to provide a high dispersion of the halide in the CuO matrix through numerous anchoring pods and thereby result in a tight and effective heterojunction. The nanocomposites obtained through this approach have been characterized structurally and morphologically using various techniques. The nanocomposites have been studied for their visible-light photocatalytic and self-cleaning ability via photodegradation of methylene blue and 4-chlorophenol as model systems. Further, their antimicrobial efficacy has been studied against E. coli and S. aureus both under dark and visible-light conditions. In addition to revealing the high potential of the CuO/AgX nanocomposites towards environmental applications, the synthetic approach presented herein can potentially be extrapolated to fabricate exotic compositions possessing enhanced efficacy.

An exotic Cretaceous kimberlite linked to metasomatized lithospheric mantle beneath the southwestern margin of the São Francisco Craton, Brazil

We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts, ilmenite, phlogopite, spinel, zircon, and uncommon minerals (titanite, calzirtite, anatase, baddeleyite and pyrochlore) of a newly discovered Late Cretaceous kimberlite (U-Pb zircon age 90.0 ± 1.3 Ma; 2σ) named Osvaldo França 1, located in the Alto Paranaíba Igneous Province (APIP), southeastern Brazil. Pyrope grains are lherzolitic (Lherz-1, Lherz-2 and Lherz-3), harzburgitic (Harz-3) and wehrlitic (Wehr-2). The pyrope xenocrysts cover a wide mantle column in the subcratonic lithosphere (66–143 km; 20–43 kbar) at relatively low temperatures (811–875 °C). The shallowest part of this mantle is represented by Lherz-1 pyropes (20–32 kbar), which have low-Cr (Cr2O3 = 1.74–6.89 wt.%) and fractionated middle to heavy rare earth elements (MREE-HREE) pattern. The deepest samples are represented by Lherz-2, Lherz-3, Harz-3, and Wehr-2 pyropes (36–43 kbar). They contain high-Cr contents (Cr2O3 = 7.36–11.19 wt.%) and are characterized by sinusoidal (Lherz-2 and Wehr-2) and spoon-like (Lherz-3 and Harz-3) REE patterns. According to their REE and trace elements, pyrope xenocrysts have enriched nature (e.g., Ce and Yb vs. Cr2O3), indicating that the cratonic lithosphere has been affected by a silicate melt with subalkaline/tholeiite composition due to their low Zr, Ti and Y concentrations. Besides minerals with typical kimberlitic signatures, such as ilmenite and zircon, the exotic compositions of phlogopite and ulvöspinel suggest an enriched component in the magma source. The formation of rare mineral phases with strong enrichment of light-REE (LREE) and high field strength elements (HFSE) is attributed to the late-stage kimberlitic melt. We propose a tectonic model where a thermal anomaly, represented by the low-velocity seismic anomaly observed in P-wave seismic tomography images, supplied heat to activate the alkaline magmatism from a metasomatized cratonic mantle source during the late-stages of Gondwana fragmentation and consequent South Atlantic Ocean opening. The metasomatism recorded by mineral phases is a product of long-lived recycling of subducted oceanic plates since the Neoproterozoic (Brasiliano Orogeny) or even older collisional events, contributing to the exotic character of the Osvaldo França 1 kimberlite, as well as to the cratonic lithospheric mantle.

Properties of planetary silicate melts by molecular dynamics simulation

Because magmatic liquids play a fundamental role in the evolution of the terrestrial planets, a precise knowledge of their physical properties is requisite to better understand the formation and the dynamics of planetary interiors. In using an improved force field for silicates (Dufils et al., 2017), we report the results of a series of molecular dynamics simulations (MD) aiming to evaluate the equation of state (EOS), the viscosity, the electrical conductivity, and the elemental self-diffusion coefficients of various planetary melts representative of the Earth, Mars, the Moon and Mercury. The agreement between MD calculations and experimental data (when they exist) is remarkable, a finding which suggests that the MD simulations can be used on trust to extend the existing laboratory data on planetary melts (as it is proposed here for some lunar, martian and mercurian basalts) or to predict the thermo-physical properties of more exotic compositions (magma oceans, lava planets). Moreover, the MD simulations show that the evolution of the viscosity and of the electrical conductivity with the pressure depends in a complex manner on temperature and composition and that is difficult to extract general trends from these behaviors. Another advantage of the MD simulations is that the transport coefficients (viscosity, conductivity, and diffusivities) being evaluated along the same numerical experiment, one is able to test the validity of the Eyring and the Stokes-Einstein equations relating viscosity and diffusivity, as also as the Nernst-Einstein equation expressing the conductivity as function of the ionic diffusivities. It is shown that the Stokes-Einstein equation is suitable to describe planetary melts of low viscosity (≤1 Pa·s), whereas the Eyring relation leads to a better estimation at high viscosities (>100 Pa·s). Concerning the electrical conductivity, the Nernst-Einstein relation fails to reproduce the conductivity values obtained by MD simulation, a result which can be explained by the crucial role played by ion-ion correlations in silicate melts.

Carbonatitic mineralogy of natural diamond-forming fluids

A suite of 20 cuboid fibrous diamonds from the Democratic Republic of Congo was studied to determine mineral phases crystallized from diamond-forming fluids that were trapped as inclusions in diamonds. We identified minerals and non-crystalline components using their compositional trends in the electron microprobe analyses of inclusions, an innovative bulk X-ray diffraction analysis and characteristic FTIR and Raman peaks. The DRC diamonds contain fluid inclusions of the Ca–carbonatitic–silicic composition. Most common phases precipitated from the fluid are found to be high-Si micas (90–30% of the solids volume), complex non-crystalline Na–Ca–Mg–Fe carbonate matter and apatite (together 5–70%), leaving a residual aqueous solution of K, Cl and carbonate ions and gaseous CO 2. A notable absence of carbonate minerals in bulk X-ray diffraction patterns combined with the vibrational spectroscopy observations on the C–O bonds indicates that C may be included in dissolved or amorphous carbonate matter. The modes of the most abundant phases are controlled by chemical trends of the bulk fluid compositions. Other relatively rare ∼ 30 minerals, including various minerals with structural and coordinated H 2O, are detected by the vibrational spectroscopic and X-ray analyses. The fluid also contains some hydrocarbons associated with the carbonate material. The presence of some rare minerals and exotic compositions of solid-solution minerals in the fluid suggest crystallization from a closed system with high salinity–high aH 2O–high aCO 2 composition that persisted to relatively low temperatures and pressures. The bulk of the fluid crystallized in the diamond stability field ( P < 7 GPa, T < 950 °C), but continued to form hydrocarbons, accessory and deuteric phases down to 200 °C and 0.2 GPa. Overall, the mineralogy of the fluid resembles carbonatite.

An integrated approach to understanding Apollo 16 impact glasses: Chemistry, isotopes, and shape

Abstract The major‐ and minor‐element abundances were determined by electron microprobe in 1039 glasses from regoliths and regolith breccias to define the compositional topology of lunar glasses at the Apollo 16 landing site in the central highlands of the Moon. While impact glasses with chemical compositions similar to local materials (i.e., Apollo 16 rocks and regoliths) are abundant, glasses with exotic compositions (i.e., transported from other areas of the Moon) account for up to ˜30% of the population. A higher proportion of compositionally exotic, angular glass fragments exists when compared to compositionally exotic glass spherules. Ratios of non‐volatile lithophile elements (i.e., Al, Ti, Mg) have been used to constrain the original source materials of the impact glasses. This approach is immune to the effects of open‐system losses of volatile elements (e.g., Si, Na, K). Four impact glasses from one compositionally exotic group (low‐Mg high‐K Fra Mauro; lmHKFM) were selected for 40Ar/39 Ar dating. The individual fragments of lmHKFM glass all yielded ages of ˜3750 ± 50 Ma for the time of the impact event. Based on the petrography of these individual glasses, we conclude that the likely age of the impact event that formed these 4 glasses, as well as the possible time of their ballistic arrival at the Apollo 16 site from a large and distant cratering event (perhaps in the Procellarum KREEP terrain) (Zeigler et al. 2004), is 3730 ± 40 Ma, close to the accepted age for Imbrium.

Valley networks on Venus

Valley networks on Venus are classified as rectangular, labyrinthic and pitted, or irregular. The venusian valley networks are structurally controlled, as indicated by the morphological patterns of valley branches, consistency between valley and fracture orientations, and associations with the deformed terrains. The morphologies resemble those of terrestrial and martian sapping valleys. Valley networks on Venus probably formed initially from fracture systems and became enlarged by low viscosity lava sapping processes. Subsurface flow of lava may locally have been assisted by surface flows. The lavas probably moved through permeable media and fractures. Venusian valley networks have a higher degree of network integration than do lunar sinuous rilles, but they are less integrated than martian and terrestrial sapping valleys. The viscosity of valley-forming lavas must have been very low, but was not low enough to exploit the permeable media so extensively as to attain a high degree of network integration. The compositions of these lavas may have been mafic to ultramafic or mafic alkaline. Alternatively, the lavas could have had more exotic compositions, such as carbonatite and sulfur. Valley networks are often associated with corona and corona-like features, which are hypothesized to be the surface expressions of mantle plumes. A plume association could mean that the lavas came from the mantle.

Radiative transfer models for Galileo NIMS studies of the atmosphere of Jupiter

Scientific results from NIMS observations of Venus have been extensively reported in the literature, while those of Jupiter have, at the time of writing, just barely commenced. The planning and interpretation of studies of these planets, with their massive atmospheres and exotic compositions (by terrestrial standards), requires a comprehensive treatment of radiative transfer in both. This paper describes work done at Oxford to develop the underlying theory and practical radiative transfer schemes, with particular reference to the NIMS wavelength range, spectral resolution, and scientific objectives for Jupiter. Equivalent work for Venus has already been reported in the literature (e.g. Kamp and Taylor, 1990) and will not be covered in detail here.

Carbon components and their isotopic compositions in the Allende meteorite

High resolution stepwise oxidations have been undertaken on two bulk fractions and on an acid residue of the Allende meteorite. The results show the existence of at least three carbon phases and suggest the possible presence of two more. The bulk of the carbon has a temperature of peak release closely corresponding to amorphous (free) carbon and an isotopic composition of approximately −20‰. At a slightly lower temperature of release we believe there to be carbon depleted in 13C. As far as variations in experimental technique allow a decision to be made, combustion of this component appears to take place under conditions which previous workers have shown to liberate exotic compositions of nitrogen and xenon.

Second Reading of Pindar: The Fourth Nemean

A few years ago, in Greece &amp; Rome of 1978, I recommended an approach to Pindar whereby he should be seen first in his more straightforward and simple compositions, on the grounds that an initial experience of the conventions of the epinician genre in odes for relatively unimportant people would both have immediate value for the understanding of those poems and also enable the student to use that familiarity at a later stage as a background for the understanding of the exotic and untypical compositions which celebrate the victories of the politically important, especially the Sicilian tyrants to whom the first three Olympians and the first three Pythians are dedicated.