Using 3D positions and kinematics of stars relative to the Sagittarius (Sgr) orbital plane and angular momentum, we identify 166 Sgr stream members observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) that also have Gaia DR2 astrometry. This sample of 63/103 stars in the Sgr trailing/leading arm is combined with an APOGEE sample of 710 members of the Sgr dwarf spheroidal core (385 of them newly presented here) to establish differences of 0.6 dex in median metallicity and 0.1 dex in [alpha/Fe] between our Sgr core and dynamically older stream samples. Mild chemical gradients are found internally along each arm, but these steepen when anchored by core stars. With a model of Sgr tidal disruption providing estimated dynamical ages (i.e., stripping times) for each stream star, we find a mean metallicity gradient of 0.12 0.03 dex Gyr(-1) for stars stripped from Sgr over time. For the first time, an [alpha/Fe] gradient is also measured within the stream, at 0.02 0.01 dex Gyr(-1) using magnesium abundances and at 0.04 0.01 dex Gyr(-1) using silicon, which imply that the Sgr progenitor had significant radial abundance gradients. We discuss the magnitude of those inferred gradients and their implication for the nature of the Sgr progenitor within the context of the current family of Milky Way satellite galaxies, and we suggest that more sophisticated Sgr models are needed to properly interpret the growing chemodynamical detail we have on the Sgr system.

The prediction of reaction pathways for solid-solid transformations remains a key challenge. Here, we develop a pathway sampling method via swarm intelligence and graph theory and demonstrate that our PALLAS method is an effective tool to help understand phase transformations in solid-state systems. The method is capable of finding low-energy transition pathways between two minima without having to specify any details of the transition mechanism a priori. We benchmarked our PALLAS method against known phase transitions in cadmium selenide (CdSe) and silicon (Si). PALLAS readily identifies previously reported, low-energy phase transition pathways for the wurtzite to rock-salt transition in CdSe and reveals a novel lower-energy pathway that has not yet been observed. In addition, PALLAS provides detailed information that explains the complex phase transition sequence observed during the decompression of Si from high pressure. Given the efficiency to identify low-barrier-energy reaction pathways, the PALLAS methodology represents a promising tool for materials by design with valuable insights for novel synthesis.

The Open Cluster Chemical Abundances and Mapping (OCCAM) survey aims to constrain key Galactic dynamical and chemical evolution parameters by the construction of a large, comprehensive, uniform, infrared-based spectroscopic data set of hundreds of open clusters. This fourth contribution from the OCCAM survey presents analysis using Sloan Digital Sky Survey/APOGEE DR16 of a sample of 128 open clusters, 71 of which we designate to be "high quality" based on the appearance of their color-magnitude diagram. We find the APOGEE DR16 derived [Fe/H] abundances to be in good agreement with previous high-resolution spectroscopic open cluster abundance studies. Using the high-quality sample, we measure Galactic abundance gradients in 16 elements, and find evolution of some of the [X/Fe] gradients as a function of age. We find an overall Galactic [Fe/H] versus R-GC gradient of -0.068 0.001 dex kpc(-1) over the range of 6 R-GC < 13.9 kpc; however, we note that this result is sensitive to the distance catalog used, varying as much as 15%. We formally derive the location of a break in the [Fe/H] abundance gradient as a free parameter in the gradient fit for the first time. We also measure significant Galactic gradients in O, Mg, S, Ca, Mn, Cr, Cu, Na, Al, and K, some of which are measured for the first time. Our large sample allows us to examine four well-populated age bins in order to explore the time evolution of gradients for a large number of elements and comment on possible implications for Galactic chemical evolution and radial migration.

Alkali metals Na, K, Rb and Cs are depleted in planetary mantles and their depletion is commonly attributed to the effect of volatility during the condensation of the first solids in the solar nebula or the high temperatures involved during planetary growth. Most models of planetary differentiation assume that alkalis behave entirely as lithophile elements and do not participate in core segregation. Here, we tested this hypothesis by determining experimentally the partitioning of Na, Cs and Rb between iron sulfide and silicate (D-sulf/sil) and combining it with available data from the literature on K, Na and Cs partitioning. Our experiments were conducted at 1-3.5 GPa, with an additional one at 8 GPa, 1600-1900 degrees C, and varying FeO contents, which lead to a relatively large range of O content in the sulfide phases (up to 13 wt%). We found maximum D-sulf/sil of 0.8, 0.4, and 0.36 for Na, Cs and Rb respectively. In addition, D-sulf/sil for Na, K, Cs and Rb increases with temperature and O content in the sulfide and decreases with FeO content in the silicate. The degree of polymerization of the silicate melt and the S content of the sulfide additionally increase D-sulf/sil for Na, K and Cs. Since the solubility of O in sulfides is correlated with the FeO content of the silicate and both have opposite effects on D-sulf/sil, varying the oxidation state of equilibrating material does not significantly affect D-sulf/sil, which is more controlled by the temperature of equilibration. We modeled core formation for Earth, Mars and asteroid Vesta, assuming that some of the accreted embryos contained immiscible sulfides, that segregated into planetary cores. Our results show that with such a scenario, significant amounts of Na, K, Cs and Rb were sequestered in planetary cores, leading to core/mantle distribution of alkalis between 4.10(-5) and 0.15. The depletion of alkalis in the mantles of Earth, Mars and Vesta could have resulted from combined effects of volatility and core segregation, but are largely due to volatile depletion in the accreting materials. (C) 2019 Elsevier Ltd. All rights reserved.

Permanent density increase of silica glass was used to calibrate pressure generation delivered by cupped sintered diamond anvils ('dimple anvils') [Haberl B, Molaison JJ, Neuefeind JC, et al. Simple modified Bridgman anvil design for high pressure synthesis and neutron scattering. High Press. Res. submitted] within the Paris-Edinburgh press between approximately 9 and 20 GPa. Raman spectral changes of recovered silica glass with increased density were used to determine the maximum pressure reached by following an established calibration curve [Deschamps T, Kassir-Bodon A, Sonneville C, et al. Permanent densification of compressed silica glass: a Raman-density calibration curve. J. Phys. Condens. Matter. 2013;25:025402]. The monotonic Raman shift of the Main Band spectral region (similar to 200-700 cm(-1)) of silica glass recovered from 9 to 20 GPa allows for continuous pressure calibration and is applicable to all presses that operate within this pressure range. Radial & axial Raman profiles were conducted to determine the pressure distribution within the sample chamber. This technique has been verified by in situ resistance measurements of the insulator-to-metal phase transition of ZnS near 15 GPa.

Numerical models of whole-mantle convection demonstrate that degassing of the mantle is an inefficient process, resulting in ca. 50% of the Ar-40 being degassed from the mantle system. In this sense the numerical simulations are consistent with the Ar-40 mass balance between the atmosphere and mantle reservoir. These models, however, are unable to preserve the large-scale heterogeneity predicted by models invoking geochemical layering of the mantle system. We show that the three most important noble-gas constraints on the geochemically layered mantle are entirely dependent on the He-3 concentration of the convecting mantle derived from the He-3 flux into the oceans and the average ocean-crust generation rate. A factor of 3.5 increase in the convecting-mantle noble-gas concentration removes all requirements for: a He-3 flux into the upper mantle from a deeper high He-3 source; a boundary in the mantle capable of separating heat from helium; and a substantial deep-mantle reservoir to contain a hidden Ar-40 rich reservoir. We call this model concentration for the convecting mantle the 'zero-paradox' concentration.

We report the first APOGEE metallicities and alpha-element abundances measured for 3600 red giant stars spanning a large radial range of both the Large (LMC) and Small Magellanic Clouds, the largest Milky Way (MW) dwarf galaxies. Our sample is an order of magnitude larger than that of previous studies and extends to much larger radial distances. These are the first results presented that make use of the newly installed southern APOGEE instrument on the du Pont telescope at Las Campanas Observatory. Our unbiased sample of the LMC spans a large range in metallicity, from [Fe/H] = -0.2 to very metal-poor stars with [Fe/H] -2.5, the most metal-poor Magellanic Cloud (MC) stars detected to date. The LMC [alpha/Fe]-[Fe/H] distribution is very flat over a large metallicity range but rises by similar to 0.1 dex at -1.0 < [Fe/H] less than or similar to -0.5. We interpret this as a sign of the known recent increase in MC star formation activity and are able to reproduce the pattern with a chemical evolution model that includes a recent "starburst." At the metal-poor end, we capture the increase of [alpha/Fe] with decreasing [Fe/H] and constrain the "alpha-knee" to [Fe/H] less than or similar to -2.2 in both MCs, implying a low star formation efficiency of similar to 0.01 Gyr(-1). The MC knees are more metal-poor than those of less massive MW dwarf galaxies such as Fornax, Sculptor, or Sagittarius. One possible interpretation is that the MCs formed in a lower-density environment than the MW, a hypothesis that is consistent with the paradigm that the MCs fell into the MW's gravitational potential only recently.

Controlling the solid-state polymerization of organic molecules to form crystalline materials remains a challenge for the synthetic chemist. In an effort to control reaction pathways through topochemistry, we have compressed the 1:1 naphthalene-octafluoronaphthalene cocrystal, C10H8 center dot C10F8. This starting material displays a unique structure wherein the molecules are aligned in a nearly sandwich-like pi-pi stacking arrangement because of the inverse polarities of naphthalene and its perfluoronated derivative. This stacking arrangement and the use of fluorine as an sp(3)-templating functional group creates favorable interactions between the molecules along the crystallographic a axis, providing topological control over the reaction pathway at high pressure. Reaction of C10H8 center dot C10F8 along the molecular stacking axis to form polymerized sp(3) rods with single-crystalline order was confirmed through in situ single-crystal X-ray diffraction and infrared spectroscopy, as well as GC-MS analysis of the recovered polymerized material, and supported by computational models. Polymerization occurs at room temperature under rapid compression without uniaxial stress indicating enhanced control through the topology of the molecular precursor.

Nanothreads are one-dimensional sp(3) hydrocarbons that pack within pseudohexagonal crystalline lattices. They are believed to lack long-range order along the thread axis and also lack interthread registry. Here we investigate the phase behavior of thiophene up to 35 GPa and establish a pressure-induced phase transition sequence that mirrors previous observations in low-temperature studies. Slow compression to 35 GPa results in the formation of a recoverable saturated product with a 2D monoclinic diffraction pattern along (0001) that agrees closely with atomistic simulations for single crystals of thiophene-derived nanothreads. Paradoxically, this lower-symmetry packing signals a higher degree of structural order since it must arise from constituents with a consistent azimuthal orientation about their shared axis. The simplicity of thiophene reaction pathways (with only four carbon atoms per ring) apparently yields the first nanothreads with orientational order, a striking outcome considering that a single point defect in a 1D system can disrupt long-range structural order.