We investigate the thermal equation of state, bulk modulus, thermal expansion coefficient, and heat capacity of MH-III (CH4 filled-ice Ih), needed for the study of CH4 transport and outgassing for the case of Titan and superTitans. We employ density functional theory and ab initio molecular dynamics simulations in the generalizedgradient approximation with a van der Waals functional. We examine the temperature range 300-500 K and pressures between 2 and 7 GPa. We find that in this P-T range MH-III is less dense than liquid water. There is uncertainty in the normalized moment of inertia (MOI) of Titan; it is estimated to be in the range of 0.33-0.34. If Titan's MOI is 0.34, MH-III is not stable at present in Titan's interior, yielding an easier path for the outgassing of CH4. However, for an MOI of 0.33, MH-III is thermodynamically stable at the bottom of an ice-rock internal layer capable of storing CH4. For rock mass fractions less than or similar to 0.2 upwelling melt is likely hot enough to dissociate MH-III along its path. For super-Titans considering a mixture of MH-III and ice VII, melt is always positively buoyant if the H2O:CH4 mole fraction is >5.5. Our thermal evolution model shows that MH-III may be present today in Titan's core, confined to a thin (approximate to 10 km) outer shell. We find that the heat capacity of MH-III is higher than measured values for pure water ice, larger than heat capacity often adopted for ice-rock mixtures with implications for internal heating.

Observations of the geochemical diversity of mid-oceanic ridge and ocean-island basalts have traditionally been attributed to the existence of large-scale mantle heterogeneity. In particular, the layered convection model has provided an important conceptual basis for discussing the chemical evolution of the Earth. In this model, a long-term boundary is assumed between a well-mixed and depleted upper mantle and a heterogeneous and more primitive lower mantle. The existence of high He-3/He-4 in ocean-island sources has been used to argue for the preservation of a primitive component in the deep mantle. Nevertheless, a primitive deep layer is difficult to reconcile with the abundant lithophile isotopic evidence for recycling of oceanic crust and the lack of preservation of primitive mantle. In addition, the widespread acceptance of geophysical evidence for whole mantle flow has made straightforward application of the layered convection model problematic. Model calculations show that whole mantle convection with present day heat flow and surface velocities is sufficiently vigorous to mix large-scale heterogeneity to an extent that is incompatible with the geochemical observations. Several concepts have been proposed in recent years to resolve the apparent conflicts between the various observational constraints and theoretical interpretations. The suggestions include the presence of deeper layering, preservation of highly viscous blobs, core mantle interaction, and strong temporal variations in mantle dynamics. Although these models generally appear to solve parts of the puzzle, at present no single model is able to account for all of the major observations. The reconciliation of conflicting evidence awaits improvements in observational and experimental techniques integrated with better model testing of hypotheses for the generation and destruction of mantle heterogeneity.

Phosphorus in Martian mantle is believed to be five to ten times more abundant than in Earth's mantle, and the distribution of this essential ingredient for life between different deep reservoirs is critical for understanding the habitability of the red planet. In this study, we investigated the behavior of phosphorus in a Martian magma ocean scenario, and measured the partition coefficient of phosphorus (D-p) between liquid metal and silicate melt within the pressure range of 3-8 GPa, temperatures between 1973 and 2173 K and oxygen fugacity ranging from -1.5 to similar to -2.5 as normalized to the iron-wustite oxygen buffer. Our results show D-p increasing with pressure but decreasing with temperature. A decrease of oxygen fugacity has a negative effect on D-p. The moderately siderophile character of phosphorus indicates that the Martian core might be an important reservoir of phosphorous. Based on our experimental results and phosphorus abundance in Martian mantle and bulk Mars, a minimum pressure of 5.8-10.4 GPa is estimated at the base of Martian magma ocean or during the impact melting if a contribution from the late accretion scenario is taken into account. The shallow Martian magma ocean would avail the preservation of volatiles after the rapid solidification of the planet.

The formation of the Moon is thought to be the result of a giant impact between a Mercury-to-proto-Earth-sized body and the proto-Earth. However, the initial thermal state of the Moon following its accretion is not well constrained by geochemical data. Here, we provide geochemical evidence that supports a high-temperature origin of the Moon by performing high-temperature (1973-2873 K) metal-silicate partitioning experiments, simulating core formation in the newly-formed Moon. Results indicate that the observed lunar mantle depletions of Ni and Co record extreme temperatures (>2600-3700 K depending on assumptions about the composition of the lunar core) during lunar core formation. This temperature range is within range of the modeled silicate evaporation buffer in a synestia-type environment. Our results provide independent geochemical support for a giant-impact origin of the Moon and show that lunar thermal models should start with a fully molten Moon. Our results also provide quantitative constraints on the effects of high-temperature lunar differentiation on the lunar mantle geochemistry of volatile, and potentially siderophile elements Cu, Zn, Ga, Ge, Se, Sn, Cd, In, Te and Pb. At the extreme temperatures recorded by Ni and Co, many of these elements behave insufficiently siderophile to explain their depletions by core formation only, consistent with the inferred volatility related loss of Cr, Cu, Zn, Ga and Sn during the Moon-forming event and/or subsequent magma-ocean degassing. (C) 2020 Elsevier B.V. All rights reserved.

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.