Several characteristics of a planet, including its internal dynamics, hinge on the composition and crystallization regime of the core, which, in turn, depends on the phase relations, melting behaviour and thermodynamic properties of constituent materials. The Fe-Si-C ternary system can serve as a proxy for core composition and formation processes under reducing conditions. We conducted laser-heated diamond anvil cell experiments coupled with in situ X-ray diffraction and electron microscopy analysis of the recovered samples, on four different starting compositions in the Fe-Si-C ternary system. Phase relations up to 200 GPa and up to 4000 K were determined. An FeSi phase with a B2 structure and iron carbides with different stoichiometries (i.e., Fe3C and Fe7C3) are the main observed phases, along with pure C (diamond) that has an extended stability field in the subsolidus regime. Carbon is largely soluble in B2-structured FeSi, whereas Si does not partition into the carbides. The melting curve determined for the starting material containing the least amount of light elements is consistent with the one for the Fe-C system. The other starting materials display higher melting temperatures than that of Fe-C, suggesting the existence of at least two different invariant points in the Fe-Si-C system. Applied to planetary interiors, our observations highlight how a small variation in light elements content would deeply affect the solidification style of a core. Bottom-up (Fe-enriched systems) and top-down regimes (C-rich systems), as well as solidification of a crystal mush (Si-enriched systems). These three crystallization regimes influence significantly the possibility of starting and sustaining a dynamo. Our results provide new insights into the differentiation of terrestrial planets in the Solar System and beyond, contributing to the study of planetary diversity. (C) 2022 Elsevier Ltd. All rights reserved.

The electrical resistivity of solid and liquid Cu and Au were measured at high pressures from 6 up to 12 GPa and temperatures similar to 150 K above melting. The resistivity of the metals was also measured as a function of pressure at room temperature. Their resistivity decreased and increased with increasing pressure and temperature, respectively. With increasing pressure at room temperature, we observed a sharp reduction in the magnitude of resistivity at similar to 4 GPa in both metals. In comparison with 1 atm data and relatively lower pressure data from previous studies, our measured temperature-dependent resistivity in the solid and liquid states show a similar trend. The observed melting temperatures at various fixed pressure are in reasonable agreement with previous experimental and theoretical studies. Along the melting curve, the present study found the resistivity to be constant within the range of our investigated pressure (6-12 GPa) in agreement with the theoretical prediction. Our results indicate that the invariant resistivity theory could apply to the simple metals but at higher pressure above 5 GPa. These results were discussed in terms of the saturation of the dominant nuclear screening effect caused by the increasing difference in energy level between the Fermi level and the d-band with increasing pressure.

On compression of alpha-cristobalite SiO2 to pressures above approximately 12 GPa, a new polymorph known as cristobalite X-I forms. The existence of cristobalite X-I has been known for several decades; however, consensus regarding its exact atomic arrangement has not yet been reached. The X-I phase constitutes an important step in the silica densification process, separating low-density tetrahedral framework phases from high-density octahedral polymorphs. It is the only nonquenchable high-density SiO(2 )phase, which reverts to the low-density form on decompression at ambient temperature. Recently, an experimental study proposed an octahedral model of SiO2 X-I with intrinsic structural defects involving partial Si site occupancies. In contrast, our new single-crystal synchrotron X-ray diffraction experiments have shown that the ideal structure of this phase should instead be described by a defect-free model, which does not require partial occupancies. The structure of cristobalite X-I consists of octahedral chains with a 4-60 degrees-2 zigzag chain geometry. This geometry has not been previously considered but is closely related to post-quartz, stishovite, and seifertite. In addition to the ideal, defect-free crystal structure, we also present a description of the defects that are most likely to form within the X-I phase. Density functional theory calculations support our observations, confirming the dynamic stability of the X-I geometry and reasonably reproducing the pressure of the phase transformation. The enthalpy of cristobalite X-I is higher than stishovite and seifertite, but X-I is favored as a high-pressure successor of cristobalite due to a unique transformation pathway. Elastic and lattice dynamical properties of the X-I phase show intermediate values between stable tetrahedral and octahedral polymorphs, confirming the bridge-role of this phase in SiO2 densification.

As evidenced by isotope geochemistry, the persistence of primitive reservoirs indicates that the earth's lower mantle is likely to be heterogeneous. Such heterogeneity could be a legacy from magma-ocean (MO) solidification. The viscosity of MO is a key parameter to constrain the solidification type of MO. Here we directly measure the viscosity of peridotite (an analog of MO composition) melt at the pressure-temperature conditions of the deep mantle, using the in situ falling sphere method. The viscosity of peridotite melt along liquidus is in the range of 38-17 mPa s at pressures from 7 to 25 GPa, which is 0.9-0.4 times of the estimation based on the viscosity of endmember compositions. Low viscosity favors fractional solidification and chemically layering of the early mantle at least to the top lower mantle, which could be a source of heterogeneity for the present mantle.

We attempted to generate ultrahigh pressure and temperature simultaneously using a multi-anvil apparatus by combining the technologies of ultrahigh-pressure generation using sintered diamond (SD) anvils, which can reach 120 GPa, and ultrahigh-temperature generation using a boron-doped diamond (BDD) heater, which can reach 4000 K. Along with this strategy, we successfully generated a temperature of 3300 K and a pressure of above 50 GPa simultaneously. Although the high hardness of BDD significantly prevents high-pressure generation at low temperatures, its high-temperature softening allows for effective pressure generation at temperatures above 1200 K. High temperature also enhances high-pressure generation because of the thermal pressure. We expect to generate even higher pressure in the future by combining SD anvils and a BDD heater with advanced multi-anvil technology.

Titanium (Ti) isotopes are emerging as a power tool for studying magmatic processes on the Earth and other planets. Pioneering studies carried out bulk-rock Ti isotopic measurements by conventional solution nebulization multi-collector inductively coupled plasma mass spectrometry (SN-MC-ICP-MS) and in situ Ti isotopic analysis via secondary ionization mass spectrometry (SIMS), which sacrificed spatial resolution and had relatively low analytical precision, respectively. In this work, a novel and robust method for in situ Ti isotopic analysis of titanium-bearing minerals (i.e., rutile and ilmenite) was presented, based on a femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometer (fs-LA-MC-ICP-MS). Very stable isotopic signals can be ensured after careful optimization of the parameters of fs-LA (e.g., fluence, spot size, and frequency), and thus a high analytical precision has been obtained for Alfa-Ti (an ultrapure Ti metal rod) under a high spatial resolution of spot diameter = 30 mu m. The within run and external reproducibility for delta Ti-49/47 (Ti-49/Ti-47 isotopic ratio, reported as delta Ti-49/47 notation, in parts in 10(3)) measurement on Alfa-Ti are 0.05 parts per thousand (2SE, internal precision of within-spot analysis) and 0.07 parts per thousand (2SD, external reproducibility of spot-to-spot analysis), respectively. A series of titanium-bearing minerals, including five potential rutile U-Pb chronological standards and four potential ilmenite Fe isotope standards, were assessed for Ti isotopic homogeneity on a 30 mu m-scale and precision of the measurement. The most homogeneous minerals were subsequently used to comprehensively evaluate the analytical accuracy and potential matrix effect. Our results show that in situ Ti isotopic analysis is susceptible to matrix effects when using fs-LA and accurate delta Ti-49/47(OL-Ti) values (calibrated against the OL-Ti reference material developed in the Origins Laboratory of the University of Chicago) can be obtained when a matrix-matched reference material is used as a bracketing standard. Therefore, well characterized matrix-matched reference materials are necessary for in situ Ti isotopic analysis. KNW rutile and PZH12-15 ilmenite characterized in this study show potential as suitable reference materials for micro-beam Ti isotopic analysis.

For several decades, economists have been warning the academic community that graduate training has been too tightly focused on careers in higher education, using the apprenticeship model in which students are trained to become tenure-track faculty at research-focused institutions. These jobs are simply not growing at the same rate as graduate admissions. In biomedical research, the mismatch in supply and demand has now been widely recognized. Other disciplines have begun these discussions, but for smaller fields, employment trends are more difficult to identify because they are subsumed in aggregated national statistics. For non-biomedical biological fields in particular, such as ecology, using biology statistics may be inappropriate since trends within the field may be obscured by the strong signal from biomedical disciplines. Here, we use the 2013 Survey of Doctorate Recipients (SDR) to investigate career paths for ecology Ph.D. recipients in the United States, and present the first fine-scale national profile of careers in ecology. Our results demonstrate that while involuntary unemployment is low for ecology Ph.D. recipients (3.3%) and job satisfaction is high, the assumptions of the prevailing apprenticeship model are inappropriate: Less than 20% of employed recent ecology Ph.D. graduates are in tenure-track positions at a Ph.D.-granting university. Accordingly, proactive steps could be taken to create more realistic expectations about graduate training and preparation for diverse careers. Further, the SDR data provide demographic profiles for ecology. Ethnic diversity has remained low in ecology (7.5% non-Caucasian for Ph.D. recipients since 2000). Gender balance in career-track positions appears to have improved by multiple metrics. However, women are overrepresented in non-tenure-track academic positions, where access to resources that support professional advancement may be limited relative to tenure-track jobs, and salary disparities appear for women in private academic institutions. Thus, while there is much good news in these data, we suggest that ecology as a field would benefit from (1) a broad analysis of the training required to make Ph.D.s best prepared for jobs outside of the research-oriented tenure track and (2) continued attention to increasing diversity and equity.