Context. Metal-rich asteroids and iron meteorites are considered core remnants of differentiated planetesimals and/or products of oxygen-depleted accretion. Aims. Investigating the origins of iron-rich planetesimals could provide key insights into planet formation mechanisms. Methods. Using differentiation models, we evaluate the interior structure and composition of representative-sized planetesimals (~200 km diameter), while varying oxygen fugacity and initial bulk meteoritic composition. Results. Under the oxygen-poor conditions that likely existed early in the inner regions of the Solar System and other protoplanetary disks, core fractions remain relatively consistent across a range of bulk compositions (CI, H, EH, and CBa). Some of these cores could incorporate significant amounts of silicon (10–30 weight%) and explain the metal fractions of Fe-rich bodies in the absence of mantle stripping. Conversely, planetesimals forming under more oxidizing conditions, such as beyond snow lines, could exhibit smaller cores, enriched in carbon, sulfur (>1 wt%), and oxides. Sulfur-rich cores, like those formed from EH and H bulk compositions, could remain partly molten, sustain dynamos, and even drive sulfur-rich volcanism. Additionally, bodies with high carbon contents, such as CI compositions, can form graphitic outer layers. Conclusions. These variations highlight the importance of initial formation conditions in shaping planetesimal structures. Future missions, such as NASA’s Psyche mission, offer an opportunity to measure the relative abundances of key elements (Fe, Ni, Si, and S) necessary to distinguish among formation scenarios and structure models for Fe-rich and reduced planetesimals.

Phlogopite and K-richterite constitute important carrier phases for H and F in Earth's lithosphere and mantle. The relative importance depends on their stabilities at high pressure and temperature, which in turn depends on bulk composition. Most previous experimental studies focused on the thermal stability of phlogopite and K-richterite were conducted using simplified chemical compositions. Here, partial melting experiments on metasomatized and carbonated, OH ± F-bearing near-natural peridotite were performed at high pressures (2 and 5 GPa) and temperatures (1,100–1,350°C) to assess the thermal stability of F-free versus F-bearing phlogopite and K-richterite. Experimental results demonstrate that the thermal stability of F-bearing phlogopite is increased by >55°C/wt.% F, relative to F-free phlogopite, whereas K-richterite is absent in all experiments with significant degrees of melting (>2%). The thermal stability of phlogopite containing several wt.% F exceeds continental and oceanic geotherms within the upper 150 km. Fluorine-rich phlogopite would therefore be stable in virtually all of the continental lithosphere, only to be decomposed during large, regional melting events such as continental break-up, thereby acting as a major long-term sink for F and/or H. This could even be the case for the oceanic asthenosphere, depending on the oceanic geotherm of the area of interest.

Venus is like Earth in terms of size, density, and composition. However, both bodies evolved so dramatically differently that Venus is characterized by an uninhabitable, run-away greenhouse atmosphere, in marked contrast to Earth's favorable habitable conditions. Understanding the extreme divergence of Venus and Earth’s evolution is of great importance for understanding our early Earth, climate change as well as for studying the habitability of Earth- and Venus-like exoplanets. The key role of Venus in these fields is reflected by at least three major explorative missions scheduled in the next decade by NASA and/or ESA to study Venus in unprecedented detail.

The processes responsible for the isotopic compositions and abundances of volatile elements in the early solar system remain highly debated. Orders of magnitude variation of (highly) volatile elements exist between different magmatic iron meteorite groups, but it is unclear to what extent their depletions can be explained by evaporation from metal melts during parent body accretion and/or subsequent break up. To this end, we present 86 new evaporation experiments with the aim of constraining the volatility of most volatile metals from metallic melts. The results confirm the previously proposed important effects of S in metal melt on the volatility of the elements of interest governed by their S-loving or S-phobic behavior. Nominally S-loving elements In, Sn, Te, Pb and Bi are significantly more volatile in Fe melt relative to FeS liquid, whereas nominally S-avoiding elements Ga and Sb are more volatile in FeS liquid relative to Fe melt, at a given pressure and temperature. The newly derived volatility sequences for S-free/poor and S-rich metallic melts were also compared with commonly used volatility models based on condensation temperatures. The results indicate significant differences between the latter, including the much more volatile behavior of Te, relative to Se, in both explored bulk compositions, which are traditionally assumed to be equally volatile. The (minimum) degree of volatile element depletion due to evaporation was quantified using the new experimental results and models. A comparison between the volatile element depletions in magmatic iron meteorites and the predicted depletions appropriate for evaporation from Fe melts shows that the latter depletions can be easily reconciled with (an) evaporation event(s). Altogether, the new data and models will provide an important framework when more accurate and precise estimates of magmatic iron meteorite bulk volatile element contents are available.

The Earth could have experienced sulfide segregation during its differentiation due to sulfur (S) saturation within a magma ocean. The relative timing of sulfide saturation during magma ocean crystallisation is strongly dependent on the solubility of S at sulfide saturation (SCSS). Here, we present SCSS data directly relevant for a deep terrestrial magma ocean obtained from laser heated diamond anvil cell experiments. Our new data, along with existing SCSS data obtained for similar compositions, was parameterised to derive a new predictive equation. Our parameterisation predicts that existing models strongly underestimate the SCSS over the P-T range of a deep magma ocean. Our SCSS models provide the S abundances required at any given stage of terrestrial accretion, and imply that sulfide saturation is much less efficient at stripping the Earth’s mantle of S during accretion than previously predicted. Applying our results to the most recent mantle S evolution models shows that the SCSS would be far too high to achieve sulfide saturation, until only perhaps the final stages of magma ocean crystallisation. To satisfy highly siderophile element systematics, either the S content of the magma ocean was considerably higher than currently assumed, or highly siderophile element abundances were affected by other processes, such as iron disproportionation.