The mantle composition of Venus is often assumed to be similar to Earth, albeit with a lower iron content to account for the density differences between the two planets. However, it has yet to be tested whether partial melting of proposed Venusian mantle compositions can produce melts that are similar to the measured basaltic rock compositions analysed in-situ during the Venera 14 and Vega 2 missions. In this study, we used Perple_X to calculate melt compositions from several bulk mantle compositions of Venus and found they were unable to reliably produce primary melt compositions that are similar to the Venera 14 or Vega 2 basalts, regardless of the oxidation state or degree of fractional crystallisation. As such, we used an iterative approach to identify new mantle compositions for Venus that are able to produce Vega 2- and/or Venera 14-like melts over a large pressure and temperature range. We found 23 mantle compositions that are similar to the terrestrial composition of KLB-1, but have a high Al2O3 and low CaO abundance, resulting in a sub-chondritic CaO/Al2O3 and SiO2/Al2O3. We recommend two of these as new mantle compositions for Venus as they were the most successful at producing Venus-like melts. Lastly, we propose that the sub-chondritic ratios of these new mantle compositions are the result of igneous processes, such as magma ocean differentiation and Ca-rich carbonatite melt extraction, that altered the mantle composition prior to the melting that produced the basalts sampled by the Venera 14 and Vega 2 missions.

Understanding Mars' deep interior is essential to reconstruct its geological history, thermal evolution, and present-day dynamics. To this end, the NASA InSight mission has provided unprecedented seismic observations. However, strong trade-offs between temperature and composition in seismic interpretations continue to limit our ability to resolve interior models. To address this challenge, we account for electromagnetic induction data from Mars Global Surveyor as an additional, independent constraint. We develop a joint probabilistic inversion framework that simultaneously fits seismic body wave arrival times, electrical conductivity, the Love number, and the moment of inertia. A key feature of our approach is the integration of Mars' long-term thermal evolution within the forward model, along with mineral physics and petrology data, to better constrain geodynamical parameters. We explore three different mantle compositions (Sanloup et al., 1999, https://doi.org/10.1016/s0031-9201(98)00175-7; Taylor, 2013, https://doi.org/10.1016/j.chemer.2013.09.006; Yoshizaki & McDonough, 2020, https://doi.org/10.1016/j.gca.2020.01.011) and consider both radially homogeneous and heterogeneous (with a basal molten layer (Samuel et al., 2023, https://doi.org/10.1038/s41586-023-06601-8)) mantle scenarios. For homogeneous mantle models, two families of solutions emerge regardless of the bulk composition: one with low Mg content and high potential temperature, which better reproduces electrical conductivity data due to a thicker lithosphere, and another with high Mg content and lower potential temperature. Models with a heterogeneous mantle reproduce electrical conductivity data less accurately, due to thinner lithospheres, and the mantle composition of Yoshizaki and McDonough (2020, https://doi.org/10.1016/j.gca.2020.01.011) appears to be less consistent with the full data set. To further refine models of Mars' interior, future efforts should focus on acquiring electromagnetic data with reduced uncertainties and seismically constraining more precisely the depth of mantle discontinuities associated with mineral phase transitions.

Ureilites are carbon-rich ultramafic achondrites that display unique textures, including strips of metal and carbon phases situated along grain boundaries and in fractures. Shock metamorphism observed in ureilites suggests an episode of brittle deformation caused by impact disruption of their parent body. The origin of carbon and metal has long been debated; in particular, whether either is endogenous or at least partly exogenous. We conducted experiments to simulate the metal-carbon textures and constrain their origin. Two model systems were investigated: (A) intrusion of FeS melt (analog for metal) into an olivine matrix containing dispersed graphite and (B) intrusion of graphite into a matrix containing dispersed FeS. After static annealing at 0.5-2 GPa and 1300 degrees C, the samples were deformed at high strain rates to simulate an impact event. The microstructures of system A most closely resembled the textures observed in medium to low-shock main group ureilites, supporting an endogenous origin of carbon and a largely exogenous origin of metal. The grain boundary linings of ureilites were formed by impactor metal that intruded along grain boundaries and mixed with locally mobilized carbon. Hence, we establish a direct connection between the metal-carbon textures in ureilites and the collision history of their parent body.

The observation of low viscosity lava flows and shield volcanoes on radar images, combined with X-ray fluorescence analyses performed by Soviet landers, strongly suggests that Venus's crust is primarily basaltic. However, near-infrared emissivity data from the Galileo and Venus Express missions indicate that crustal plateaus may be compositionally distinct and contain higher proportions of felsic minerals than the surrounding plains. The hypothesis that these highlands are felsic has led to suggestions of past oceans on Venus, as the generation of such rocks on Earth typically requires significant amounts of water. We use thermodynamic modeling to explore the conditions under which silica-rich melts can form without water, through deep melting of basaltic crust in thickened crustal roots or during crustal recycling in the mantle. Felsic melts containing 60-65 wt.% can form at depths 60 km. In regions of thickened crust, such melts would be produced along a thermal gradient of 10 K/km, whereas for higher gradients (15 K/km), melting occurs at depths where quartz is not yet present (50 km), resulting in silica-poor melts. The presence of small amounts of water allows similar felsic melts (in composition and volume) to form at moderately shallower depths (40 km). Our results indicate that large volumes of felsic melt could form even under anhydrous conditions on Venus, via crustal thickening or recycling mechanisms such as subduction or lithospheric dripping. This supports the idea that part of Venus's highlands could be the product of remelted basaltic crust, regardless of whether surface water existed in the past.

Enstatite chondrites are potential source material for the accretion of Mercury due to their reduced nature and enrichment in volatile elements. Understanding their melting properties is therefore important to better assess a scenario where Mercury formed from these chondrites. Here, we present experimental data on the partial melting of a modified EH4 Indarch enstatite chondrite, which was adjusted to have 18 % more metallic Si than SiO2 in mass, yielding an oxygen fugacity of 3.7 +/- 0.6 below iron-w & uuml;stite redox buffer and 12 wt% Si in the metal. Experiments were performed from 0.5 to 5 GPa using piston cylinder and multi-anvil apparatuses. Results indicate that the stability field of enstatite expands relative to olivine. This expansion is likely due to the presence of Ca-S and Mg-S complexes in the silicate melt, which enhance SiO2 activity and promote enstatite crystallization. Silicate melts present a correlation between Ca and S concentrations, like the global patterns seen on Mercury's surface but with higher sulfur abundances. Additionally, sulfides show enrichment in Mg and Ca, up to 22 and 13 wt% respectively, the main remaining cations being Fe, Cr and Mn. These high Mg and Ca contents are observed at low temperatures and high silica content in the silicate melt, respectively. Partial melting of this reduced EH4 chondrite yields a large range of silicate melt compositions, due to the Mg- and Ca-rich sulfides which act as significant residual phases. High-pressure melts (2 to 5 GPa, 160-400 km depth in Mercury) are Mg-rich, similar to those in Mercury's high-magnesium region (HMR), while low-pressure melts (0.5 to 1 GPa, 40-80 km depth) are Si-rich, comparable to the northern volcanic plains (NVP). Results suggest that a large fraction of Mercury's surface aligns compositionally with these melts, implying that Mercury's mantle could predominantly have a pyroxenitic composition. However, regions with differing compositions, such as aluminum-rich areas, like the Caloris basin, suggest local variability in mantle geochemistry. The HMR chemistry indicates melting at pressures up to the base of Mercury's mantle, possibly due to a large impact. Our study also explores whether the surface compositions could result from mixing processes like impact gardening or polybaric melting and magma mixing. The findings suggest that areas such as the intercrater plains and heavily cratered regions could be mixtures of melts from different pressures, ranging from 0.5 to 5 GPa, which corresponds to the crust-mantle to core-mantle boundaries. Overall, our results show that if Mercury formed from materials similar to enstatite chondrites, batch melting of its primitive pyroxenite mantle would yield magmas with compositions resembling those of most rocks observed on the surface. While the exact olivine content of the mantle remains uncertain, the residual mantle is likely enstatite-rich due to the extensive stability of enstatite relative to olivine in sulfur-rich reduced systems.

IntroductionDuring the early stages of lunar evolution, the large amount of heat available from the lunar forming impact led to extensive melting of the Moon’s silicate mantle giving rise to a magma ocean. The fractional crystallization of the latter resulted in an inhomogeneous mantle with cumulate layers of increasing density towards the surface, due to iron enrichment in the residual magma ocean [1]. Such an unstable density distribution is prone to overturn and sets the stage for the subsequent thermo-chemical history of the mantle. In particular, this initial stage can significantly affect mixing of mantle material and the partial melt production, as well as the melt composition during the later stages of lunar evolution. The melt composition is directly linked to the composition of lunar basalts, whose variability at the lunar surface indicates complex melting processes and the presence of various mantle reservoirs.In this study we model the solid-state convection, mixing and partial melt production in the lunar mantle. We investigate the effects of an initially layered mantle as a consequence of the fractional crystallization of the lunar magma ocean. To this end, we combine geodynamical models using the mantle convection code GAIA with petrological calculations that provide information about the composition and structure at the end of lunar magma ocean (LMO) solidification.While previous mantle convection studies investigated lunar mantle melting in a purely homogeneous mantle [2] or with a localized KREEP layer [3,4], here we combine petrological and geodynamical models to investigate the degree of heterogeneity that can be produced by melting and mixing an initially heterogeneous lunar mantle.Petrological modelingThe petrological model computes the crystallization sequence of the LMO. For the calculations in the deeper part of the mantle we use FOXMTR, for the shallower part alphaMELTS, as this approach is in good agreement with experimental data for the whole solidification process. We assume fractional crystallization of a spherical shell with an initial thickness of 1350 km, and a bulk lunar mantle composition from [5].During crystallization the temperature is lowered stepwise and all minerals (except plagioclase) are assumed to accumulate at the bottom of the LMO and equilibrate with the melt at the respective pressure conditions. Plagioclase is assumed to float to the surface and form anothoritic crust.The resulting mantle structure is characterized by 5 compositional layers, where the predominant minerals are olivine, orthopyroxene, clinopyroxene, clinopyroxene and ilmenite (ilmenite bearing cumulates = IBC) and plagioclase (crust), respectively. For each of those layers an average density is calculated from the density profile after crystallization, as well as the change in density as a function of depletion. The solidus and liquidus temperature profiles are calculated for the average layer compositions using alphaMELTS.The depths and densities of the compositional layers, the initial temperature profile, the solidus and liquidus profiles for each layer and the density-depletion functions of each layer composition are provided in the form of read-out tables.The results from the petrological model are shown in figure 1. The densities of the mantle layers increase with radius due to the enrichment of iron in the melt during crystallization. The crust has a comparative low density (figure 1a).The temperature profile follows the crystallization temperatures of the cumulates. In figure 1b the temperature profile is shown together with the individual layers and their corresponding solidi and liquidi. From olivine to IBC the solidus and liquidus temperatures decrease because of the crystallization sequence (figure 1b).Geodynamical modellingWe use the mantle convection code GAIA [6] in a 2D quarter cylinder geometry to model the thermochemical evolution of the compositionally heterogeneous lunar mantle. The nondimensional conservation equations of mass, momentum, thermal energy, and composition are solved under the extended Boussinesq approximation. We use an Arrhenius law to calculate the temperature- and depth-dependent viscosity.To track the composition of the mantle we use a particle-in-cell (PIC) method [7], where tracer particles carry information about material properties such as density, melting temperature, degree of depletion, amount of heat producing elements etc. Our models use between 35 to 50 particles per cell that are advected at each time step according to the velocity field.In our simulations we account for core cooling and radioactive decay, as well as mechanical mixing. In addition, we consider the effects of melting by accounting for the latent heat during mantle melting, as well as solidus and density increase/decrease due to mantle depletion [8]. During melting, low melting point components are extracted from the mantle as the melt rises to the surface to form the basaltic crust. This leads to an increase of the solidus temperature requiring a higher temperature to further melt the residual mantle material. Gradual changes in density and solidus temperatures are extracted from the precomputed read-out tables of the petrological modelling.Summary and Outlook Our coupled petrological-geodynamical model of mantle convection and melt production will be used to investigate the thermochemical evolution of an initially heterogeneous lunar mantle structure (figure 2). Preliminary results show a smaller amount of melt produced in a heterogeneous mantle in comparison to a homogeneous one. We will present the distribution of the compositional layers as a function of time and compare the amount of melting produced from each individual component.With this approach we aim to provide a link between melting processes in the interior of the Moon and the compositional diversity of basalts observed at the lunar surface. This in turn will help us to put constraints on the LMO crystallization and the subsequent thermochemical history of the Moon. AcknowledgementsWe thank Brian Doherty for his contribution to the numerical implementation.I.B. and S.S. were supported by the German Research Foundation (Deutsche Forschungsgemeinschaft) SFB-TRR170, (subprojects C4 and A5).References[1] Elkins-Tanton et al., Annu. Rev. Earth Planet Sci. 2012; [2] Ziethe et al., PSS, 2009; [3] Laneuville et al., JGR, 2013 [4] Laneuville et al., JGR, 2018; [5] O’Neill et al., 1991; [6] Hüttig et al., PEPI, 2013; [7] Plesa et al., IGI Global, 2013; [8] Doherty et al., AGU, 2018.

The crustal plateaus on Venus are highlands characterized by strongly deformed terrains, called tessera, and correspond to the stratigraphically oldest features on the planet [1]. Investigations of the gravity and topography signatures of the plateaus show that they are compensated by thickening of the crust and are consistent with being in an Airy isostasy state [2,3]. Recent crustal thickness estimates indicate that the base of the crust under plateaus can reach depths larger than 50 km [4], which correspond to pressures large enough to generate eclogite. A key property of eclogite is that it has a density larger than the surrounding mantle. This negatively buoyant material causes delamination and recycling of the crust. Hence, the depth of eclogite formation is commonly used to define the maximum possible crustal thickness of planets, and of Venus in particular [e.g. 5].Nevertheless, the process of delamination of high-density eclogite under thickened crustal regions has not been numerically studied in detail using thermodynamic models that calculate the densities consistent with Venusian petrology and basalt-to-eclogite phase change. In this study, we combine depth-dependent crustal density profiles obtained with Perple_X considering the surface composition constrained by Vega 2 (see [6] for details) and the finite-element software COMSOL multiphysics that couples heat transfer, creeping flow, and the level set modules to investigate the delamination process. We focus in particular on the Western Ovda Regio, as this process could be relevant for the crustal structure suggested at this location by gravity and topography studies [5].Western Ovda Regio, shown in Figure 1, has a unique topographic signature among all crustal plateaus. It presents a high-elevation rim, reaching up to 2 km altitude in the eastern part, and a collapsed center heavily embayed by the volcanic plains. The processes that caused this central collapse is not well-understood and cannot be explained by viscous relaxation alone [e.g., 7, 8]. In this study we investigate if Western Ovda could have experienced delamination of a high-density eclogitic material, which is thought to have formed at the base of the plateau as a consequence of the basalt-to-eclogite phase transition.Our first results show that generating negatively buoyant eclogitic crust is not enough to trigger recycling. If the temperature is cold, this eclogitic crust would be trapped in the so-called stagnant lid, an immobile layer that forms as a consequence of the temperature-dependence of the viscosity. When considering a linear thermal gradient of 5 K/km throughout the crust, negatively buoyant material can be found below 40 km depth, but this material is only recycled if the crust reaches about 100 km thickness. On the other hand, if the temperature is high enough, (i.e., about 1200K), the material is able to flow, and the deeper crustal layer could delaminate. For a thermal gradient of 10 K/km, the stagnant lid is thin enough to allow for high density materials that form at depths of around 60 km to delaminate. In conclusion, our results show a delicate interplay between the thickness of the crust, the thermal conditions of the lithosphere and the density contrast between the crust and mantle. In scenarios where delamination occurs, the models indicate that the high surface topography indeed collapses, as we expected. Next steps in this study will include testing a wider range of model setups, in particular testing different lithospheric thermal gradients. In addition, we will extend our analysis by estimating the gravity signature from these models and comparing them with the observations. References[1] Ivanov and Head, 2011. PSS; [2] Grimm, 1994. Icarus; [3] Kucinskas and Turcotte, 1994. Icarus; [4] Maia and Wieczorek, 2022. JGR: Planets; [5] James et al., 2013. JGR: Planets; [6] Collinet et al., 2024. EPSC 2024 (this meeting); [7] Nunes et al., 2004. JGR; [8] Nunes and Phillips, 2007. JGR

The Martian crust has long been thought to be made of relatively uniform FeO-rich mafic rocks but, over the last two decades, new meteorite discoveries, orbital and in-situ analysis of Martian rocks have painted a more nuanced picture. Alkaline rocks, silica-rich rocks and possible anorthosites have been identified at several locations [e.g., 1]. The basaltic rocks, still believed to account for the majority of the upper crust, also display a chemical variability that could be tied to the thermal-evolution of the planet’s interior [2]. Those various lithologies indicate that Mars had a rich geologic history and that the crust was shaped by a wide range of igneous processes.The abundant basalts likely result from decompression melting of the upper mantle and represent a “secondary” crust that would have covered any “primary” crust formed during the crystallization of the magma ocean. The compositions of the melts transferred from the mantle to the crust are controlled by the pressure and temperature of melting and are best constrained with experiments [3]. However, experiments represent snapshots at fixed P-T and usually simulate isobaric batch melting processes that are not completely analogous to actual melting conditions in the mantle. An alternative is to use thermodynamic models [4, 5], which can be used to simulate polybaric and fractional melting. Unfortunately, those models show several important shortcomings. For example, pMELTS [4] systematically overestimates the FeO content of the melts produced at high pressure. To simulate partial melting of the Martian mantle, other workers have adopted a hybrid approach using pMELTS and corrections based on experiments [1-2]. This type of hybrid approach is still problematic because the magnitude of the offsets relative to experiments are pressure-dependent and larger than previously anticipated (e.g., up to 8 wt.% (Fig. 1a) vs. 3 wt.% [1-2] for FeO).Here, we present a simple empirical model that reproduces the composition of experimental melts more accurately (Fig. 1b, 1d) and can also be used to simulate various melting behaviors. It is similar in concept to melting models developed for the near-fractional polybaric melting of the Earth mantle at mid-oceanic ridges [6]. The concentrations of minor and major incompatible elements (TiO2, Al2O3, Na2O, K2O and P2O5) into the melt are first calculated from experimentally determined partition coefficients between minerals and melt [3]. The concentration of the other major elements (SiO2, FeO and MgO) are then deducted from the concentration of incompatible elements and pressure. CaO is treated as a major compatible element or as an incompatible element, based on whether clinopyroxene is present in the residue (Fig. 1d).The new empirical model calculates the composition of “instantaneous melts” formed by isobaric or polybaric continuous melting with an adjustable critical melt fraction (i.e. melt fraction retained in the residue). Compared to batch and aggregate melts (symbols and solid lines in Fig. 1), instantaneous melts (dashed lines in Fig. 1) become enriched in FeO, MgO and CaO and depleted in incompatible elements (e.g., K2O, Na2O, Al2O3) as melting progresses. Instantaneous melts can be pooled and directly compared to the composition of the Martian crust. They could also be used as input for more advanced modelling of igneous differentiation (e.g. involving fractional crystallization, magma mixing, etc.) to account for the formation of the more evolved Martian igneous rocks. In a future step, the new model will be used to incorporate an accurate treatment of compositional changes due to melting and depletion of the mantle in global thermochemical evolution models.Fig. 1. Composition of mantle melts from a primitive mantle composition [7] at different but constant pressure (isobaric melting). (a, c) pMELTS model. (b, d) Empirical model. In this example, both models simulate continuous melting (near-fractional melting) with a 1% retained melt fraction. The dashed lines represent the “instantaneous melts” constantly removed from the residue. The solid lines represent the total pooled or “aggregate melts”, the sum of all previous instantaneous melts. Under isobaric conditions, aggregate melts are analogous to batch melts. Symbols (exp. C15) represent batch melts from the experiments of [3]. pMELTS accurately reproduces the experimental melt compositions at 0.5 GPa but not at higher pressure.­­References:[1] Sautter et al. (2015) Nat. Geosci. 10.1038/ngeo2474, [2] Baratoux et al. (2011) Nature 10.1038/nature09903, [3] Collinet et al. (2015) EPSL 10.1016/j.epsl.2015.06.056, [4] Ghiorso et al. (2002) G3 10.1029/2001GC000217, [5] Holland et al. (2018) J. Petrol 10.1093/petrology/egy048, [6] Till et al. (2012) JGR 10.1029/2011JB009044, [7] Dreibus and Wänke (1985) Meteoritics

Moderately siderophile elements (MSEs) are potential tracers of the thermodynamic conditions prevailing during planetary core formation because their metal-silicate partition coefficients (D-met/sil) vary as a function of P, T, and oxygen fugacity (fO(2)). Those properties result in the production of planetary mantles with unique MSE depletion signatures. Among the MSEs, Ni and Co are reliable barometers in magma oceans because their D-met/sil values are strongly correlated with pressure, decreasing by almost 3 orders of magnitude between 1 bar and 100 GPa. Current pressure-dependent expressions of D-met/sil were calibrated based on experiments performed under relatively oxidizing conditions, mostly at fO(2) slightly below the iron-wustite Fe-FeO buffer (IW), which is relevant to the mantles of Earth and Mars. However, planets and asteroids formed under a wide range of redox conditions, from Mercury, the most reduced (similar to IW - 5.5), to the most oxidized angrite parent body (IW - 1.5 to IW + 1). In this study, we performed and analyzed 38 metal-silicate partitioning experiments over a wide range of pressures (1 bar to 26 GPa) and oxygen fugacities (IW - 6.4 to IW - 1.9) to expand the available Ni and Co D-met/sil values to reducing conditions. We then parameterized 255 Ni and 194 Co D-met/sil values as a function of T (1573-5700 K), P (1 bar to 100 GPa), and fO(2) (IW - 6.4 to IW + 0.2). We also modeled the evolution of Ni and Co D-met/sil values along the liquidus of a chondritic mantle at various P and fO(2) conditions to investigate the thermodynamic conditions of various planetary bodies' magma oceans. The P and fO(2) conditions we obtained for Earth, Mars, the Moon, and Vesta are consistent with previous studies using similar methods, and the pressure during core formation is strongly correlated to planetary size. Finally, we also applied our model to several achondrite parent bodies; our results indicate a wide variety of objects, from the asteroid-sized, oxidized angrite parent body to the planet-sized, highly reduced aubrite parent body.

Venus is often referred to as the Earth’s twin due to its similar size and mass. While Venus surface is young (~750 Myr old on average), there is no direct evidence for large tectonic plates as we have on Earth. Most of the surface is thought to be basaltic in composition, although some regions, the so-called tessera, that cover about 8% of Venus’ surface have been suggested to be more felsic, potentially resembling continental crust on the Earth (Gilmore, 2015). The crustal composition carries valuable information about planetary differentiation processes and can be used to constrain the interior composition. Several mantle compositions have been proposed based on the evolution of the Solar System (condensation (e.g., Lewis, 1972) and chondrite models (Morgan and Anders, 1980)) or based on the similarities and differences between Venus and Earth (e.g., difference in mean density and similarity in radius (BVSP, 1981)). However, it is yet to be assessed if these mantle compositions can reproduce the composition of Venus surface rocks. During the 1980’s, three landers (Venera 13 and 14, and Vega 2) successfully completed missions on the surface of Venus, collecting and analysing surface rock material (Surkov et al., 1984, 1986). The surface rock compositions were determined using X-ray fluorescence and then compared to a suite of Earth rocks analysed in the same way. The results show that Venus’ surface is best described as consisting of alkaline basalts (Venera 13) and tholeiitic basalts (Venera 14 and Vega 2) (Fegley, 2014). However, the data compiled from these missions contain large uncertainties and several elements, such as Na, have not been measured (Treiman, 2007). Although these analyses are limited, they remain the most accessible data we have about Venus’ surface, as there have been no further surface-based missions to Venus and there are no known Venus meteorites. A combination of thermodynamic modelling with the Venera and Vega lander data can be used to constrain the composition of Venus’ mantle. Perple_X allows for primary melt compositions to be calculated using a combination of thermodynamic data and Gibbs energy minimisation (Connolly, 2005). Hence, in this study, proposed Venus mantle compositions (BVSP, 1981) and a terrestrial mantle composition (Davis et al., 2009) will be used to generate partial melt compositions in Perple_X (Connolly, 1990, 2005, 2009) in conjunction with the internally consistent database described in Holland, Green, and Powell (2018, 2022). These partial melt compositions will be compared with the Venera 14 and Vega 2 lander data (Treiman, 2007) with a focus on the abundance of major elements SiO2, Al2O3, FeO, MgO, and CaO. Further investigations will evaluate if fractional crystallisation may have played a role in the generation of Venera 14 and Vega 2 rocks. Our models are a necessary step towards understanding the interior of Venus, its magmatic differentiation, and the link to surface composition. Combined with thermal evolution models (Herrera et al., this meeting), our work will provide a useful basis for the interpretation of future measurements of VERITAS and EnVision missions, that aim to characterize the types of rocks and minerals on Venus’ surface with unprecedented detail. References: Basaltic Volcanism Study Project, 1981, Basaltic volcanism on the terrestrial planets: New York, Pergamon, 286 p.Connolly, J., 1990, Multivariable phase diagrams; an algorithm based on generalized thermodynamics: AJS, v. 290, no. 6, p. 666–718, doi: 10.2475/ajs.290.6.666.Connolly, J., 2005, Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation: EPSL, v. 236, 1-2, p. 524–541, doi: 10.1016/j.epsl.2005.04.033.Connolly, J., 2009, The geodynamic equation of state: What and how: Geochem, Geophys, v. 10, no. 10, doi: 10.1029/2009GC002540.Davis, F.A., et al., 2009, The composition of KLB-1 peridotite: American Mineralogist, v. 94, no. 1, p. 176–180, doi: 10.2138/am.2009.2984.Fegley, B., 2014, 2.7 - Venus, in Heinrich D. Holland, Karl K. Turekian, eds., Treatise on Geochemistry (Second Edition), Second Edition ed.: Oxford, Elsevier, p. 127–148.Gilmore, M.S., et al., 2015, “VIRTIS emissivity of Alpha Regio, Venus, with implications for tessera composition.” In: Icarus 254, pp. 350–361, doi: 10.1016/j.icarus.2015.04.008.Holland, T.J.B., et al., 2018, Melting of Peridotites through to Granites: A Simple Thermodynamic Model in the System KNCFMASHTOCr: Journal of Petrology, v. 59, no. 5, p. 881–900, doi: 10.1093/petrology/egy048.Holland, T.J.B., et al., 2022, A thermodynamic model for feldspars in KAlSi 3 O 8 −NaAlSi 3 O 8 −CaAl 2 Si 2 O 8 for mineral equilibrium calculations: Journal of Metamorphic Geology, v. 40, no. 4, p. 587–600, doi: 10.1111/jmg.12639.Lewis, J.S., 1972, Metal/silicate fractionation in the solar system: EPSL, v. 15, no. 3, p. 286–290, doi: 10.1016/0012-821X(72)90174-4.Morgan, J.W., and Anders, E., 1980, Chemical composition of Earth, Venus, and Mercury: PNAS, v. 77, no. 12, p. 6973–6977, doi: 10.1073/pnas.77.12.6973.Surkov, Y.A., et al., 1984, New data on the composition, structure, and properties of Venus rock obtained by Venera 13 and Venera 14: LPSC, v. 89, B393-B402, doi: 10.1029/JB089iS02p0B393.Surkov, Y.A., et al., 1986, Venus Rock Composition at the VEGA 2 Landing Site: LPSC, v. 91, 215-E218.Treiman, A.H., 2007, Geochemistry of Venus' Surface: Current limitations as future opportunities, in Esposito, L.W., et al., eds., Exploring Venus as a Terrestrial Planet: Washington, D. C., American Geophysical Union. Geophysical Monograph Series, p. 7–22.

The observation of low viscosity lava flows and shield volcanoes on radar maps, combined with in-situ X-ray fluorescence analyses performed by three Soviet landers, strongly suggests that Venus' crust is primarily basaltic. Still, some of the most intriguing features of Venus are its crustal plateaus, characterized by heavily deformed terrains, which have long been suggested to bear a superficial resemblance to Earth’s continental crust and mountain ranges. Infra-red emissivity spectra from the Galileo and Venus Express missions tend to support the presence of a larger fraction of felsic minerals in the plateaus compared to the surrounding basaltic plains [1-2].On Earth, flux melting of the mantle wedge at subduction zones, followed by fractional crystallization or partial melting of hydrous basalts, are believed to be the two primary mechanisms generating the large volumes of intermediate to felsic rocks that make up the continental crust. By contrast, igneous differentiation of water-poor basaltic melts typically yields negligible amounts of felsic melts. The possibility that highland plateaus are dominated by intermediate to felsic rocks will be evaluated by the EnVision and Veritas missions, in the hope of providing evidence for the presence of water oceans and, therefore, habitable conditions in Venus' distant past.In this work, we show, using thermodynamic calculations (Perple_X), that the melting of dry eclogite is another viable mechanism that can produce large volume of intermediate to felsic melts, in the absence of water. An average basaltic crust, of composition identical to the ones analyzed at the Venera 14 and Vega 2 landing sites, would transform into a quartz eclogite at a depth of 50 to 60 km. Partial melting of this material can produce 15-25 % of dacitic melts. The crust of Venus could have reached this depth under crustal plateaus, according to gravity and topography investigations [3]. It has also been suggested that the current young surface of Venus could indicate that abundant basaltic material was recycled to the mantle [4]. Remelting of this material could fuel occasional but large-scale magmatic events and account for the formation of felsic crustal plateaus in the absence of water. Confirmation that crustal plateaus are dominantly felsic by future missions might therefore not necessarily indicate that wetter and more hospitable conditions prevailed on early Venus. [1] G. L. Hashimoto et al. (2008) Felsic highland crust on Venus suggested by galileo near-infrared mapping spectrometer data. J. Geophys. Res.: Planet 113, E00B24.[2] M. S. Gilmore, N. Mueller, J. Helbert (2015) VIRTIS emissivity of Alpha Regio, Venus, with implications for tessera composition. Icarus 254, 350–361.[3] J. S. Maia, M. A. Wieczorek (2022) Lithospheric structure of Venusian crustal plateaus. J. Geophys. Res.: Planet. 127, e07004.[4] S. E. Smrekar, A. Davaille, C. Sotin (2018) Venus Interior Structure and Dynamics. Space Science Reviews 214(5), 88.

In this presentation, we will summarize the conclusions of three recent articles [1-3] describing partial melting experiments of ordinary and carbonaceous synthetic chondrites (H, LL, CI, CM and CV). The experiments highlight the role of alkali elements on the melting processes of chondritic planetesimals and provide insights into the distribution of “moderately volatile elements” in the early solar system. They were performed at 2–13 MPa (CO pressure) in a Molybdenum-Hafnium Carbide Pressure Vessel. This approach prevented the loss of alkali elements during experiments, a common limitation of previous studies using gas-mixing furnaces.Alkali-rich planetesimals, similar in composition to CI, H and LL chondrites, started to melt at low temperature (1040 ºC) and produced silicate melts with high alkali, SiO2 and Al2O3 contents [1], similar in composition to “trachyandesite achondrites” such as GRA 06128 [4] and ALM-A [5]. In addition, the main groups of primitive achondrites (i.e., brachinites, acapulcoites-lodranites and ureilites), which represent mantle residues, all produced similar low-degree melts (550 samples), represent the residual mantle of a planetesimal that was catastrophically disrupted and quenched while in the process of melting [3]. Our experiments show that the Ureilite Parent Body (UPB) produced a total of 16-24 wt.% silicate melt as small increments (< 5%). Following the extraction of melts rich in SiO2, Al2O3 and alkalis, the residual mantle produced melts poor in alkalis but rich in CaO. The sampled portion of the UPB reached temperatures as high as 1300 ºC, but the rapid extraction of silicate melts preserved primordial heterogeneities in O, C and Cr isotopes as well as in intrinsic fO2.Trachyandesite achondrites, ureilites and other major groups of primitive achondrites were all derived from planetesimals that were initially rich in alkali elements (i.e., not depleted relative to the Sun’s photosphere). They all display nucleosynthetic anomalies characteristic of the inner solar system. Therefore, the depletion of alkalis in other meteorite parent bodies of the inner solar system (e.g., Vesta) likely results from processes that occurred during partial melting. Similarly, the depletion of alkalis in terrestrial planets could result from a secondary loss of alkalis associated with partial melting of the planets building blocks, rather than from the incomplete condensation of the solar nebula.References:[1] Collinet and Grove (2020a), GCA, https://doi.org/10.1016/j.gca.2020.03.005, [2] Collinet and Grove (2020b), GCA, https://doi.org/10.1016/j.gca.2020.03.004, [3] Collinet and Grove (2020c), MAPS, https://doi.org/10.1111/maps.13471, [4] Day et al., (2009), Nature, https://doi.org/10.1038/nature07651, [5] Bischoff et al. (2014), PNAS, https://doi.org/10.1073/pnas.1404799

The martian surface is predominantly covered by FeO-rich basalts and their alteration products.Several samples, either analyzed in situ by rovers or recovered as meteorites, might represent primitive (i.e.near-primary) basaltic melts that can shed light on the mineralogy, the bulk composition, and the temperature of their mantle sources.We recently developed a new melting model, called MAGMARS, that can predict the melt compositions of FeO-rich mantles and the martian mantle in particular (Collinet et al., submitted to JGR:P).It represents a more accurate alternative to pMELTS (Ghiorso et al., 2002, G3), which systematically overestimates the FeO and MgO content of martian melts and underestimates the SiO2 content (by up to 8 wt.%).MAGMARS can simulate near-fractional and batch melting of various mantle compositions.For example, MAGMARS can produce melts identical to the Adirondack-class basalts by near-fractional melting, between 2.3 and 1.7 GPa, of a depleted mantle with a potential temperature (Tp) of 1390°C (˜7 wt.% melt fraction).For this study, MAGMARS is applied to all other martian basalts from which the primary melt compositions can be inferred in order to constrain their mantle sources: the Columbia hills basalts, igneous rocks from Gale crater, shergottites, nakhlites and Northwest Africa (NWA) 7034/7533.We find that a few basaltic clasts in the pre-Noachian polymict regolith breccia NWA 7034/7533 are the only samples with bulk compositions that could represent melts derived from a primitive mantle.The Columbia hills basalts (Gusev crater), alkali-rich rocks from Gale crater, nakhlites and enriched shergottites are most easily reproduced by melting depleted mantle reservoirs that were re-fertilized to different degrees in alkalis by fluids or melts (i.e.metasomatized sources).Most martian basalts, with the exception of depleted shergottites, can be produced from martian mantle reservoirs with Mg# comprised between 75 and 81.From this sample set, the melting conditions of the martian mantle seem to remain relatively stable through time (Tp = 1400 ± 100 ºC and P = 2 ± 0.5 GPa) but the depleted nature of all mantle sources sampled after the pre-Noachian points towards an early crust-mantle differentiation.source of shergottites) undeniably represents the hottest regions of the Martian mantle.Overall, the sampled primary melts, while not fully representative, seem to be in line with global thermal evolution models.

Abstract The composition of basaltic melts in equilibrium with the mantle can be determined for several Martian meteorites and in‐situ rover analyses. We use the melting model MAGMARS to reproduce these primary melts and estimate the bulk composition and temperature of the mantle regions from which they originated. We find that most mantle sources are depleted in CaO and Al2O3 relative to models of the bulk silicate Mars and likely represent melting residues or magma ocean cumulates. The concentrations of Na2O, K2O, P2O5, and TiO2 are variable and often less depleted, pointing to the re‐fertilization of the sources by fluids and low‐degree melts, or the incorporation of residual trapped melts during the crystallization of the magma ocean. The mantle potential temperatures of the sources are 1400–1500°C, regardless of the time at which they melted and within the range of the most recent predictions from thermochemical evolution models.

Angrites are silica-undersaturated achondrites formed very early in the history of the Solar System, and the most volatile-depleted known meteorites. As such, the study of angrites can provide critical insights into the early stages of planetary formation, melting and differentiation. Yet, understanding the origins of angrites and the nature of their parent body has long been hindered by the initially small number of specimens available. Here, we leverage (i) the rapidly growing number of known angrites, and (ii) equilibrium crystallization experiments at various pressure, temperature and oxygen fugacity conditions (P-T-fO(2)), to revisit the petrogenesis of angrites and constrain key features of the angrite parent body (APB), such as its composition and size. We observe that quenched (i.e., volcanic) angrites define two compositional groups, which we show are readily related by fractional crystallization. This crystallization trend converges on an olivine-clinopyrox ene-plagioclase (Ol + Cpx + Plag) multiple saturation boundary, whose composition is sampled by D'Orbigny, Sahara 99555 and NWA 1296. Using the observation that some quenched specimens represent primitive angritic melts, we derive a self-consistent bulk composition for the APB. We find that this composition matches the proposed Mg/Si ratio of 1.3 derived from the angrite delta Si-30 values, and yields a core size (18 +/- 6 wt%) in agreement with the siderophile elements depletion in the APB mantle. Our results support a primary control of nebular fractionation (i.e., partial condensation) on the composition of the APB. To establish the liquid phase equilibria of angrites, a series of 1 atmosphere and high-pressure crystallization experiments (piston cylinder and internally heated pressure vessel) were performed on a synthetic powder of D'Orbigny. The results suggest that the APB was a large (possibly Moon-sized) body, formed from materials condensed at relatively high-temperature (similar to 1300-1400 K), and whose fO(2) changed from mildly reducing (similar to IW-1.5) to relatively oxidizing (similar to IW+1 +/- 1) in the similar to 3 Myr between its core formation and the crystallization of D'Orbigny-like (Group 2) angrites. Based on its timing of accretion and differentiation, its composition, redox, and size, we argue that the APB represents the archetype of the first-generation of refractory-enriched planetesimals and embryos formed in the innermost part of the inner Solar System (<1 AU), and which accreted in the telluric planets. (C) 2022 Elsevier Ltd. All rights reserved.

Introduction The early Moon was covered by a global lunar magma ocean (LMO) whose solidification set the initial stage for the subsequent thermochemical evolution. Equilibrium solidification leads to a homogeneous initial mantle composition, while fractional solidification leads to a layered heterogeneous mantle composition. This difference is crucial for the subsequent thermochemical evolution of the lunar mantle and the amount of secondary crust produced. Estimates of the thickness of the secondary crust, which consists of Mg-suite rocks and basaltic lava flows, and the composition of these rocks from surface measurements and Apollo samples can be combined with models of the interior dynamics to gain insight into the evolution of the lunar mantle. In our study we model the solid state convection in the lunar mantle and focus on the mixing and partial melt production during convection. We consider both a homogeneous initial mantle composition, as it was used in previous studies (e.g., Ziethe et al., 2009), and a heterogeneous mantle composition that formed by fractional crystallization of the LMO. We compute the amount of partial melt and compare our results to estimates of the Moon’s secondary crust. This allows us to constrain parameters such as the initial temperature for the homogeneous case and the temperature dependence of the viscosity for the heterogeneous case. Our models can provide critical information about the location and timing of partial melt, and for the more realistic heterogeneous case, also about the components that undergo melting. Petrological modeling For the initial mantle composition in the homogeneous lunar mantle case we chose KLB-1 peridotite (Zhang and Herzberg, 1994). The solidus, liquidus and density change due to mantle depletion were calculated with alphaMELTS. The initial temperature profiles vary from a cold to an intermediate temperature following Laneuville et al. (2013) (Figure 1b). For the heterogeneous lunar mantle case we follow the approach described by Schwinger and Breuer (2021) to compute the fractional solidification of the LMO using the bulk lunar mantle composition of O‘Neil (1991). The resulting compositional structure of the mantle consists of 5 layers, for which the predominant minerals are shown in Figure 1c. For each of these layers we calculate an average density, solidus and liquidus profiles, and their changes due to mantle depletion. The initial temperature profile follows the crystallization temperatures of the cumulates (Figure 1d). Geodynamical modeling We use the mantle convection code GAIA (Hüttig et al., 2013) to model the thermochemical evolution of the lunar mantle for both the homogeneous and heterogeneous case. We solve the conservation equations of mass, linear momentum, thermal energy, and composition using the extended Boussinesq Approximation in a 2D quarter cylinder geometry. The temperature- and depth-dependent viscosity follows an Arrhenius law, and we track material properties (e.g., melting temperature, density, degree of depletion, amount of heat producing elements) employing a particle-in-cell method (Plesa et al., 2013). All simulations consider core cooling and radioactive decay. Additionally, we account for latent heat consumption during mantle melting and the increase of solidus and density changes due to mantle depletion (Breuer et al., 2018). Our models track the timing and depth of the melting events and the components that melt. Produced melt forms the secondary crust and successful models must fit the secondary crust thickness with values between 2 to 10 km. This range accounts for basaltic lava flows that comprise less than 1% of today's crust (Head, 1976) and the Mg-suite rocks that may comprise 6% to 30% (Tompkins and Pieters, 1998, Wieczorek and Zuber, 2001). Though, recent findings show that at least some rocks of the Mg-suite are impact melts (White et al., 2020). Results For the homogeneous case, our models show that a relatively cold initial potential mantle temperature of 1501 - 1547 K is required to match the secondary crust estimates of 2 to 10 km. The cold temperatures correspond to abnormally small initial magma ocean depths of only 106 - 145 km and are not able to produce early mantle melting as required to explain the oldest ages of basalts (Figure 2a). For the heterogeneous case the initial temperature profile is determined by the crystallization temperatures and is thus a fixed parameter. In this case, the IBC cumulates with their high density and low solidus temperature can significantly affect convection and subsequent partial melting. We consider a reference viscosity of 1e21 Pa s. For models with a strong temperature-dependence of the viscosity (i.e., activation energy of 300 kJ/mol) the IBC remains trapped beneath the crust, and only 1.3 km of secondary crust are produced. In contrast, if we consider a lower temperature-dependence of the viscosity (i.e., activation energy of 83 kJ/mol), then up to 61% of IBC sinks into the mantle, producing an average secondary crust thickness of up to 2.9 km (Figure 2b). Conclusion and Outlook Our coupled petrological-geodynamical models indicate that a heterogeneous mantle composition yields more comparable results to estimates of the secondary crustal thickness than a homogeneous mantle. In the heterogeneous case, at least part of the IBC layer needs to be recycled into the mantle to match the estimates, indicating a low activation energy e of the viscosity, a low reference viscosity or additional mechanisms to destabilize the IBC layer. In the homogeneous case, our models show that only cold initial temperatures can produce a secondary crust thickness comparable to the estimates - but these are not consistent with the timing of secondary crust formation. In future work, we will investigate the composition of the partial melt over time using the thermodynamic software Perple_X and compare our results to the composition of mare basalts. In addition, we plan to test the consequences of a heterogeneous shallow magma ocean on the thermochemical evolution and mantle melt production. Future missions that could return additional information about the thickness and composition of the secondary crust would greatly help to improve our numerical models and constrain the thermochemical history of the Moon. Acknowledgements I.B. and S.S. were supported by DFG SFB-TRR170, (subprojects C4 and A5).

DIFFERENTIATION OF THE UREILITE PARENT ASTEROID. C. A. Goodrich, M. Collinet, M. Jercinovic, T. Prissel, H. Tang, L. Tafla, E. Young, P. Jenniskens, and M. H. Shaddad. Lunar & Planetary Institute, USRA, Houston TX 77058 USA (goodrich@lpi.usra.edu); Institute of Planetary Research, DLR, Berlin 12489 Germany; Dept. Geosciences, Univ. Mass., Amherst MA 01003 USA; Jacobs-JSC, Houston TX 77058 USA; Dept. Earth & Planetary Sci., Univ. California, Los Angeles CA 90095 USA; SETI Institute, Mountain View, CA 94043 USA; Dept. Physics & Astronomy, Univ. Khartoum, Khartoum 11115 Sudan.

MS‐MU‐012, a 15.5 g clast from the Almahata Sitta polymict ureilite, is the first known plagioclase‐bearing main group ureilite. It is a coarse‐grained (up to 4 mm), equilibrated assemblage of 52% olivine (Fo 88), 13% orthopyroxene (Mg# 89.2, Wo 4.5), 11% augite (Mg# 90.2, Wo 37.3), and 14% plagioclase (An 68), plus minor metal and sulfide. The plagioclase grains have been secondarily remelted and internally recrystallized, but retain primary external morphologies. Melt inclusions occur in olivine. Rounded chadocrysts of olivine and orthopyroxene are enclosed in augite grains. In terms of texture, mineralogy, major and minor element mineral compositions, and oxygen isotopes, MS‐MU‐012 is virtually identical to the archetypal Hughes‐type main group ureilites, with the significant addition of primary plagioclase. We conclude that MS‐MU‐012 formed as a cumulate in a common lithologic unit with the Hughes‐type ureilites. Based on reconstructed compositions of melts trapped in olivine, orthopyroxene, and augite in the Hughes‐type samples, we infer that the parent magma of the Hughes unit originated as a late melt in the incremental melting of the ureilite parent body (UPB), near the end of the melting sequence, but was not completely extracted from the mantle like earlier melts and was emplaced in an intrusive body. MELTS calculations indicate that olivine began to crystallize at ~1260 °C, followed shortly thereafter by co‐crystallization of orthopyroxene and augite. Plagioclase began to crystallize at ~1170–1180 °C. Graphite was buoyant in the melt and became heterogeneously distributed in flotation cumulates. Residual silicate liquid was extracted from the cumulate pile and could have crystallized to form the “labradoritic melt lithology” (with plagioclase of An ~68‐35), which is partially preserved as clasts in polymict ureilites. The final equilibration temperature recorded by the Hughes unit was ~1140–1170 °C, just before catastrophic disruption of the UPB. MS‐MU‐012 provides a critical missing link in the differentiation history of this asteroid.