Exploring Pyroxene-Rich Planetary Mantles: Experimental Insights from CB Chondrites

Under the reducing conditions of the inner region of stellar nebulae (Cartier and Wood, 2019), thermodynamic calculations have shown that rocky planetary mantles could be dominated by pyroxenes instead of olivine (Cioria et al., 2024). This mineralogical composition would have geodynamic implications such as low mantle viscosity and reduced solidus temperature compared to peridotitic mantles, as on Earth and Venus. In this preliminary work, we perform high pressure- temperature experiments in a Paris-Edinburgh press to investigate the subsolidus and melting phase relations of CB (Bencubbin like) bulk silicate composition at conditions corresponding to the crust- mantle interface and core-mantle boundary in Mercury. The objective is also to experimentally constrain and anchor the previous thermodynamic calculations of Cioria et al. (2024), focusing particularly on the mineral constituents of the crusts and mantles of reduced bodies.

Synthetic samples (CB1, CB2) have been prepared using the average bulk silicate fraction of CB chondrites as proposed by Malavergne et al. (2010) and Brown and Elkins-Tanton (2008). Starting powders were prepared from high purity oxide mixtures, according to Cioria et al., 2024 Model 2, with 67.9 wt% SiO₂, 0.2 wt% TiO₂, 3.1 wt% Al₂O₃, 1.1 wt% Cr₂O₃, 0.3 wt% FeO, 0.1 wt% MnO, 25 wt% MgO, 2.6 wt% CaO, 0.1 wt% Na₂O and 0.01 wt% K₂O. Paris-Edinburgh press experiments were conducted in standard 10/3.5mm (sample CB1) and 7/2.4 mm (sample CB2) cell assemblies, using boron epoxy transmitting medium and graphite furnace heaters. Powders were loaded into a double capsule with a graphite inner wall and boron nitride outer wall. Samples were subjected to 2 GPa–1300K (CB1) and 5 GPa–1700K (CB2), for durations ranging from 3hours to 24 hours, respectively; in order to achieve equilibrium conditions. These P-T ranges are the same used in Cioria et al. (2024) as boundary conditions in thermodynamic modelling. Recovered charges were mounted, polished, and optical observations shown that run-products have coarse-grained texture, where large, euhedral crystals are embedded in a fine-grained matrix. The detailed analysis is under progress and scanning electron microscopy (SEM) are planned to characterize textures, phase assemblages, major element distributions, and constrain oxygen fugacity conditions - fO₂ - using relevant phase equilibria buffers. Results will be compared with thermodynamic models (Cioria et al., 2024) and experimental studies by Berthet et al., (2009) and Boujibaar et al. (2025), to understand any differences between products derived from CB and EH compositions, respectively. Further works will involve synthesis under larger P-T conditions as well as investigating sulfur-bearing compositions under various oxygen fugacity conditions in order to assess the building materials and differentiation processes of Mercury-like planets and small bodies in the early solar system.

Acknowledgments

G.M. and C.C. acknowledge support from the Italian Space Agency (2022-16-HH.1-2024). 

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