^1,2Xiaofeng Lu, ³Olivier Namur, ¹Yongjiang Xu, ⁴Bernard Charlier, ¹Yanhao Lin
Earh and Planetary Science Letters 690, 120123 Open Access Link to Article [https://doi.org/10.1016/j.epsl.2026.120123]
¹Lin Earth and Planetary Laboratory, Center for High pressure Science and Technology Advanced Research, Beijing, 100193, China
²School of Earth Sciences and Resources, China University of Geosciences, Beijing, 100083, China
³Earth and Environmental Sciences, KU Leuven, 3001, Leuven, Belgium
⁴Department of Geology, University of Liège, 4000, Sart Tilman, Belgium
Copyright Elsevier

Chemical data from the MESSENGER spacecraft reveal that Mercury’s lavas are unusually sulfur-rich, suggesting highly reduced conditions during their formation. As a major volatile, sulfur profoundly affects the physical and chemical properties of silicate melts, potentially impacting key processes such as magma ocean crystallization and mantle melting. Here we conducted near-liquidus experiments (1650–2000 °C and 3–5 GPa) to quantify the effect of sulfur on phase relations in olivine- and orthopyroxene-saturated mafic compositions representative of Mercury’s mantle. Our results show that elevated sulfur contents (up to 6 wt.% S) can depress the liquidus of Mercurian mantle by up to 200 °C. The liquidus depression of silicate melt is positively correlated with sulfur concentrations, negatively correlated with pressure, and compositionally-sensitive with a larger S-effect on higher Mg/Si melt. Using a newly developed parameterization for sulfur-bearing melting, we show that the mantle potential temperatures required to produce the volcanic provinces are lower than previously estimated. Furthermore, modeling of magma ocean cooling and crystallization dynamics indicates that sulfur-induced liquidus depression can extend solidification timescales by tens to hundreds to thousands of years, especially beneath an insulating graphite crust (>100 m). Using viscosity models, we find that sulfur also reduces the critical crystal size for settling, thus promoting fractional crystallization and formation of a chemically stratified mantle. These findings provide critical constraints on Mercury’s interior structure and show that sulfur lowers the solidus and liquidus, enhances melt production, and helps to explain Mercury’s fertile mantle, extensive crustal formation, and diverse surface lavas.

¹Ioana-Bogdana Radu, ¹Cécile Deligny, ¹Henrik Skogby, ²Roland Stalder, ¹Jeremy J. Bellucci, ¹Martin J. Whitehouse
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [10.1016/j.gca.2026.06.002]
¹Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
²Institut für Mineralogie und Petrographie, Universität Innsbruck, Innsbruck 6020, Austria
Copyright Elsevier

Water on Mars has traditionally been estimated from hydrogen contents in melt inclusions and apatite, yet these may not account for magma degassing or post-crystallization dehydration. Pyroxenes offer an alternative approach, as they incorporate trace amounts of water during crystallisation via charge-balancing structural defects, that are retained after dehydration enabling experimentally reversing water loss and constraining magmatic water contents. Here we report the first hydrothermal rehydration experiments on pyroxene from both nakhlites and shergottites. The treated nakhlite augites contain 140‒185 ppm H2O, consistent with previous values for Nakhla (130 ± 26 ppm) and within the range of terrestrial basaltic pyroxene. Using clinopyroxene-melt partition coefficients, this corresponds to 1.59 ± 0.03‒1.83 ± 0.10 wt% average H2O in the nakhlite magma, slightly higher than previous estimates (0.69‒1.42 wt% H2O). The complex hydrothermal history of nakhlites, including evidence for magmatic degassing and interaction with H2O-poor, Cl-rich fluids, suggests these estimates may represent minimum values of the nakhlite magmatic water content. Assuming a low degree of partial melting, the nakhlite mantle source is expected to contain 80‒91 ppm H2O, overlapping previous estimates (59‒184 ppm), and comparable to Earth’s MORB mantle (54‒330 ppm). This is consistent with a common magmatic source for all nakhlites, and broadly consistent with water estimates for the chassignite mantle source (39‒252 ppm), suggesting that any exogenous fluid assimilation had a negligible effect on the net water budget. In contrast, shergottite pigeonites show no detectable water by Fourier Transform Infrared Spectroscopy, consistent with expected water contents of ∼ 2‒11 ppm H2O derived from published source compositions for depleted and enriched shergottites. Together, these results refine our understanding of Martian magmatic water content, consistent with a common, more water-rich mantle source for nakhlites, distinct from the more heterogeneous and generally drier shergottite mantle reservoirs