¹J ZhangZhou,²Yuan Li,³Proteek Chowdhury,⁴Sayan Sen,^5,6Urmi Ghosh,²Zheng Xu,⁷Jingao Liu,⁸Zaicong Wang,⁹James M.D. Day
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.11.029]
¹Key Laboratory of Geoscience Big Data and Deep Resource of Zhejiang Province, School of Earth Sciences, Zhejiang University, Hangzhou, China
²State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
³Earth, Environment and Planetary Sciences, Rice University, TX 77005, USA
⁴Zuckerberg Institute for Water Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion 8499000, Israel
⁵Environmental and Biochemical Sciences, The James Hutton Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
⁶Department of Geology and Geophysics, Indian Institute of Technology (IIT) Kharagpur, 721302 Kharagpur, India
⁷State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing), Beijing, China
⁸State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
⁹Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA
Copyright Elsevier

The sulfur content at sulfide saturation (SCSS) of a silicate melt can regulate the stability of sulfides and, therefore, chalcophile elements’ behaviors in planetary magma oceans. Many studies have reported high-pressure experiments to determine SCSS using either linear or exponential regressions to parameterize the thermodynamics of the system. Although these empirical equations describe the effects of different parameters on SCSS, they perform poorly when predicting laboratory measurements. Here, we compiled 542 published analyses of experiments performed on a range of sulfide and silicate compositions at varying P–T conditions (<24 GPa, <2673 K). Using empirical equations, linear regression, Random Forest algorithms, and a hybrid algorithm employing empirical fits to P–T conditions and the Random Forest algorithm for compositions, we developed several SCSS models and compared them to laboratory measurements. The Random Forest and hybrid models (R² = 0.82–0.91, mean average error [MAE] < 746 ppmw S, residual mean standard error [RMSE] < 972 ppmw S), significantly outperform previous empirical models (R² = 0.28–0.69, MAE = 622–1,170 ppmw S, RMSE = 1,070–1,744 ppmw S), whereas linear regression performs moderately well, i.e., between the classic and machine learning models. We applied our hybrid model to predict SCSS during magma ocean solidification on Earth, Mars, and the Moon, and we compared our model results to expected S contents in the residual magma oceans calculated by mass balance. Our results confirm that during early accretion, sulfides precipitated from magma oceans and into the outer cores of Earth and Mars, but not the Moon. Subsequently, once the respective magma oceans began precipitating minerals with increasingly FeO-rich and SiO₂-, Al₂O₃-, and MgO-depleted compositions, the increasing S concentration in the residual magma was offset by temperature and compositional effects on SCSS, preventing sulfide precipitation during intermediate stages of crystallization. Sulfides precipitated late during magma ocean crystallization, but failed to percolate through the underlying crystalline mantle, significantly contributing to the modern bulk-silicate sulfur abundances of Earth, Mars, and the Moon. Our calculations suggest that late-stage sulfide precipitation occurred at shallow depths of 120–220 km, 40–320 km, and <10 km in the magma oceans of Earth, Mars, and the Moon, respectively.

^1,2F. Jourdan,^1,2T. Kennedy,²L. Foreman,¹C. Mayers,³E. Eroglu,^4,5A. Yamaguchi
Geoochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2023.11.027]
¹Western Australian Argon Isotope Facility, John de Laeter Centre, TIGeR, Curtin University, Australia
²Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Australia
³School of Molecular and Life Sciences, Curtin University, Australia
⁴National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
⁵Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo 190-8518, Japan
Copyright Elsevier

In this study, we investigate the 40Ar/39Ar systematics of nineteen diogenites thought to come from deep crustal levels of asteroid 4 Vesta. We applied both Electron Backscattered Diffraction (EBSD) and 40Ar/39Ar and methods to the unbrecciated diogenite LAP 031381. We obtained three plateau ages resulting in a combined weighted mean age of 4441 ± 15 Ma (P = 0.16). The EBSD analyses suggest that LAP 031381 displays minimal evidence of shock and, when combined with petrography observations, diffusion modelling and 40Ar/39Ar data, these results suggest that the crustal volume that initially contained this diogenite, reached a temperature of ca. 630 °C at ∼ 4.44 Ga. This corresponds to a linear cooling rate of ∼ 5 °C / Ma for a crystallization age of 4550 Ma. Independent thermal models suggest that these conditions were present at a depth of 60 to 65 km at 4.44 Ga.

The other eighteen diogenites yielded 40Ar/39Ar results that indicate that they have been variously shocked by impact events and seven of them yielded plateau ages ranging from 2413 ± 189 Ma to 84 ± 162 Ma. We combined these results with 40Ar/39Ar ages from eucrites and howardites and propose that the HED (Howardite, Eucrite, Diogenite) meteorites recorded impact events at the surface of Vesta until ∼ 3.4 Ga when they were then ejected during a large collision. The eucrites, diogenites and howardites were then recombined into small rubble pile asteroids which probably make up a large part of the Vestoid family. After ejection, the K/Ar system in plagioclase crystals ceased in most cases to be fully reset by impact events as the temperature spikes reached during small impacts lack enough energy to trigger significant 40Ar* diffusion. On the other hand, ultra-transient and high-temperature – sensitive pyroxene crystals kept a more systematic record of small impacts until recent time. 38Arc cosmochron cosmogenic exposure ages on diogenites mostly range from 51 ± 7 Ma to 0 ± 1 Ma and when combined with other HED cosmochron ages, suggest that almost all the HED meteorites were continuously ejected from secondary rubble pile asteroids mostly between 50 Ma and present.