Spacecraft-returned samples from the C-type asteroid (162173) Ryugu and the B-type asteroid (101955) Bennu record various processes from presolar chemistry and early-stage aqueous alteration processes on their parent planetesimals to ongoing geological processes on their surfaces. Hydrous magnesium phosphate is a common phosphate in Bennu, whereas it is present but rare in Ryugu. Because phosphates in both asteroids formed during aqueous alteration on their parent planetesimals, the difference in the abundance of hydrous magnesium phosphate indicates that the two asteroids experienced different aqueous chemistries. Hydrous magnesium phosphate grains in both asteroid samples show varying degrees of dehydration, which may have been resulted from heating at later evolutionary stages, such as solar heating in their Earth-crossing orbits. This study aims to elucidate the dehydration behavior of hydrous magnesium phosphate on near-Earth asteroids, we conducted kinetic dehydration experiments on MgHPO₄·3H₂O (newberyite), a likely precursor mineral phase before dehydration, under low-pressure conditions (~200 and ~10^–4 Pa). Dehydration of MgHPO₄·3H₂O occurs efficiently at lower temperatures under pressures lower than at 1 atm, likely due to the more effective escape of H₂O molecules from the sample. The dehydration reaction stalled at different extents of dehydration depending on temperature, suggesting that the reaction rate decreases significantly as dehydration progresses. We demonstrate that this reaction stall in the experiments reflects an increase in the activation energy for dehydration caused by a decrease in the coordination number of Mg atoms as H₂O molecules detach. The kinetic dehydration model developed in this study successfully reproduces the temporal evolution of the degree of dehydration at each temperature ranging from 50 to 300°C. The dehydration model suggests that hydrous magnesium phosphates on the surfaces of Ryugu and Bennu would experience only partial dehydration under their current orbits over their dynamical lifetimes as near-Earth asteroids. The model also suggests that the samples from Ryugu and Bennu have never been heated above 190°C—the radiation equilibrium temperature in a Venus-crossing orbit—which could place a constraint on their orbital evolution after migration from the main asteroid belt.

Iron exhibits variable redox states between Fe²⁺ and Fe³⁺, governing key processes from Earth’s deep interior to surface environments. Quantitative evaluation of Fe valence is therefore essential in geoscience; however, partial fluorescence yield XANES (PFY-XANES), a widely used nondestructive technique, often suffers from thickness or self-absorption effect in samples with high Fe concentrations. In this study, we applied the inverse PFY (IPFY) method, to geological samples to assess its reliability in Fe valence determination. Iron L_III-edge IPFY-XANES and conventional PFY-XANES analyses were performed on olivine, wadsleyite, bridgmanite, and two biotite samples with FeO contents of 6.6–37.7 wt.%. Conventional PFY-XANES overestimated Fe³⁺/ΣFe by approximately 10% relative to Mössbauer spectroscopy, whereas IPFY-XANES yielded values differing by less than 3%, showing excellent agreement. The inverse spectra also exhibited reduced pre- and post-edge tailing, confirming suppression of thickness effects. These results demonstrate that IPFY-XANES provides more accurate determination of the valence state of Fe, particularly for Fe-rich or thick samples. Because many geoscientific specimens such as asteroidal return samples, meteorites, or recovered high-pressure samples cannot be destructively prepared, the IPFY approach offers a powerful and reliable method for nondestructive Fe valence analysis across a wide range of natural materials.

Theoretical estimates of equilibrium platinum-isotope (¹⁹⁸Pt/¹⁹⁴Pt) fractionation including both the nuclear volume component of the field shift effect and mass dependent fractionation are reported. Field shift (nuclear volume) fractionations in the related elements iridium, osmium, and ruthenium are also reported. For Os, Ir, and Pt, field shift fractionation is predicted to be no more than ~0.1–0.2‰ per amu at geochemically relevant temperatures, with mass-dependent fractionation in Pt-bearing species having a larger range at 25°C (up to 0.6‰ per amu for Pt^IV-oxides relative to Pt⁰ and Pt^II). However, the mass-dependent component decreases more rapidly at higher temperatures, scaling in proportion to 1/T² while the field shift component scales as 1/T. Field shift fractionations in the other platinum group elements are qualitatively similar, but typically smaller on a per amu basis. Field shift fractionation for all elements studied show a pattern of isotopes with larger charge volumes (more massive isotopes, for these elements) being most concentrated in species with the smallest positive oxidation states studied (Pt^II, Ir^III, Ru^IV, and Os^IV), and less concentrated in species with higher oxidation states (Pt^IV, Ir^IV, Ru^{VI to VIII}, and Os^{VI to VIII}). Field shift fractionations of platinum, osmium, and iridium isotopes in metals are dependent on alloy composition, favoring smaller (less massive) isotopes in iron-rich compositions. In contrast, mass dependent fractionation tends to concentrate massive isotopes in species with high oxidation states, and shows little sensitivity to alloy composition. A hypothetical Pt-substituted olivine is predicted to show total equilibrium ¹⁹⁸Pt/¹⁹⁴Pt fractionation of approximately +0.1‰ relative to the iron-rich alloy Fe₇Pt at core-mantle differentiation temperatures near 2000 K, and +0.06‰ near 3000 K. While it is unclear which modeled species (if any) is the best analog for platinum in a silicate melt, Pt-substituted olivine, native platinum, and solid PtS are all capable of producing an isotopically heavy pre-late-veneer mantle. Only a Rayleigh-type Pt-olivine vs. Fe₇Pt model is able to approach the previously observed 0.4–0.6‰ fractionation between chondrites and the early Archean mantle.