Equilibrium cerium isotope fractionations in cerium-bearing minerals and aqueous species are estimated using electronic structure calculations that include both nuclear volume and mass dependent effects. As with europium and uranium, the nuclear volume effect in redox reactions goes in the opposite direction from equilibrium mass-dependent fractionation for ¹⁴²Ce/¹⁴⁰Ce because of the larger nuclear charge radius of the ¹⁴²Ce nucleus. Mass-dependent effects dominate ¹³⁶Ce/¹⁴⁰Ce and ¹³⁸Ce/¹⁴⁰Ce fractionations because ¹³⁶Ce, ¹³⁸Ce, and ¹⁴⁰Ce share very similar nuclear charge radii. ¹⁴²Ce/¹⁴⁰Ce is predicted to be lower in most Ce(IV)-bearing species than in coexisting Ce(III)-bearing species, particularly at high temperatures. However, species with Ce:Zr substitution, such as zirconium-rich compositions along the stetindite-zircon (CeSiO₄-ZrSiO₄) solid solution series and a model Ce-subsituted baddeleyite (CeZr₃O₈), may show higher ¹⁴²Ce/¹⁴⁰Ce at ambient to low-T metamorphic temperatures because of higher effective force constants acting on the smaller, more snug substitution sites. Ce(III)P(V) charge-coupled substitution into zircon is likewise associated with high ¹⁴²Ce/¹⁴⁰Ce relative to other Ce(III) species. ¹³⁶Ce/¹⁴⁰Ce and ¹³⁸Ce/¹⁴⁰Ce fractionations will tend to favor more massive isotopes in Ce(IV)-bearing species, by 0.1–1.0‰ at 25°C and 0.01–0.1‰ at 727°C depending on the species present. The models predict ~0.3‰ higher ¹⁴²Ce/¹⁴⁰Ce in Ce(III)-bearing solution than coexisting Ce(IV)-solids at ambient temperatures, roughly agreeing with measurements. Zircon in equilibrium with typical silicate melts is predicted to be slightly enriched in ¹⁴⁰Ce relative to ¹⁴²Ce, ¹³⁸Ce, and ¹³⁶Ce. Supplementary calculations based on ¹⁴¹Pr-Mössbauer spectroscopy literature suggest somewhat (~1/3) smaller nuclear volume fractionation effects than the electronic structure models.

While the process of spheroidal weathering in silicate rocks has been studied intensively, much less is known about this process in carbonate rocks. Here we report an unusual spheroidal weathering phenomenon within terra rossa overlying the dolomitic limestone bedrock of the Yunnan-Guizhou Plateau in southwestern China. The spheroidal weathering products are characterized by iron shells. Restoration of the protolith reveals that the parent rock of the spheroidal weathering products is silty carbonate rock, rather than pure carbonate rock. Our results also show that the formation conditions of the spheroidal weathering include the following: (1) the outer iron shell supports the weathered rock body and prevents collapsing; (2) the silty carbonate rock contains sufficient silicate components to form the matrix and framework of the spheroidal weathering products; and (3) the inward diffusion of external Fe(II) promotes rindlet formation. The inward diffusion of Fe(II) and the associated ferrolysis weathering are responsible for the quasi-synchronous Fe-(hydr)oxide precipitation and dissolution of carbonate minerals. Regarding hydrogeological conditions, spheroidal weathering may occur in a paleo-perched saturated zone or in zone of fluctuating paleo-phreatic water in the epikarst zone. This redox transition zone facilitates sustainable ferrolysis weathering and rindlet formation. In general, the development of spheroidal weathering patterns comprises four stages: (1) development of incipient joints and fractures, (2) formation of an initial outer iron shell, (3) early rindlet development, and (4) a late karstification process (with inactive spheroidal weathering).

Groundwater and river water are essential in geochemical transportation from land to sea. Freshwater discharge to the coastal sea as groundwater sometimes has large geochemical fluxes comparable to river water; however, it is more challenging to monitor groundwater than river water. In this study, we assessed the carbon cycle including an underground system in a highly porous coastal area along Mt. Chokai, northern Japan, where abundant submarine spring water emerges. Groundwater and river water chemistries are generally characterized by silicate weathering, reflecting the andesitic lava that occupies the basin. Groundwater discharge was determined using mass balance calculations, including precipitation, river water discharge, and evapotranspiration. Considering the underground dissolved bicarbonate flux from land to ocean, the CO₂ consumption by silicate weathering in the western foothills of Mt. Chokai is calculated to be 8.8 ± 0.15 t-C/km²/yr, which is significantly larger than the case where only river water flux was evaluated (4.7 ± 0.25 t-C/km²/yr). Therefore, considering underground flux when estimating the amount of chemical weathering in coastal watersheds is crucial.

Manganese (Mn) is a crucial element in global metallic cycling owing to its redox-sensitive dynamics; however, determining the redox state of abundant Mn-microparticles in deep-sea oxic sediments is challenging because of high-energy beam irradiation sample processing, such as focused-ion-beam and/or electron beam processing. Our study demonstrates an improved sample preparation method for accurately determining the redox state of Mn-microparticles in deep-sea oxic sediments. We could enrich Mn-microparticles without altering their redox state by employing a novel beam-free approach, as confirmed by scanning transmission X-ray microscopy (STXM) analyses. Our methodological advancements revealed that the Mn-microparticles predominantly existed in the Mn⁴⁺ oxidation state. The new technique, which allows us to accurately determine the redox state of Mn in these particles, provides critical insights into the geochemical processes of Mn oxide precipitation and the environmental conditions that favor its formation and preservation, thereby highlighting the potential of these particles as indicators of oxic conditions in geological environments.

We measured Raman spectra of calcite at various exposure times using a spectrometer with two gratings (1800 and 1200 grooves/mm) to evaluate the precision of the relative ratio of Raman peaks (¹²C¹⁶O¹⁶O¹⁸O/¹²C¹⁶O₃), which is attributed to ¹⁸O/¹⁶O. Preliminary calculations showed that the uncertainty in the Raman peak ratio is a function of the Raman peak intensity and the pixel resolution. The relation between the Raman peak ratio and the exposure time obtained with two gratings showed that the precisions of both the intensity and area ratio of the Raman peaks improved with increasing exposure time for both gratings, which is consistent with the calculations. However, for exposure time greater than approximately 90 s, the rate of improvement in precision with increasing exposure time decreased for both gratings. Continuous monitoring of the Raman peak ratio for more than 60 hr revealed that the decrease in precision improvement can be attributed to external factors such as room temperature fluctuation. Nevertheless, we obtained the best precision values of 2.69 ± 0.84‰ and 3.26 ± 0.74‰, respectively, for the ¹²C¹⁶O¹⁶O¹⁸O/¹²C¹⁶O₃ intensity and area ratio when measuring using a 1200 grooves/mm grating and 900 s exposure time. Those findings are 70 times better than the best precision achieved to date. They might be applicable to studies using natural calcites with large oxygen isotopic variation.

Minerals formed in high-temperature environments are distributed along faults on Yonaguni Island. Volcanic activity is thought to be a possible heat source. In this study, samples from the carbonate veins distributed around the basement rocks and faults of Yonaguni Island were melted in vacuum, and the concentrations and isotope ratios of the noble gases in the furnace were investigated. The observed isotope ratios suggest that the noble gases were dominated by crustal helium, with little influence from mantle helium. These results suggest that there was some kind of non-volcanic heat supply at Yonaguni Island (e.g., eruption of non-volcanic hot springs).

Organic compounds such as amino acids may have been delivered to the early Earth via carbonaceous chondrites and interplanetary dust particles. Mineralogical and petrological evidence has shown that liquid water was formed from water ice in the early stages of the formation of the parent bodies of carbonaceous chondrites, due to the heat from the radioactive decay of short-lived radionuclides such as ²⁶Al. In previous research, amino acid precursors were produced from aqueous solutions containing formaldehyde and ammonia by hydrothermal experiments and gamma-ray irradiation experiments to evaluate organic formation during aqueous alteration processes in meteorite parent bodies. However, the differences in the effects and reactions of gamma rays and heating, as well as the synergetic effects, are not well understood. In this study, both heating and gamma irradiation were applied to aqueous solutions containing formaldehyde, methanol, and ammonia to investigate the synergistic effects of heating and gamma rays on amino acid formation. The results showed that gamma irradiation followed by heating was more efficient in producing amino acid precursors compared to heating followed by gamma irradiation. The characteristic features of UV-visible and fluorescence spectra of the experimental products were consistent with Maillard-type reactions, suggesting that Maillard-type reactions led to the formation of amino acid precursors.