Close to a thousand presolar silicate grains have been identified since their initial discovery in interplanetary dust particles (IDPs) just over ten years ago. Studies have shown that silicates are the most abundant type of presolar grain other than nanodiamonds, with abundances of ~200 ppm in the most primitive meteorites and upwards of ~400 ppm in anhydrous IDPs. The oxygen isotopic compositions of presolar silicates are similar to those of presolar oxides, with the majority of the grains originating in low-mass red giant or asymptotic giant branch stars of close-to-solar metallicity. The vast majority of the grains are ferromagnesian silicates with high Fe concentrations. This, together with TEM studies indicating that many presolar silicates have amorphous structures with heterogeneous and non-stoichiometric compositions, suggests that conditions in the stellar environments in which these grains formed were variable and rapidly changing, with grain condensation under non-equilibrium kinetic conditions. Presolar silicates also reflect secondary processes taking place in the solar nebula and the parent bodies of the meteorites in which they are found. Abundance variations within individual meteorites provide constraints on secondary processes, and both thermal metamorphism and aqueous alteration result in changes to the elemental compositions of the grains. Studies of presolar silicates complement those of other presolar grain types, providing additional constraints on stellar environments and nucleosynthetic processes.
The presence of water on Earth has played important roles in shaping the solid regions of the planet as well as in the origin and evolution of life. This paper addresses three fundamental aspects of Earth’s water; (1) the quantity of water on the surface and in the interior that Earth possesses, (2) the length of time that surface oceans have been present, and (3) the mechanism(s) by which this water was supplied or generated. From geochemical and geophysical analysis, and high-pressure experimental works, the water content in the Earth’s mantle can be estimated to be from one to ten times the present ocean mass. Although it is difficult to estimate the water content in the Earth’s core, recent high-pressure experimental work indicates copious amounts of hydrogen in the core. From geological and geochemical evidence, the Earth’s surface oceans appear to have existed since very early in the Earth’s history, perhaps even since the Earth’s formation. However, changes in the ocean volume throughout the Earth’s history have not been well determined. Several possible water sources and supply mechanisms have been proposed, in association with theories regarding planet formation in our solar system. Since there are several uncertainties concerning the process of planet formation, the origin of the Earth’s water is still in question.
Unlike most terrestrial rocks, some meteorites and their mineral components show isotope compositions that are clearly associated with nucleosynthetic processes in stars other than the Sun. Although isotope anomalies of nucleosynthetic origin have been an active research area of isotope cosmochemistry for the past few decades, only recently have planetary-scale isotope anomalies been discovered and confirmed for elements other than the noble gases. The discovery of isotope anomalies of nucleosynthetic origin, especially at the planetary scale, provides new constraints on the evolution of the young Solar System. They are the most direct evidence for tracing the origin of extraterrestrial samples, understanding their genetic relationships, and deciphering the stellar environment of Solar System formation. In this paper, we review the progress that has been made in the field of nucleosynthetic isotope anomalies and their cosmochemical significance.
Remote sensing data from orbiter missions have proposed that ground ice may currently exist on Mars, although the volume is still uncertain. Recent analyses of Martian meteorites have suggested that the water reservoirs have at least three distinct hydrogen isotope compositions (D/H ratios): primordial and high D/H ratios, which are approximately the same and six times that of ocean water on Earth, respectively, and a newly identified intermediate D/H ratio, which is approximately two to three times higher than that in ocean water on Earth. We calculate the evolution of the D/H ratios and the volumes of the water reservoirs on Mars by modeling the exchange of hydrogen isotopes between multiple water reservoirs and the atmospheric escape. The D/H ratio is slightly higher in the topmost thin surface-ice layer than that in the atmosphere because of isotopic fractionation by sublimation, whereas the water-ice reservoir just below the exchangeable topmost surface layer retains the intermediate D/H signature found in Martian meteorites. We propose two possible models for constraining the volume of the ground ice considering the observed D/H ratios and geomorphological estimates of Paleo-oceans. The first assumes that the atmospheric loss is dominated by the Jeans escape. In this case, the volume of ground ice should be larger than the total volume of the observable surface ice that mainly occurs as polar layered ice deposits. The other model assumes diffusion-limited atmospheric loss in which the interactive evolution of the multiple water reservoirs naturally accounts for the observed D/H ratios. In this scenario, a large volume of ground ice does not necessarily exist currently on Mars as opposed to the perspective view proposed on the basis of recent orbiter missions.
In order to understand the influence of coal-forming materials and slight differences in coal maceral composition on the isotopic composition of thermogenic coalbed gas, closed-system isothermal pyrolysis experiments were performed on forest and herbaceous swamp peats and on coals with slight differences in maceral compositions. The carbon and hydrogen isotopic compositions of the hydrocarbon gases (methane, ethane and propane) generated during the pyrolysis of the samples were determined. The results showed that the carbon and hydrogen isotopic compositions of the hydrocarbon gases generated from the herbaceous swamp peat were lighter than those generated from the forest swamp peat. At pyrolysis intervals from peat to vitrinite reflectance values (Ro) of 2.5%, 3.5% and 5.5%, this difference in the average δD value of the generated methane was 18‰, 11‰ and 9‰, and in the δ13C value of methane was 3.4‰, 3.8‰ and 1.8‰, respectively. The reason for the difference of trend can be explained as follows. The methane generated from the coal containing slightly higher exinite content had noticeably lighter carbon isotopic composition. However, the hydrogen isotopic composition of the generated methane was primarily related to coal-forming environmental factors, such as latitude, δD value of rainwater, and diagenetic fluid of freshwater or seawater. These results provide a basis for studying the isotopic geochemistry of coalbed gas.
Fifteen water and gas samples from hot spring sites in Chungcheong Province, South Korea, were analyzed to determine their chemical compositions and noble gas isotopic ratios. These hot spring water samples were grouped into three chemical types: Ca-HCO3, Ca(Na)-HCO3, and Na-HCO3. The wide range of 3He/4He ratios in the hot spring waters (0.036–1.76 × 10–6; R/Ra = 0.026–1.26) indicates that the He originates from both mantle and crustal sources in the forms of 3He and 4He, respectively, with 4He dominating. The 4He/20Ne isotopic ratios and the concentrations of mantle-derived 3He suggest that geothermal waters originate from a common, deep-seated source containing mantle-derived and crustal He that mixes with shallow groundwater containing atmospheric noble gases as it rises to the surface.