Ko Hashizume

Oxygen Isotopic Compositions of Asteroidal Materials Returned from Itokawa by the Hayabusa Mission

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in 16O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.

Nitrogen and argon signatures in 3.8 to 2.8 Ga metasediments: Clues on the chemical state of the Archean ocean and the deep biosphere

Abstract N and Ar elemental and isotopic analyses were conducted on Archean metasediments of Isukasia, West Greenland and Pilbara Craton, Western Australia, in order to investigate the N isotopic evolution during the first half of Earth’s history. The selected samples are deep-sea sediments and hydrothermal deposits having ages from 3.8 to 2.8 Ga and affected by different degrees of metamorphism. The release patterns of N and Ar obtained by high-resolution stepped combustion show the occurrence of at least two trapped components. The first is released at 600°C and it is likely contained in fluid inclusions. N is released together with primordial 36Ar and shows a δ15N value of −1.3 ± 1.0‰, close to that of modern atmospheric N2 (δ15N = 0‰). This component is well preserved in hydrothermal-vent silica deposits of North Pole, Pilbara Craton, and nitrogen may represent ammonium salt dissolved in deep-sea hydrothermal fluids. The second N component, released at temperatures higher than 1000°C, is accompanied by radiogenic 40Ar∗, and shows a δ15N value of −7.4 ± 1.0‰ in a kerogen-rich chert from North Pole, Pilbara Craton. This N is likely biogenic and negative 15N values may reflect a metabolic isotopic fractionation induced by chemosynthetic bacteria using inorganic NH4+ contained in hydrothermal fluids. This 15N-depleted biogenic component may occur in Isukasia Banded Iron Formation (δ15N ∼ −1.7‰), but further data are needed to confirm such a hypothesis. In all other samples, metamorphic-induced Rayleigh distillation has altered the pristine N isotopic signature.

PROTOSOLAR CARBON ISOTOPIC COMPOSITION: IMPLICATIONS FOR THE ORIGIN OF METEORITIC ORGANICS

New ion probe isotopic measurements of carbon trapped within the 50 nm thick surface layer of lunar regolith grains strongly suggest that solar wind C is depleted in 13C by at least 10% relative to terrestrial C. In order to account for the general 13C enrichment of planetary C relative to solar C, we propose that the main carriers of C in these objects, i.e., organics, were formed in an environment that allowed a strong isotopic enrichment of 13C in the solid phase. Such an environment is most likely a dense and warm circumstellar or interstellar gas medium, which could well correspond to the nebula surrounding the proto-Sun, where isotopic fractionation could be triggered by photochemical reactions.

Nitrogen Isotopes on the Moon: Archives of the Solar and Planetary Contributions to the Inner Solar System

The two isotopes of nitrogen, 14N and 15N, have relative abundances extremely variable among solar system reservoirs such as planets and their atmospheres, primitive and differentiated meteorites, and comets. Expressed in the delta notation (o 15N = [[ 15N/14Nlsample/l 15N/14N]standardI) x 1000, in parts per mil, or %0, where the standard is atmospheric N having 15N114N = 0.003676), o15N ranges from -250%0 (the lower limit of lunar soil values) up to 1600%0 (measured in the meteorites benccubinites). The lunar surface constitutes a unique archive of the past corpuscular (solar and meteoritic) contributions to planetary surfaces. Nitrogen trapped in the lunar regolith presents a highly variable isotopic composition, which represents either secular variation of the solar wind composition although this possibility conflicts with the apparent isotope stability over time of other solar wind volatile elements, or more likely different contributions from solar corpuscular radiation and non-solar sources. In this case, the solar nitrogen component is depleted by more than 24% in 15N, whereas non-solar, planetary sources (meteorites, micrometeorites, possibly comets) are enriched in the heavy isotope of nitrogen by :::: I0% on average. Variations in the nitrogen isotopic composition oflunar soils are explained by a secular change in the strength of the planetary flux, and a correlation between N isotopic compositions and surface exposure age for different soils suggest that the planetary contribution to the inner solar system might have increased in the last 0.4 Gy. The variability of the N isotope composition among solar system objects might be due to incomplete equilibration of nitrogen isotopes from different host phases of pre-solar origin. Alternatively, it could result from mixing between 15N-depleted protosolar nitrogen originally present in the gas and presolar solid (organic?) compounds enriched in 15N.

Nitrogen Isotopic Analyses at the Sub-Picomole Level Using an Ultralow Blank Laser Extraction Technique

Publisher Summary This chapter presents a new analytical procedure aimed to measure the isotopic composition of sub-picomole quantities of nitrogen. Two major points have been improved with respect to standard analytical procedures, the level of the hot blank and that of interfering species, especially that of N 2 H. The hot blank is lowered by minimizing the hot area during the extraction procedure using a defocused laser beam as a heating device. The amount of the interfering species N 2 H arising from the mass-spectrometer ion source was lowered by optimizing the ion source condition. The aim of this development is to enable isotopic analysis of single mineral grains using the smallest quantity of nitrogen with a precision sufficient to resolve isotopic variations of nitrogen in extraterrestrial samples. There are certain limiting factors also. Hydrocarbon is mass-resolved with mass-spectrometer. Isotopic analyses of sub-picomole quantities of N 2 with a precision typically of + 10% is done. The required amount of samples for nitrogen isotopic analyses, in the case of the lunar regolith, is reduced by a factor of ∼10 -5 . Hence, mass-spectrometry system performs nitrogen isotopic analyses within limits imposed only by the counting statistics.

Biogenic nitrogen and carbon in Fe-Mn-oxyhydroxides from an Archean chert, Marble Bar, Western Australia

To quantify and localize nitrogen (N) and carbon (C) in Archean rocks from the Marble Bar formation, Western Australia, and to gain insights on their origin and potential biogenicity, we conducted nuclear reaction analyses (NRA) and carbon and nitrogen isotope ratio measurements on various samples from the 3460-Myr-old Fe-rich Marble Bar chert. The Marble Bar chert formed during the alteration of basaltic volcanoclastic rocks with Fe- and Si-rich hydrothermal fluids, and the subsequent precipitation of magnetite, carbonates, massive silica, and, locally, sulfides. At a later stage, the magnetite, sulfides, and carbonates were replaced by Fe-Mn-oxyhydroxides. Nuclear reaction analyses indicate that most of the N and C resides within these Fe-Mn-oxyhydroxides, but a minor fraction is found in K-feldspars and Ba-mica dispersed in the silica matrix. The N and C isotopic composition of Fe-oxides suggests the presence of a unique biogenic source with δ15NAIR values from +6.0 ± 0.5‰ to 7.3 ± 1.1‰ and a δ13CPDB value of −19.9 ± 0.1‰. The C and N isotope ratios are similar to those observed in Proterozoic and Phanerozoic organic matter. Diffusion-controlled fractionation of N and C released during high combustion temperatures indicates that these two elements are firmly embedded within the iron oxides, with activation energies of 18.7 ± 3.7 kJ/mol for N and 13.0 ± 3.8 kJ/mol for C. We propose that N and C were chemisorbed on iron and were subsequently embedded in the crystals during iron oxidation and crystal growth. The Fe-isotopic composition of the Marble Bar chert (δ56Fe = −0.38 ± 0.02‰) is similar to that measured in iron oxides formed by direct precipitation of iron from hydrothermal plumes in contact with oxygenated waters. To explain the N and C isotopic composition of Marble Bar chert, we propose either (1) a later addition of N and C at the end of Archean when oxygen started to rise or (2) an earlier development of localized oxygenated environments, where biogeochemical cycles similar to modern ones could have developed.

Biogeochemical Cycles of Sulfur and Nitrogen in the Archean Ocean and Atmosphere

Early geodynamic processes modeled the surface of the Earth, providing suitable environments and energy sources for the development of life. Studying the ancient biogeochemical cycles is thus a valuable way to understand the physico-chemical conditions of the early environments, such as the redox state of the primitive oceans and the atmosphere. To investigate early biogeochemistries, the measurement of stable isotopes of S, N, and C from ancient sedimentary rocks is crucial. Here we review the sulfur and nitrogen isotopic signatures measured in Archean rocks that suggest the antiquity of sulfate-reducing bacteria and chemolithotrophs living in a low-oxygen environment.

ISOTOPICALLY ANOMALOUS NITROGEN IN H-CHONDRITE METAL

The cause of the nitrogen isotopic anomalies observed by Hashizume and Sugiura (1995) among bulk equilibrated ordinary chondrites has been investigated. The bulk nitrogen isotopic anomalies can be explained by the isotopically anomalous nitrogen dissolved in taenite and/or tetra-taenite (γ/γ′ phase FeNi metal). Nitrogen concentrations in taenite range between 5 and 45 ppm, and in most of the samples within 10 ± 5 ppm. Nitrogen isotopic ratios trapped in taenite vary with samples in a range of −40 < δ15N < +120%o. No systematic correlation was observed between the amounts and the isotopic ratios. Another rather minor nitrogen reservoir seems to exist in nonmagnetic fractions, although the exact host phase has not yet been identified. The isotopic ratios of nitrogen trapped in nonmagnetic minerals generally do not coincide with those in taenite. We consider that the nitrogen dissolved in taenite was trapped there at the end of the thermal metamorphic event when the interior of the H-chondrite parent body was cooled down to 500°C, the closure temperature of nitrogen in γ-Fe,Ni. The concentration and isotopic ratios of nitrogen trapped in taenite reflect the nitrogen equilibrium system within the parent body at ⩾500°C. The nitrogen isotopic anomalies in taenite cannot be explained by an initially isotopically homogeneous nitrogen reservoir or by processes such as mass-dependent isotopic fractionation.

Early Life Record from Nitrogen Isotopes

Biological activity fractionates the nitrogen isotopes in a peculiar way, making them a reliable biosignature and an accurate paleoenvironmental proxy. Nitrogen has been ignored for long time, being extremely fragile compared to the more stable graphitic forms of C; however, N has an advantage over other isotopic systems such as those of C and S. The dominant source of N at the surface of the Earth, that is, the atmospheric triple-bonded N2, is so stable that only a very limited number of metabolic processes can bridge the abiotic and biotic world. Therefore we can draw relatively simple flux models for N. In this contribution, we review the N isotopic record in the last 4 billions years. Large isotopic shifts recorded by nitrogen are related to specific metabolic changes as a direct response to major environmental stress such as the rise of oxygen in the atmosphere and the evolution of nitrifiers and denitrifiers in the ocean. These isotopic changes are not unique but well correlated with those of C and Fe, indicating that nitrogen can be successfully used for modeling the interplay of changing microbial metabolisms over Earth’s history and relate them to precise environmental changes.

The isotopic composition of solar nitrogen and the heterogeneity of the solar system

The isotopic composition of solar nitrogen is a long standing issue that received recently new impetus. The analysis of nitrogen isotopes in lunar samples and in Jupiter show that solar nitrogen is depleted in 15N by 30% relative to terrestrial. The systematic enrichment of 15N in terrestrial planets and bulk meteorites requires the contribution of 15N-rich compounds to the total nitrogen in planetary materials. Most of these compounds are possibly of an interstellar origin that never equilibrated with the 15N-depleted protosolar nebula N2.