Laurent Zimmermann

Isotopic Compositions of Cometary Matter Returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single 17O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is 16O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.

Volatiles (He, C, N, Ar) in mid-ocean ridge basalts: Assesment of shallow-level fractionation and characterization of source composition

Abstract To document the characteristics of volatiles in the terrestrial mantle, the abundances and the isotopic ratios of carbon, nitrogen, helium, and argon have been analyzed in 45 mid-ocean ridge basalts (MORB) glasses from the Mid-Atlantic Ridge between 24°N and 36°N, the East Pacific Rise at 21°N, 13°N, and 17–19°S, the Red Sea (18–20°N), the Indian Ocean near the Triple Junction, and the central North Fiji basin. Gases were extracted by crushing and subsequently were split for purification and analysis. Static mass spectrometry was used for N, He, and Ar, and conventional dynamic mass spectrometry was used for C. The data confirm the occurrence of near-constant He isotope ratios (3He/4He = 8.53 ± 0.79 Ra; n = 36) and C isotope ratios (δ13C = −5.2 ± 0.7‰ vs. PDB; n = 21), and of a light nitrogen component in the convective mantle. Overall, the δ15N signature of MORB (mean δ15N = −3.3 ± 1.0‰ vs. ATM, for samples with 40Ar/36Ar > 1000; n = 19) does not drastically differ from that of diamonds, some of which being presumably ≥3 Ga, and is thought to represent the nitrogen isotopic ratio of the asthenospheric mantle. A common mantle source for diamond-bearing magmas and MORB magmas is unlikely, and this similarity may imply either active exchange of volatiles between the respective mantle sources or homogeneous distribution of nitrogen isotopes in these sources during most of Earth’s history. Variations of the 4He/40Ar∗ ratios as well as 40Ar/36Ar ratios are consistent with fractional crystallization–assimilation–degassing taking place in the depth range of 3 to 6 km. The corrected average C/N of the MORB mantle is 535 ± 224, significantly higher than potential cosmochemical and geochemical end-members. C/He and C/N ratios corrected for fractional crystallization–assimilation–degassing fractionation increase with the degree of MORB enrichment (e.g., increasing K2O/TiO2) and are thought to reflect carbon heterogeneities in the mantle source, as a result of fractional recycling of a (fluid?) component extremely enriched in carbon (C/N > 3000). The N/3He ratios are less variable than the C/3He ratios, suggesting limited recycling of nitrogen. Such limited recycling is required to preserve a N isotope ratio in the convective mantle distinct from that of the atmosphere–hydrosphere sediments. Overall, neon, argon, nitrogen, and carbon abundances in the mantle appear to be chondritic, rather than solar, although the neon isotopic signature of the mantle indicates contribution of a solar component. This apparent discrepancy may reflect mixing between a solar-type component mostly seen at present in light rare gases and a chondritic-type component and has strong implications for the origin of terrestrial matter and the processes of their accretion.

Nitrogen Isotopic Composition and Density of the Archean Atmosphere

Same As It Ever Was Nitrogen constitutes approximately 78% by volume of Earths atmosphere and is a key component in its chemical and physical characteristics. It is not clear whether N2 has been so abundant throughout Earths geological history. Marty et al. (p. 101, published online 19 September) analyzed the isotopic compositions of nitrogen and argon from fluid inclusions trapped in hydrothermal quartz formed 3 to 3.5 billion years ago. The partial pressure and isotopic composition of atmospheric N2 were similar to todays. Thus, other factors are needed to explain why liquid water existed on Earths surface despite the Sun being 30% less luminous. Earth’s Archean atmosphere contained roughly as much nitrogen between 3.0 and 3.5 billion years ago as it does today. Understanding the atmosphere’s composition during the Archean eon is fundamental to unraveling ancient environmental conditions. We show from the analysis of nitrogen and argon isotopes in fluid inclusions trapped in 3.0- to 3.5-billion-year-old hydrothermal quartz that the partial pressure of N2 of the Archean atmosphere was lower than 1.1 bar, possibly as low as 0.5 bar, and had a nitrogen isotopic composition comparable to the present-day one. These results imply that dinitrogen did not play a significant role in the thermal budget of the ancient Earth and that the Archean partial pressure of CO2 was probably lower than 0.7 bar.

The Noble Gases as Geochemical Tracers: History and Background

This chapter describes the discovery of the noble gases and the development of the first instrumentation used for noble gas isotopic analysis before outlining in very general terms how noble gases are analysed in most modern laboratories. Most modern mass spectrometers use electron impact sources and magnetic sector mass filters with detection by faraday cups and electron multipliers. Some of the performance characteristics typical of these instruments are described (sensitivity, mass discrimination). Extraction of noble gases from geological samples is for the most part achieved by phase separation, by thermal extraction (furnace) or by crushing in vacuo. The extracted gases need to be purified and separated by a combination of chemical and physical methods. The principles behind different approaches to calibrating the mass spectrometers are discussed.