A contribution to the stable carbon isotope geochemistry of iron meteorites

Deines, Peter; Wickman, Frans E.

doi:10.1016/0016-7037(75)90001-0

Cited by 50 publications

(27 citation statements)

References 14 publications

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“…Cohenite in iron meteorites is depleted in 13 C by 11.5-12.5‰ relative to coexisting graphite (33)(34)(35). Carbon dissolved in taenite (Fe−Ni alloy) from several types of meteorites is also depleted in 13 C relative to graphite (34,36).…”

Section: Resultsmentioning

confidence: 95%

Carbon-bearing iron phases and the carbon isotope composition of the deep Earth

Juske

Polyakov

2014

Proc. Natl. Acad. Sci. U.S.A.

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The carbon budget and dynamics of the Earth’s interior, including the core, are currently very poorly understood. Diamond-bearing, mantle-derived rocks show a very well defined peak at δ13C ≈ −5 ± 3‰ with a very broad distribution to lower values (∼−40‰). The processes that have produced the wide δ13C distributions to the observed low δ13C values in the deep Earth have been extensively debated, but few viable models have been proposed. Here, we present a model for understanding carbon isotope distributions within the deep Earth, involving Fe−C phases (Fe carbides and C dissolved in Fe−Ni metal). Our theoretical calculations show that Fe and Si carbides can be significantly depleted in 13C relative to other C-bearing materials even at mantle temperatures. Thus, the redox freezing and melting cycles of lithosphere via subduction upwelling in the deep Earth that involve the Fe−C phases can readily produce diamond with the observed low δ13C values. The sharp contrast in the δ13C distributions of peridotitic and eclogitic diamonds may reflect differences in their carbon cycles, controlled by the evolution of geodynamical processes around 2.5–3 Ga. Our model also predicts that the core contains C with low δ13C values and that an average δ13C value of the bulk Earth could be much lower than ∼−5‰, consistent with those of chondrites and other planetary body. The heterogeneous and depleted δ13C values of the deep Earth have implications, not only for its accretion−differentiation history but also for carbon isotope biosignatures for early life on the Earth.

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Section: Resultsmentioning

confidence: 95%

Carbon-bearing iron phases and the carbon isotope composition of the deep Earth

Juske

Polyakov

2014

Proc. Natl. Acad. Sci. U.S.A.

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“…This is consistent with an earlier report (Bashir et al, 1996). Deines and Wickman (1975) reported C concentrations of 0.5 wt% in taenite in iron meteorites. But according to the phase diagram of Romig and Goldstein (1 978), this is too high for C in solid-solution in taenite at low temperatures.…”

Section: Carbon and Nitrogen Concentrations In Various Phasesmentioning

confidence: 99%

Ion probe measurements of carbon and nitrogen in iron meteorites

Sugiura

1998

Meteorit & Planetary Scien

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Abstract-Carbon and nitrogen distributions in iron meteorites, their concentrations in various phases, and their isotopic compositions in certain phases were measured by secondary ion mass spectrometry (SIMS).Taenite (and its decomposition products) is the main carrier of C, except for IAB iron meteorites, where graphite and/or carbide (cohenite) may be the main carrier. Taenite is also the main carrier of N in most iron meteorites unless nitrides (carlsbergite CrN or roaldite (Fe,Ni)4N) are present. Carbon and N distributions in taenite are well correlated unless carbides and/or nitrides are exsolved.There seem to be three types of C and N distributions within taenite.(1) These elements are enriched at the center of taenite (convex type). (2) They are enriched at the edge of taenite (concave type). (3) They are enriched near but some distance away from the edge of taenite (complex type). The first case (1) is explained as equilibrium distribution of C and N in Fe-Ni alloy with M-shape Ni concentration profile. The second case (2) seems to be best explained as diffusion controlled C and N distributions. In the third case (3), the interior of taenite has been transformed to the a phase (kamacite or martensite). Carbon and N were expelled from the a phase and enriched near the inner border of the remaining y phase. Such differences in the C and N distributions in taenite may reflect different cooling rates of iron meteorites.Nitrogen concentrations in taenite are quite high approaching 1 wt% in some iron meteorites. Nitride (carlsbergite and roaldite) is present in meteorites with high N concentrations in taenite, which suggests that the nitride was formed due to supersaturation of the metallic phases with N. The same tendency is generally observed for C (ie., high C concentrations in taenite correlate with the presence of carbide and/or graphite).Concentrations of C and N in kamacite are generally below detection limits.Isotopic compositions of C and N in taenite can be measured with a precision of several permil. Isotopic analysis in kamacite in most iron meteorites is not possible because of the low concentrations. The C isotopic compositions seem to be somewhat fractionated among various phases, reflecting closure of C transport at low temperatures. A remarkable isotopic anomaly was observed for the Mundrabilla (IIICD anomalous) meteorite. Nitrogen isotopic compositions of taenite measured by SIMS agree very well with those of the bulk samples measured by conventional mass spectrometry.

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“…It is an issue of kinetics whether cohenite or graphite appears as a C-bearing condensed solid phase. Cohenite is often observed among iron meteorites enriched in C (Deines andWickman, 1973, 1975) and sometime among unequilibrated ordinary chondrites, although often nonstoichiometric (e.g. Hutchison et al, 1987).…”

Section: Chemical Equilibriummentioning

confidence: 99%

Transportation of gaseous elements and isotopes in a thermally evolving chondritic planetesimal

Hashizume

Sugiura

1998

Meteorit & Planetary Scien

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Abstract-The behavior of H, C, N and their isotopes in a thermally evolving planetesimal was evaluated by numerical simulation. Transportation of heat and gas molecules, and the chemical equilibrium involving these elements, were simulated. Our modeled planetesimals initially contain homogeneous amounts of radioactive heat source (26A1); and H, C, and N in forms of organic materials, graphite, and in some models, water ice. Vaporized gas molecules were transported from the interior of the planetesimal to its surface, although their transportation efficiencies were quite different among the three elements, primarily due to differences in their affinities to metallic Fe. Significant portions of these elements were redistributed into metallic Fe when the planetesimal was heated at 600 "C and above. Nitrogen showed the most prominent siderophile characteristics, resulting in fairly large concentrations of N trapped in metallic Fe, which is consistent with observations by Hashizume and Sugiura (1997). Efficiency of C transportation crucially depended on 0 fhgacity. To realize effective C transportation, it was necessary to assume an oxidizing condition (logfO2 > logfOZ,(FIF) + 1) in the initially accreted material. Water vapor, generated at the interior of the planetesimal and transported to its near surface, formed a water-rich layer under certain conditions, providing an environment sufficient for aqueous alteration of chondritic materials to occur. Variations in isotopic ratios of N in taenite observed among equilibrated ordinary chondrites can be explained by our gas transportation model. It is required, however, that carriers of isotopically anomalous N, perhaps presolar grains, were initially localized on a large spatial scale within a single planetesimal, which possibly suggests incorporation of preaccretionary objects as large as 0.1 x of the final mass of the ordinary chondrite parent body.

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A contribution to the stable carbon isotope geochemistry of iron meteorites

Cited by 50 publications

References 14 publications

Carbon-bearing iron phases and the carbon isotope composition of the deep Earth

Carbon-bearing iron phases and the carbon isotope composition of the deep Earth

Ion probe measurements of carbon and nitrogen in iron meteorites

Transportation of gaseous elements and isotopes in a thermally evolving chondritic planetesimal

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