Pristine Interstellar Graphite-Bearing Assemblage Is Finally Found In Situ InUnequilibrated Chondritic Meteorite

May 29, 1998

Since about ten years scientists have discovered interstellar graphite grains in primitive chondritic meteorites. The grains were identified after extraction from meteorites using a complex method of successive chemical dissolution of almost all the meteorite material. These grains have never been located in situ and hence there was no indication of the nature of their eventual associated mineral phases.

An international group of Meteoritists (S. Mostefaoui, P. Hoppe, and A. El Goresy) at the Max Planck Institute of Nuclear Physics in Heidelberg and at the Max Planck Institute of Chemistry in Mainz have used different techniques to successfully find in situ interstellar graphite with its associated pristine mineral phases in an unequilibrated chondrite. The results of this study, published in the Mai 29, 1998 issue of "Science", showed that graphites of a fine-grained structure have large D and 15N excesses (dD~ 1500 per mil and d15N~ 1300 per mil). The excesses are the largest D and 15N excesses ever observed in situ in a well characterized phase in a meteorite, and are suggestive of an interstellar origin of the graphite.

In a fine-grained chondrule-free clast in the unequilibrated ordinary chondrite Khohar (UOC L3 type) we found a spherical object (~100mm in diameter) consisting of a fine-grained, Ni-poor metal aggregate and graphite. The clast consists mostly of small (<10mm) silicate grains and metal particles (Fig. 1A). The bulk chemical composition of the clast, obtained by 20 broad beam electron microprobe (EMP) analyses, differ from the bulk compositions of both, the Khohar chondrite and the L ordinary chondrite group. The abundances of the major oxides SiO2, MgO, and FeO are similar to those of the bulk composition of the Orgueil (CI) carbonaceous chondrite.

The survey of the metal-graphite assemblage with optical microscope and scanning electron microscope (SEM) showed that it consists of small (£2mm) metal particles of kamacite (a-phase) and taenite (g-phase) and a very fine-grained (<1mm) birefringent graphite (Fig. 1C). The graphite is abundant (~50 volume %) and fills interstices between the metal particles (Fig. 1D). This much graphite with its coexisting metal particles is unlikely to be produced by the breakdown of carbides. Ni and Co contents of kamacite and taenite are not chondritic: (Ni in kamacite = 0.5 to 2.89 wt. % and in taenite = 28 to 32 wt. %) (Co in kamacite <0.01 wt. % and in taenite ~ 0.89 wt. %). Analysis of individual metal particles revealed that some of them have Ni contents as low as ~ 0.5 weight %, suggesting the presence of nearly pure metallic Fe.

Analysis of the graphite with secondary ion mass spectrometry (SIMS) revealed maximum H and N concentrations of 4100 and 250 ppm, respectively, although most analyses gave lower concentrations. The C isotopic ratios are not significantly different from the terrestrial ratio, and d13C values lie between -21 " and -51 ". The N isotopic ratios, are more variable and anomalous, with d15N values of up to 1330 " (Fig. 2). The highest d15N values were found in graphites with low N concentrations. The H isotopic ratios are also variable, with positive dD values that range from close to normal (dD = 0 ") up to anomalous (dD = 1500 ") (Fig. 2). The highest dD values are found in graphites with high N concentrations, indicating the presence of two isotopically distinct phases, one N rich with high D abundance and other N poor with high 15N content.

The D and 15N excesses reported here are the largest D and 15N excesses ever measured in situ in a well-characterized phase in a meteorite. The isotopic characteristics are suggestive of an interstellar origin, probably by ion-molecule reactions at low temperature in the interstellar molecular cloud from which the solar system formed.

Graphite is considered an important constituent of carbonaceous dust detected in the interstellar medium. The anisotropism observed with the optical microscope in the Khohar assemblage indicates that most of the C is present as crystalline graphite. However, although all or most of the C in the assemblage is graphite, we do not know in which forms H and N are sited in the graphite; therefore, the enrichments in D and 15N may not be genetically associated with the graphite. Some portions of the Khohar chondrite show different signs of shock and a proposed scenario would call for the release of H and N by shock waves from their original D- and 15N-rich phases which in turn would be trapped in graphite. However, such a scenario predicts a positive correlation between 15N and D enrichments. The H and N isotopic signatures in the Khohar assemblage are not correlated, and the metal assemblage as well as the other metal grains in the clast do not show any shock metamorphic effects. Accordingly, we argue that, although the chondritic part in Khohar shows different degrees of shock metamorphism, the N and H isotopic compositions in the graphite in the clast were not generated by a dynamic impact event.

The presence of metal particles having nonchondritic chemical composition with particles of nearly pure metallic iron, as well as their association with graphite having interstellar isotopic signatures in Khohar was unexpected. The comparison of the compositions of the metal particles with experimental data for the Fe-Ni-Co system at equilibrium showed that the particles are out of equilibrium. This suggests that metal particles and graphite from different sources were probably mixed and compacted to form this spherical object in khohar. The mechanism, location, and period of mixing are unknown. Such clasts with no chondritic features are good candidates for an in situ search for extrasolar material in meteorites.


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