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Organic Globules in the Tagish Lake Meteorite: Remnants of the Protosolar Disk

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Science  01 Dec 2006:
Vol. 314, Issue 5804, pp. 1439-1442
DOI: 10.1126/science.1132175

Abstract

Coordinated transmission electron microscopy and isotopic measurements of organic globules in the Tagish Lake meteorite shows that they have elevated ratios of nitrogen-15 to nitrogen-14 (1.2 to 2 times terrestrial) and of deuterium to hydrogen (2.5 to 9 times terrestrial). These isotopic anomalies are indicative of mass fractionation during chemical reactions at extremely low temperatures (10 to 20 kelvin), characteristic of cold molecular clouds and the outer protosolar disk. The globules probably originated as organic ice coatings on preexisting grains that were photochemically processed into refractory organic matter. The globules resemble cometary carbon, hydrogen, oxygen, and nitrogen (CHON) particles, suggesting that such grains were important constituents of the solar system starting materials.

Carbonaceous chondrite meteorites contain rare micrometer-sized mineral grains from evolved stars (stardust) (1). These meteorites also contain remnants of interstellar organic matter, marked by anomalous H and N isotopic compositions. This material has undergone complex histories of processing, dilution, and isotopic exchange with solar system materials, obscuring its original chemical and physical state. Rare microscopic inclusions with highly anomalous H and N isotopic compositions occur in meteorites and interplanetary dust particles (IDPs), suggesting that some interstellar organic materials have survived intact (24). However, analytical limitations have left the nature of these materials poorly known.

Tagish Lake is a meteorite whose chemistry and mineralogy are intermediate between CI and CM2 carbonaceous chondrites (5). It was collected immediately after its fall was witnessed, minimizing terrestrial contamination (5). It has been linked to outer belt asteroids from its orbit, reflectance spectrum, hydrated mineralogy, and abundant carbonaceous matter, having 2.6 weight percent organic carbon (57). Tagish Lake organic matter often occurs as submicrometer, hollow globules (8). Similar objects were first observed in meteorite extracts in 1961 (9) and have recently been reported in several carbonaceous chondrites (10). However, owing to analytical and sample limitations, the origins of these objects have not been well understood.

We performed coordinated in situ micro-structural, chemical, and isotopic studies of Tagish Lake globules to establish whether their origins were products of chemical processes in the meteorite parent body, the solar nebula, a cold molecular cloud, or circumstellar environment. By determining their sources, the globules can provide direct probes of primordial chemical processes.

Fresh samples of Tagish Lake matrix were sectioned by ultramicrotomy in high-purity S into 50- to ∼70-nm-thick sections (11). We observed numerous, mostly submicrometer, hollow organic globules in carbonate-free sections of the meteorite (Fig. 1). Although the average concentration was about one per 100 μm2, aggregates of two to five globules are common. The globule diameters (140 to 1700 nm) vary significantly more than their wall thickness (100 to 200 nm). All but one of the globules appeared hollow in thin sections. High-resolution transmission electron microscopy (TEM) imaging and electron energy-loss spectroscopy (EELS) show that the globules consist of structurally amorphous C that lacks long-range order or development of graphite-like domains. The distribution of C, N, and O in the globules was obtained using energy-filtered TEM (EFTEM) imaging. The walls and cores of the organic globules are almost always free of other matrix materials.

Fig. 1.

EFTEM images of Tagish Lake organic globules. (A) Bright-field TEM image of three organic globules (G15-1 1.3 μm, G15-2 0.7 μm, and G15-3 0.55 μm) embedded in saponite matrix. (B) Carbon K-edge EFTEM image of the area shown in Fig. 3A, showing carbon-containing material in high contrast. Globule G15-1 (left) shows micro-structure that may be subgrains within its wall. N and H isotopic images of these globules are shown in boxed areas of Fig. 3, A and B. (C) Carbon K-edge EFTEM image of globule G8-3 showing a typical hollow structure of an individual organic globule. N and H isotopic images of this globule are shown in boxed areas of Fig. 3, C and D. (D) This carbon postedge EFTEM image of globule aggregate G21-1 reveals its internal structure, indicating that it has incorporated several ∼50-nm globules, shown by arrows.

Twenty-six of the globules identified by TEM and 1100 μm2 of surrounding matrix material were subjected to C and N isotopic imaging with a NanoSIMS 50L ion microprobe [(11), table S1]. Remarkably, all of the globules had elevated 15N/14N ratios, with δ15N values ranging from 200 to 1000‰, significantly exceeding bulk 15N/14N ratios of CI and CM2 chondrite meteorites (7, 12) (Fig. 2) and Tagish Lake organic matter (77‰) (7). Although accounting for only ∼1.5% of the area analyzed, the globules accounted for 80% of the highly 15N-rich material (δ15N > 400‰). Eight of these globules were also measured for H isotopic compositions, and all were D-rich, with δD values ranging from 1800 to 8100‰. All globules are clearly spatially resolved in H and N isotopic images from the surrounding matrix material, with the exception of two globules that are adjacent to D-rich matrix material (Fig. 3). The C isotopic compositions of the globules had a narrower range (δ13C∼ –77 to +16‰), generally below the value of bulk Tagish Lake organic matter (–9‰) (7).

Fig. 2.

Carbon and nitrogen isotopic compositions of Tagish Lake organic globules compared with ranges observed among whole-rock samples of CI1 (δ15N=31 – 52‰), CM2 (δ15N=13 – 47‰), whole rock Tagish Lake (diamond), and Tagish Lake organic matter (square) (7, 12). The anomalous CM2 meteorite Bells has a bulk δ15N value of 335‰ (12).

Fig. 3.

(A) Nitrogen isotopic image of section G8-3 containing a uniformly 15N-enriched globule aggregate. The box is the field of view of Fig. 1A. (B) Hydrogen isotopic image of the globule aggregate in Fig. 3A. The aggregate is uniformly enriched in D/H and is adjacent to D-rich matrix. (C) Nitrogen isotopic image of section G15 containing three globules with differing 15N-enrichments. The boxes are fields of view of Fig. 1A and Fig. 1B. (D) Hydrogen isotopic image of the globules in Fig. 1C. All three globules are D-rich, but the magnitudes of the H and N isotopic anomalies are not correlated.

The fact that the globules all exhibited N and H isotopic anomalies that greatly exceeded the surrounding meteorite matrix rules out their possible formation in the Tagish Lake parent body. The globules have highly variable H and N isotopic ratios, even those within a few μm of each other (Fig. 3, C and D), making it unlikely that the isotopic variations resulted primarily from parent body processing. However, globules that are attached to each other have similar H and N isotopic compositions (Fig. 3, A and B), suggesting that the globules aggregated before incorporation into the Tagish Lake parent body.

The isotopic compositions of the globules are indicative of mass fractionation during chemical reactions at low temperatures (10 to 50 K). In cold molecular clouds, ion-molecule chemical reactions are promoted by cosmic ray ionization. Gas-phase molecules in cold interstellar clouds have D/H ratios enriched by factors of 102 to 105 relative to HD/H2 (13). Grain surfaces are also predicted to become enriched in D/H via single-atom addition of D (14). D-enrichment may also have occurred in the protoplanetary disk in the region of the Kuiper Belt [>30 astronomical units (AU)], possibly reaching values of protostellar cores at distances of >100 AU (15). Nitrogen isotopic fractionation is expected at extremely low T (10 K), with 15N/14N ratios of polycyclic aromatic hydrocarbon (PAH) molecules and grain surfaces predicted to be enhanced by up to a factor of two (16). These conditions occur in cold molecular clouds and at the outermost regions of protoplanetary disks (>100 AU). The N isotopic anomalies in the globules likely resulted from chemical fractionation, not from nucleosynthesis, because they are enriched in both D/H and 15N/14N and also lack the large C isotopic anomalies characteristic of evolved stars. Their narrow range of 13C/12C ratios limits the degree of accompanying C isotopic fractionation to <50‰ (Fig. 2), consistent with expectations that significant C isotopic fractionation would not be preserved (14, 17).

Carbonaceous chondrites contain a diversity of organic compounds, including hundreds of solvent extractable species and a dominant insoluble macromolecular material (18). These materials are moderately enriched in D and/or 15N (typically δD <1000‰; δ15N <200‰), with significant isotopic variations among different classes of organics and meteorites (19). In general, however these anomalies are well below those expected for cold molecular cloud materials, probably reflecting histories of alteration, mixing, and isotopic exchange of original interstellar organics with solar system materials.

Isotopic measurements of meteorites and IDPs at micrometer scales have revealed a much greater isotopic variability compared with values of bulk organic extracts, in some cases reaching values of cold molecular cloud molecules (δD = 50,000‰) (3, 4). In several cases, D- and 15N-rich “hotspots” were associated with carbonaceous materials (4, 20, 21). Here we show that in the Tagish Lake meteorite, the primary carriers of the most highly anomalous H and N are distinctive submicrometer organic globules.

Since forming, these globules experienced wide ranges of thermal and chemical conditions, ultimately residing in an aqueous solution that formed the phyllosilicates matrix of the Tagish Lake parent body (5). Hydrothermal alteration occurred among many carbonaceous chondrites, leaving variable imprints on their organic matter (18, 22). Tagish Lake organic matter is unusually poor in soluble species that may have been lost by low-temperature chemical oxidation, resulting in the production of carbonate (23). Because the organic globules are only found in carbonate-free regions of Tagish Lake, this suggests that the globules are susceptible to chemical oxidation. Interestingly, two of the globules are adjacent to D-rich matrix material (Fig. 3, B and D) that may have originated from the globules. This is not observed in the N isotopic images, implying that the 15N enrichments are carried by lower-solubility phases than the D-rich material, such as insoluble macromolecular material, PAHs, or amines. This would support the prediction that PAHs obtain the strongest 15N enrichments in protostellar cores (16).

Ice grains in cold molecular clouds are either primarily H2O-rich polar ices containing CO2, CH3OH, H2CO, and NH3, or nonpolar ices containing CO, CO2, N2, and O2 (24, 25). The globules are more likely to have formed by condensation of polar molecular ices, where abundant free H (H/H2 > 1) was available to form hydrocarbons and deuterate molecules on grain surfaces. The isotopic variations of the globules may reflect the sensitivity of H and N isotopic fractionation to temperature, radiation environment, and chemical composition.

Interstellar ice analogs are readily converted into complex refractory organic compounds by exposure to ultraviolet (UV) radiation (26, 27). The penetration depth of UV radiation is remarkably similar to the wall thickness of the organic globules (100 to 200 nm). The interiors of these objects may have been preexisting ice grains that were protected from radiation processing by the organic mantle. These cores remained more volatile than the coatings and would have volatilized at a later stage, leaving the organic globules with hollow cores.

Alternatively, the hollow structures of the globules may have resulted from asteroidal aqueous alteration. This is suggested by experimental production of hydrophobic, vesicle-rich materials from UV-irradiated interstellar ice analogs exposed to alkaline solutions, similar to conditions proposed for aqueous alteration of carbonaceous chondrites (28, 29). However, these materials are generally much larger and weaker than the Tagish Lake organic globules.

Whatever their formation process, the organic globules very likely originated at the outer regions of the protosolar disk in the region of the Kuiper Belt or in the preceding cold molecular cloud, well beyond the influence of the Sun and the nascent planetary system. Consequently, similar organic globules should also have been incorporated into cometary parent bodies. Interestingly, many particles detected during the Giotto and Vega encounters with comet Halley were primarily composed of the elements C, H, O, and N (CHON particles) (30). The size range (40 to 2000 nm) and bulk compositions of CHON particles match the properties of the organic globules studied here, suggesting that such grains were prevalent throughout the protoplanetary disk. Microscopic organic globules may thus have been a common form of prebiotic organic matter delivered to the early Earth by comets and meteorites. Further studies of these objects may elucidate whether their composition and membrane-like structures were important building blocks for the origin of life.

Supporting Online Material

www.sciencemag.org/cgi/content/full/314/5804/1439/DC1

Materials and Methods

Fig. S1

Table S1

References and Notes

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