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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Sample Preparation
  6. Analytical Techniques
  7. Results
  8. Discussion
  9. Conclusions
  10. References

Abstract– Coordinated in situ transmission electron microscopy and isotopic measurements of carbonaceous phases in interplanetary dust particles were performed to determine their origins. Five different types of carbonaceous materials were identified based on their morphology and texture, observed by transmission electron microscopy: globular, vesicular, dirty, spongy, and smooth. Flash heating experiments were performed to explore whether some of these morphologies are the result of atmospheric entry processes. Each of these morphologies was found to have isotopically anomalous H and N. Rare C isotopic anomalies were also observed. The isotopic and morphological properties of several of these phases, particularly the organic globules, are remarkably similar to those observed in other extraterrestrial materials including carbonaceous chondrites, comet 81P/Wild 2 particles collected by the Stardust spacecraft, and Antarctic micrometeorites, indicating that they were widespread in the early solar system. The ubiquitous nature and the isotopic anomalies of the nanoglobules and some other morphologies strongly suggest that these are very primitive phases. Given that some of the isotopic anomalies (D and 15N excesses) are indicative of mass fractionation chemical reactions in a very cold environment, and some others (13C and 15N depletions) have other origins, these carbonaceous phases come from different reservoirs. Whatever their origins, these materials probably reflect the first stages of the evolution of solar system organic matter, having originated in the outermost regions of the protosolar disk and/or interstellar cold molecular clouds.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Sample Preparation
  6. Analytical Techniques
  7. Results
  8. Discussion
  9. Conclusions
  10. References

Interplanetary dust particles (IDPs) are C-rich complex assemblages of primitive solar system materials that are collected in the Earth’s stratosphere (Brownlee et al. 1976; Sandford 1987). IDPs have been divided into two groups based in their texture and morphology: chondritic porous (CP) and chondritic smooth (CS). CP IDPs are dominated by anhydrous minerals, including forsterite, enstatite, GEMS (glass with embedded metal and sulfides) grains, Fe-Ni sulfides, and carbonaceous matter. CS IDPs are dominated by hydrated silicates but may also contain anhydrous crystalline silicates, carbonates, sulfides, and carbonaceous matter (Bradley 2003, 2010). The CP IDPs are considered the most primitive extraterrestrial materials available for laboratory study based on their unequilibrated mineralogy, chemistry, and isotopic signatures (Bradley 2003, 2010). Moreover, CP IDPs escaped the thermal processing and water–rock interactions that modified the original chemistry and mineralogy of even the most primitive meteorites. CP IDPs are thus essentially pristine aggregates of nebular and presolar materials and probably preserve some interstellar phases that have not survived in meteorites (Keller et al. 2004).

Most IDPs are very carbon-rich with an average C content of 10–12 wt% (Schramm et al. 1989) and the carbonaceous phases seem to be made of organic material (Thomas et al. 1993; Flynn et al. 2003), including aromatic and aliphatic compounds (Clemett et al. 1993; Keller et al. 2004). Past studies have shown that these carbonaceous phases commonly have H and N isotopic anomalies (Messenger 2000; Aléon et al. 2003; Keller et al. 2004) certifying that these organic materials are indigenous to the IDPs and not the product of terrestrial contamination. Indeed, H and N isotopic anomalies in IDPs have been ascribed to low-temperature chemical reactions (Messenger 2000; Keller et al. 2004; Floss et al. 2006) likely occurring in a presolar cold molecular cloud.

The organic matter observed in IDPs seems to be similar to some of the carbon-rich materials observed in carbonaceous chondrites and Antarctic micrometeorites (AMMs) in terms of chemistry (Pizzarello et al. [2006] and references therein), and isotopic compositions (Alexander et al. 1998, 2007, 2010; Huang et al. 2005, 2007; Busemann et al. 2006; Duprat et al. 2010). However, differences in the chemical and isotopic compositions of organic materials among meteorites and IDPs indicate that primordial organic materials have been altered to varying degrees by nebular and parent body processes (Pizzarello et al. 2006). H and N isotopic compositions are highly heterogeneous in primitive meteorites and IDPs, with the largest anomalies often occurring in micrometer-scale “hotspots” (positive heavy isotope anomalies) or “coldspots” (negative heavy isotope anomalies) (McKeegan et al. 1987; Messenger 2000; Floss et al. 2004). Because these tiny inclusions show such large anomalies and are clearly not isotopically equilibrated with their surroundings, it has been suggested that they are well-preserved samples of ancient organic matter. However, owing to their small size, the nature of these phases has been poorly constrained.

Coordinated isotopic and transmission electron microscopy (TEM) studies of primitive meteorites and IDPs have revealed new details about the chemical and physical properties of isotopically anomalous hotspots. For example, it was recently shown that the H and N isotopic hotspots in the Tagish Lake meteorite are mostly associated with morphologically distinct organic nanoglobules (Nakamura-Messenger et al. 2006). Organic nanoglobules are typically submicrometer, roughly spherical objects that are often hollow and are almost always isotopically anomalous. They have been reported in a number of different carbonaceous chondrites, CP IDPs, AMMs, and Wild 2 cometary particles (Maurette et al. 1995; Matrajt 2001; Nakamura et al. 2002; Busemann et al. 2006, 2009; Garvie and Buseck 2004; Nakamura-Messenger et al. 2006; Matrajt et al. 2008; Messenger et al. 2008; DeGregorio et al. 2010a, 2010b). Given their distinct morphology and pronounced isotopic anomalies, it is likely that these nanoglobules originated in the outermost regions of the protosolar disk or in a presolar cold molecular cloud.

In the present study, we investigated the texture, morphology, and isotopic compositions of the carbon-rich phases in IDPs by coordinated isotopic imaging and TEM. In addition to organic nanoglobules, we identified other distinct morphologies of carbonaceous material. We studied these morphologies in terms of their textures, their isotopic compositions, and their spatial distributions. We investigated their origin by comparing the above characteristics to other, similar carbonaceous materials found in meteorites. The nature and organic chemistry of these carbonaceous phases are a matter of another separate study, to be published elsewhere.

In this article, we first describe the six IDPs we investigated during this study. We define and describe the different morphologies found on the basis of the TEM observations and electron energy loss spectroscopy mapping. We detail the isotopic compositions found for each type of morphology based on nano secondary ion mass spectrometry (NanoSIMS) analyses. We compare both the morphologies and the isotopic compositions observed in our IDPs to other materials such as carbonaceous chondrites and other IDPs studied in the past. We discuss the possible origins of these different morphologies and finally we describe heating experiments that we performed to evaluate and rule out an atmospheric entry heating origin.

Samples

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Sample Preparation
  6. Analytical Techniques
  7. Results
  8. Discussion
  9. Conclusions
  10. References

We investigated six IDPs whose scanning electron microscope (SEM) micrographs are shown in Fig. 1. Five of these six IDPs were hand-picked in our laboratory from flags W7154, W7259, and U2-20; therefore they do not have NASA curatorial names and we nicknamed them after Indians from Latin America or personal invented names: IDP Chocha (Fig. 1A) comes from flag W7154 and is an anhydrous ultracarbonaceous particle made of >95% carbon and anhydrous minerals, mainly olivines, pyroxenes (diopside), and Fe-Ni sulfides (pyrrhotite and pentlandite). IDP Plin (Fig. 1B) comes from flag W7154 and is a hydrated porous particle, where both anhydrous minerals (olivines, Fe-Mg spinel, Fe-Ni oxides, glass, and magnetite rims) and hydrated ones (mainly saponite) are present. IDP Chuki (Fig. 1C) comes from flag W7259 and has a combination of porous and smooth textures and it is a hydrated particle. IDP Nayeli (a Zapotecan Indian name, Fig. 1D) comes from flag W7259 and is a hydrated smooth particle, dominated by saponite and some cronstedtite, although it also has some anhydrous minerals (Fe-Ni sulfides, olivines, pyroxenes) and vesicular glass. IDP GS-4 (curatorial name L2055-R-1,2,3,4,5 cluster #7) is a Grigg–Skjellerup timed-collection (Messenger 2002) particle whose SEM image is not available. It is an ultracarbonaceous particle made of >90% carbon and anhydrous minerals (mainly olivines, diopside, and Fe-Mg carbonates). IDP GCA is an anhydrous giant cluster particle from flag U2-20, dominated by Mg silicates and sulfides. It also has glass. Two subfragments of this particle were picked up for this study: Cielo and Tetouille coming from the fine-grained fraction of the cluster. SEM images of them are not available. We did not search for solar flare tracks; however, we only found magnetite rims in one particle, Plin. We did not find any signs of graphitization as has been reported for other IDPs (Keller et al. 2004). Therefore, it appears that Plin was moderately heated and the other particles escaped significant heating during atmospheric entry.

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Figure 1.  SEM images of four of the six IDPs discussed in this study. A) IDP Chocha. B) IDP Plin. C) IDP Chuki. D) IDP Nayeli. Scale bars are 1 μm.

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We also investigated insoluble macromolecular organic matter from the primitive carbon-rich CI chondrite Orgueil. This insoluble organic matter (IOM), also known as acid residue, is generally obtained by demineralizing pieces of meteorite in HF/HCl (Gardinier et al. 2000). The residue we analyzed in our study was kindly provided by the Museum of Paris. We chose to compare the morphologies of the carbon found in our IDPs with those present in the acid residue of Orgueil, because Orgueil is a meteorite where nanoglobules have already been reported (Garvie and Buseck 2006a).

Sample Preparation

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Sample Preparation
  6. Analytical Techniques
  7. Results
  8. Discussion
  9. Conclusions
  10. References

Particles were embedded in acrylic and microtomed using a 45° diamond knife and a Leica Ultracut S ultramicrotome. The sections, typically 50–70 nm thick, were placed onto C-thin films attached to Cu or Au TEM grids. Grids were placed on a home-made washer where condensed chloroform vapor was used to dissolve and remove acrylic from the sections, following the technique described by Matrajt and Brownlee (2006). The Orgueil residue was embedded and microtomed in the same way as were the IDPs.

Analytical Techniques

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Sample Preparation
  6. Analytical Techniques
  7. Results
  8. Discussion
  9. Conclusions
  10. References

Transmission Electron Microscopy

All microtome sections were studied with a 200 kV Tecnai F20 field-emission STEM, equipped with a bright field CCD Orius camera used to study at high magnification the state of the carbon and the morphologies. The probe current was 3 nA. A Gatan imaging filter (GIF) detector was used to acquire carbon maps under the energy-filtered imaging mode (EFTEM). The elemental maps were acquired using the standard 3-window method to correct for the background contribution to the C edge (285 eV). This method consists of acquiring two images in front of the 285 eV edge, the so-called pre-edge images (acquired at 252 and 272 eV) and one image above the edge, the so-called postedge image (acquired at 294 eV). The pre-edge images are used to subtract the background in the postedge window, assuming a power law background of the form AEr, where A and r are fitting parameters and E is the energy loss. The background from the two pre-edge images is extrapolated to the postedge image and it is then subtracted from it, which results in a final image, the elemental map. To minimize sample damage due to beam exposure, the carbon maps were typically collected for 3–5 s with 4000× magnification.

Nano Secondary Ion Mass Spectrometry

Interplanetary dust particle grids with sections where carbon was located by TEM surveys were subjected to H, C, and N isotopic imaging with the Johnson Space Center (JSC) NanoSIMS 50L ion microprobe. For C and N isotopic measurements, images of 12C, 13C, 16O, 12C14N, 12C15N, and 28Si were acquired simultaneously in multidetection mode with electron multipliers and by rastering a 16 keV, 1 pA Cs+ ion beam focused to 100 nm. We acquired 20–30 images of each sample over periods of 5–8 h. Sample charging was mitigated by use of an electron flood gun during the analysis. Nearby 5–10 μm sized grains of 1-hydroxyl benzotriazole hydrate were measured as C and N isotopic standards. In addition, in postanalysis data processing we checked the C isotopic composition of the amorphous C film substrate in each image scan to evaluate the stability of the instrumental mass fractionation during the analysis.

For H isotopic measurements, we acquired images of H, D, 12C, 12CH, and 16O in multidetection with electron multipliers. We used a 2.5 pA Cs+ primary ion beam rastered over a 10 μm field of view. For each sample, we acquired at least 15 sequential sets of images. The measurements were performed with entrance slits removed (low mass resolving power) after centering the secondary ion beam on the entrance slit. An electron flood gun was used for charge compensation. Nearby grains of terrestrial kerogen were measured as external H isotopic standards. The ion images obtained had a spatial resolution of approximately 250 nm. Isotopic variations in the samples and isotopic ratios of regions of interest were determined with image processing software developed at JSC. The stated errors take into account counting statistical errors and reproducibility of isotopic measurements of standards.

We coordinated the isotopic and chemical/morphological data in two ways after aligning bright-field TEM and energy-filtered C maps of the IDPs with the isotopic images. First, we delineated regions of interest in the TEM images and obtained morphology-specific isotopic ratios. Second, we inspected isotopic ratio images for isotopically distinct hotspots or coldspots and identified the carrier phases in the TEM images. For the purposes of this study, we define these hotspots and coldspots to be contiguous regions within isotopic ratio images that are clearly isotopically distinct from their surroundings, having delta values at least a factor of two above the surrounding material.

Heating Experiments

We performed a series of heating experiments to evaluate whether the various forms of carbonaceous materials we observed in the IDPs may have been formed or altered as a result of atmospheric entry heating. A microtomed section of particle Cielo placed on a Cu grid was heated for 10 s at 620 °C, 660 °C, 750 °C, and 850 °C inside a tube-shaped ceramic furnace with a N2 flow passing through to avoid oxidation. To accurately monitor the temperature, a thermocouple was placed inside the furnace at the time of the experiment. The duration of heating was chosen to mimic the flash heating time to which IDPs are exposed when entering the atmosphere (Love and Brownlee 1991; Flynn 2001). The grid was observed in the TEM to monitor the fate of the textures of the carbonaceous material after each heating sequence and before raising the temperature to the next level.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Sample Preparation
  6. Analytical Techniques
  7. Results
  8. Discussion
  9. Conclusions
  10. References

Transmission Electron Microscopy

Carbon Maps

We obtained carbon maps like the ones shown in Figs. 2–5. The bright, white areas in the maps are the carbonaceous regions of the particle. The darker areas are not carbonaceous; they are typically mineral grains or thicker areas that cannot be measured with the GIF. The carbon is clearly heterogeneously distributed within the particles and there are clear differences in the carbon distribution between particles. For example, in some particles like Chocha (Fig. 2) or GS-4 (Fig. 3), carbon makes up approximately 95% of the section of the particle, i.e., it covers most of the particle’s area. By contrast, the carbon in other particles is located as discrete spots (Figs. 4 and 5) and only makes up approximately 10–20% of the area of the section of the particle.

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Figure 2.  Particle Chocha. A) Bright field micrograph of an ultramicrotomed section. B) EFTEM carbon map of section shown in (A). The white areas are the carbon-rich regions of the particle, which account for approximately 95% of the area of this section.

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image

Figure 3.  Particle GS-4. A) Bright field micrograph of an ultramicrotomed section. B) EFTEM carbon map of section shown in (A). The white areas are the carbon-rich regions of the particle, which account for approximately 90% of the area of this section.

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Figure 4.  Particle Cielo. A) Bright field micrograph of an ultramicrotomed section. B) EFTEM carbon map of section shown in (A). The white areas are the carbon-rich regions of the particle, which account for approximately 10–20% of the area of this section and are heterogeneously spread.

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Figure 5.  Particle Tetouille. A) Bright field micrograph of an ultramicrotomed section. B) EFTEM carbon map of section shown in (A). The white areas are the carbon-rich regions of the particle, which account for approximately 10–15% of the area of this section and are heterogeneously spread.

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General State of the Carbon

Once carbon was located in the map, we used the bright field mode of the TEM to study the structural state of the carbon (i.e., amorphous, graphitized, poorly graphitized) by looking at the presence of lattice fringes, and the different textures of the carbonaceous material. All the samples studied contain carbon. In all cases, the carbon was present as an amorphous phase (Fig. 6), i.e., no evidence of the 0.34 nm lattice fringes typical of graphite or graphite-like (onion-rings, poorly graphitized carbon [PGC]) areas were ever observed. In general, graphite and graphite-like crystalline textures are linked to thermal metamorphism of organic carbonaceous material (Buseck and Bo-Jun 1985). However, lack of these observable heating effects by TEM does not mean that the carbonaceous materials found in our samples were not exposed to flash heating during atmospheric entry. It just means that these carbonaceous materials are refractory, and most likely they are “nongraphitizing” compounds. Nongraphitizing carbonaceous compounds have been observed in low-grade metamorphic rocks (Buseck and Bo-Jun 1985) and in IOM of a range of carbonaceous, ordinary, and enstatite chondrites (Cody et al. 2008). This type of organic material typically shows no evidence of homogeneous development of a graphite structure, even after heating to 3000 °C for 15–20 min (Buseck and Bo-Jun 1985).

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Figure 6.  High magnification (350 kX) TEM image of a carbonaceous area in IDP Chocha, showing the amorphous nature of the carbon.

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Morphologies and Textures of the Carbonaceous Material

The carbonaceous material displayed a variety of morphologies and textures in the samples studied. For example, carbon sometimes formed spherical objects, the so-called nanoglobules; other times carbon was found in extended smooth areas and even in chunks that contained sulfides and other mineral grains embedded inside the carbonaceous structures. After an extensive survey, we identified five distinct morphologies of carbonaceous material: globular, spongy, vesicular, smooth, and dirty. There is some overlap in the appearance of these morphologies but in most cases they can be very well distinguished. Table 1 summarizes the morphologies found in each of the samples studied.

Table 1.   Morphologies found in interplanetary dust particles and Orgueil residue.
SampleGlobularSpongyVesicularSmoothDirty
OrgueilYesYesYesYesNo
ChochaNoYesYesYesYes
PlinYesYesYesYesYes
NayeliYesYesNoYesYes
ChukiYesYesYesYesNo
GS-4YesYesYesYesYes
GCAYesYesYesYesYes

Globular. The shape is roundish, although it may not be perfectly circular. The globules may be filled or hollow. We have seen elongated globules, partially rounded, and partially filled globules. Figure 7 shows examples of the variety of globules found during this study. Their sizes typically vary from 30 to 500 nm. They can be found attached to a mineral grain, between several grains, surrounded by mineral grains, inside vesicular glass, near the edge of the particle. The shapes and sizes of these globules are very similar to the globules reported by other investigators in carbonaceous chondrites (see the Discussion section).

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Figure 7.  Bright field TEM micrographs of different types of globules observed in IDPs. A) Filled globule. B) Hollow globule. C) Elongated globule. D) Partially filled globule. E) Partially hollow globule. F) Globule between mineral grains. G) Tiny globules attached together (grape shape). H) Double globule. I) Globules in Orgueil residue. The arrows indicate the globules in the field of view.

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Spongy. This type of carbonaceous material looks like a mesh or a network of tiny voids, similar to a sponge. It appears sometimes elongated so it looks more like lace. Voids vary in size from just a few nanometers to a few dozens of nanometers and there can be just a few (5–7) voids linked together or the structure can look more like a lacey carbon film, similar to the one that is typically used to coat TEM grids. Figure 8 shows examples of the different spongy areas found throughout the samples investigated. Spongy carbonaceous material has not been previously recognized, perhaps because it has been identified as vesicular or globular carbon (see the Discussion section). Spongy carbon may resemble clusters of globules, but globules usually occur as individual objects or in small clusters (two to three globules in one field of view or like a small cluster of grapes). By contrast, spongy structures contain up to several dozens of tiny hollow globules or voids linked together. Also, the voids are always hollow, whereas in the globular morphology globules may be filled. It is sometimes hard to distinguish one morphology from another as can be seen in Figs. 9B and 9I, because they can be adjacent to each other within just a few nanometers or even sometimes in physical contact (Fig. 9I).

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Figure 8.  Bright field TEM micrographs of different types of spongy structures observed in IDPs. A) Spongy area in middle of particle. B) Spongy structure in middle of particle. C) Spongy structure linking two grains. D–H) Spongy structure between grains. I) Compacted spongy structure. J) Spongy structure at the edge of a grain. K) Spongy area in Orgueil residue (all the field of view). The arrows indicate the spongy areas in the field of view.

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Figure 9.  Bright field TEM micrographs of examples where two or more different morphologies are found together within few nm or even in physical contact. A) Spongy and vesicular morphologies in contact. B) Spongy and globular morphologies within a few nm of each other. C) Spongy and vesicular morphologies in contact. D) Spongy and smooth in contact. E) Spongy and smooth within a few nm of each other. F) Spongy and dirty in contact. G) Spongy and smooth in contact. H) Spongy and smooth in contact in Orgueil residue. I) Spongy, smooth, and globular in contact in Orgueil residue.

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Vesicular. This type of morphology does not have a particular shape; it is characterized by being a solid piece of carbonaceous material containing small vesicles. The vesicles are 10–30 nm rounded voids that may or may not extend through the chunk of carbon. Figure 10 shows examples of vesicular areas found in this study. The vesicles look similar to those reported in previous studies of carbonaceous chondrites and IDPs (see the Discussion section). The main difference between vesicular morphologies and spongy or globular ones is that the voids are found on or in a shapeless smooth chunk of carbon, whereas globules are roundish and stand alone and spongy has the shape of a mesh. Also, the size of the vesicles is usually smaller than the voids in spongy or globular morphologies.

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Figure 10.  Bright field TEM micrographs of different types of vesicular structures observed in IDPs. A–D) Vesicular morphology found at the edge of the particle. E–G) Vesicular morphology found between grains. H) Vesicular morphology showing vesicles of different sizes. I) Vesicular morphology in Orgueil residue. The arrows indicate the vesicular areas in the field of view.

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Smooth. The area does not have a regular shape and it has a smooth, uniform solid surface with no interior texture. It can vary in size from 50 to >500 nm. Smooth carbonaceous chunks can be found between mineral grains, surrounding mineral grains (at their edge), connecting mineral grains, inside mineral grains (typically inside vesicular glass), or just standing alone near mineral grains but not necessarily in contact with them. Figure 11 shows some examples of the smooth areas we have seen in this study. This morphology is similar to the vesicular one except that it does not contain any vesicles.

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Figure 11.  Bright field TEM micrographs of different types of smooth structures observed in IDPs. A) Smooth area at the edge of the particle. B) Smooth area surrounding a mineral grain. C,D) Individual carbonaceous smooth grains slightly separated from the main particle. E–G) Smooth area connecting mineral grains. H–J) Smooth area between mineral grains. K) Smooth morphology in Orgueil residue. The arrows indicate the smooth areas in the field of view.

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Dirty. The area does not have a regular shape and contains tiny embedded mineral grains, mostly sulfides. The size of the area can vary from 50 to >500 nm. The sulfides are typically <50 nm but sometimes crystals larger than 500 nm are found inside the smooth carbonaceous piece. Figure 12 shows some examples of the dirty morphologies observed in this study. Dirty areas like these have not been reported in other materials, perhaps because the GIF detector and its capabilities to map carbon at the nanometer scale were not available at the time of past studies. The main difference between this morphology and the smooth and vesicular ones is the presence of the mineral grains and the lack of vesicles.

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Figure 12.  Bright field TEM micrographs of different regions where dirty structures were observed in IDPs. A–L) Different examples of dirty areas found in the samples studied. No dirty morphologies were found in the Orgueil residue. The arrows indicate the dirty morphologies in the field of view.

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Isotopic Imaging

Each of the IDPs analyzed contained isotopically anomalous carbonaceous material, with H, C, and N isotopic compositions falling in the range of previously analyzed IDPs and chondritic meteorites (Table 2). The bulk C isotopic compositions of the IDPs are within the range of terrestrial materials. IDPs Chuki, Nayeli, and Plin had normal bulk N isotopic compositions, whereas IDPs Tetouille, Chocha, and GS4 showed moderate bulk δ15N enrichments of approximately 180‰, 230‰, and 500‰, respectively. The spatial variations in the C and N isotopic compositions of the IDPs also differed (Figs. 13–17). We compare the C and N isotopic variability of 300 nm N-rich regions within the IDPs in Fig. 18. IDPs having normal bulk N isotopic compositions showed nearly uniformly normal C and N isotopic compositions, whereas IDPs with bulk δ15N enrichments were almost uniformly anomalous. Among the 15N-rich IDPs, there are also subtle differences in the C and N isotopic distributions. IDP Chocha (Fig. 13) displays a narrower distribution of N isotopic compositions compared with the other 15N-rich IDPs and appears to have systematically lower δ13C values among the 300 nm bins. There is also a suggestion of an anticorrelation in the C and N isotopic compositions of regions within IDP GS4.

Table 2.   H, C, and N isotopic compositions of the interplanetary dust particles and their morphologically distinct inclusions as identified by TEM, identified by the column “material.” Carbonaceous regions identified as the carrier of H, C, or N isotopic hotspots or coldspots are indicated in the “Hotspots & coldspots” column. Only two hotspots/coldspots were not associated with previously characterized carbonaceous inclusions and thus their material descriptions are blank. Carbonaceous inclusions not measured for H or C and N are also left blank.
SampleMaterialArea (μm2)CN/Cδ13C ± 1σ (‰)δ15N ± 1σ (‰)δD ± 1σ (‰)Hotspots & coldspots
Chocha B 8CBulk35.30.41−30 ± 6234 ± 7490 ± 20 
Chocha B 8CBulk C310.46−30 ± 6237 ± 7470 ± 20 
Chocha B 8C-1Smooth0.70.55−45 ± 21314 ± 32  
Chocha B 8C-2Spongy3.80.71−31 ± 8286 ± 18  
Chocha B 8C-3Vesicular11.50.71−25 ± 9241 ± 9  
Chocha B 8C-4Vesicular0.10.3429 ± 39970 ± 160 15N
Chocha 3 2B S1Spongy0.35   300 ± 120 
Chocha 3 2B S2Spongy1.3   120 ± 60 
Chocha 3 2B S3Spongy2.3   690 ± 54 
Chocha 3 2B V1Vesicular3.1   1160 ± 60+δD
Chocha 3 2B V2Vesicular1.2   1130 ± 90+δD
Chocha 3 2B V3Vesicular0.3   1100 ± 170+δD
Chocha 3 2B-1 0.18   −600 ± 100−δD
Chuki 5-4DBulk1110.3−11 ± 76 ± 5−210 ± 23 
Chuki 5-4DBulk C12.70.5−8 ± 1334 ± 9−185 ± 65 
Chuki 5-4D-1Globular0.150.42 ± 23915 ± 86960 ± 300+δ D, +δ15N
Chuki 5-4D-2Vesicular0.151.75 ± 37−56 ± 37  
Chuki 5-4D-3Vesicular0.11.25 ± 43−47 ± 64  
Chuki 5-4D-4Smooth0.091.59 ± 44447 ± 88 15N
Chuki 5-4D-5 0.20.92 ± 42414 ± 62 15N
GS4 2-6aBulk3.50.7−8 ± 11500 ± 15  
GS4 2-6aBulk C3.00.78−8 ± 9530 ± 15  
GS4 2-6a-1Globular0.080.7−5 ± 17429 ± 37  
GS4 2-6a-5Smooth0.30.78 ± 14550 ± 27  
GS4 2-6a-8Smooth0.170.811 ± 19501 ± 44  
GS4 2-6a-3Spongy0.370.8−60 ± 121360 ± 32 15N
GS4 2-6a-4Spongy0.220.73 ± 14471 ± 25  
GS4 2-6a-6Spongy0.150.510 ± 14397 ± 31  
GS4 2-6a-7Spongy0.340.84 ± 11558 ± 34  
GS4 2-6a-2Vesicular0.070.53−29 ± 27990 ± 95 15N
Nayeli 5-4DBulk1030.3−47 ± 3−6 ± 5  
Nayeli 5-4DBulk C12.70.5−51 ± 5−11 ± 10  
Nayeli 5-4D-1Globular0.121.07 ± 30208 ± 57  
Plin 2-7BBulk12.90.34−2 ± 9−13 ± 6  
Plin 2-7BBulk C0.70.34−8 ± 1642 ± 18  
Plin 2-7B-1Globular.020.66−11 ± 36240 ± 84  
Plin 2-7B-2Globular.031.6−51 ± 59400 ± 100 15N
Tetouille 1 5E ABulk29.30.69−9 ± 5180 ± 4  
Tetouille 1 5E ABulk C7.20.8−6 ± 6364 ± 11  
Tetouille 1 5E BBulk5.10.66−5 ± 10139 ± 7  
Tetouille 1 5E BBulk C0.90.54−4 ± 12289 ± 32  
Tetouille 1 5E CBulk1.280.41−3 ± 9116 ± 29  
Tetouille 1 5E CBulk C0.80.39−8 ± 9171 ± 30  
Tetouille 1 5E A-3Dirty0.070.7−19 ± 16−135 ± 45 −δ15N
Tetouille 1 5E A-5Dirty0.190.9−19 ± 12748 ± 21 15N
Tetouille 1 5E A-6Dirty0.121.443 ± 28654 ± 46 15N
Tetouille 1 5E A-7Dirty0.121.332 ± 20653 ± 40 15N
Tetouille 1 5E A-8Dirty0.071.421 ± 22334 ± 36  
Tetouille 1 5E A-10Dirty0.050.6−35 ± 16317 ± 42  
Tetouille 1 5E A-12Dirty0.060.6−5 ± 17179 ± 35  
Tetouille 1 5E A-14Dirty0.091.1−32 ± 21555 ± 30 15N
Tetouille 1 5E B-1Dirty0.100.88−4 ± 21199 ± 53  
Tetouille 1 5E B-5Dirty0.050.8217 ± 2492 ± 40  
Tetouille 1 5E C-4Dirty0.160.35−1 ± 1177 ± 30  
Tetouille 1 5E C-5Dirty0.130.28−21 ± 1195 ± 35  
Tetouille 1 5E B-7Dirty0.061.7−122 ± 261400 ± 100 15N, −δ13C
Tetouille 1 5E A-4Globular0.060.6−11 ± 17431 ± 59  
Tetouille 1 5E A-17Globular0.060.8−1 ± 22552 ± 55 15N
Tetouille 1 5E B-3Globular0.040.7430 ± 24579 ± 70  
Tetouille 1 5E A-11Smooth0.200.8−20 ± 9311 ± 35  
Tetouille 1 5E A-16Smooth0.170.7−3 ± 10183 ± 21  
Tetouille 1 5E C-1Smooth0.050.44−21 ± 25355 ± 71  
Tetouille 1 5E C-2Smooth0.080.52−18 ± 15209 ± 57  
Tetouille 1 5E C-3Smooth0.070.540 ± 28308 ± 47  
Tetouille 1 5E A-15Smooth0.051.631 ± 2128 ± 27  
Tetouille 1 5E A-1Spongy0.090.5−41 ± 26188 ± 34  
Tetouille 1 5E A-9Spongy0.070.6−21 ± 13590 ± 110 15N
Tetouille 1 5E A-13Spongy0.030.4−123 ± 23377 ± 86 −δ13C
Tetouille 1 5E B-2Spongy0.050.75−16 ± 32204 ± 71  
Tetouille 1 5E A-2Vesicular0.210.7−33 ± 14481 ± 45 15N
Tetouille 1 5E B-6Vesicular0.160.62−4 ± 18263 ± 26  
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Figure 13.  EFTEM carbon map of an ultramicrotome section of IDP Chocha (left). δ15N isotopic image of the same section (right). The color scale bar shows the range of isotopic values measured in this sample.

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Figure 14.  EFTEM carbon image of an ultramicrotome section of IDP Chocha (top). δD isotopic image of the same section shown above (bottom). The color scale bar corresponds to the δD value in the H isotopic image. The outlines of seven morphologically distinct regions identified by BF and EFTEM images are shown in the δD image, including three spongy regions (V1–V3) and four vesicular regions (S1–S4). The shapes of the carbonaceous morphologies identified in TEM imaging correspond very well to isotopically distinct subregions in the H isotopic image.

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Figure 15.  NanoSIMS and TEM maps of particle Chuki. A) EFTEM carbon map of the ultramicrotomed section investigated. B) 28Si map of the particle, used to help in the alignment of the carbon map with the isotopic map. C) δ15N isotopic map of the section shown in (A). D) δD isotopic map of section shown in (A). The color scale bars show the range of isotopic values measured in this sample. The circles show the four areas in this sample where we found carbon. The linear bright features are artifacts produced during image acquisition. The arrows point to the only area where simultaneous 15N and D enrichments were measured.

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Figure 16.  NanoSIMS and TEM maps of particle GS-4. A) EFTEM carbon map of the ultramicrotomed section investigated. B) Bright field image of the particle investigated. C) δ15N isotopic map of section shown in (A). D) δ13C isotopic map of section shown in (A). The color scale bars show the range of isotopic values measured in this sample.

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Figure 17.  NanoSIMS and TEM maps of particle Tetouille. A) δ13C isotopic map of section shown in (C). B) δ15N isotopic map of section shown in (C). C) EFTEM carbon map of the ultramicrotomed section investigated. The color scale bars show the range of isotopic values measured in this sample. The arrow points the area where simultaneous 15N and 13C anomalies were found.

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Figure 18.  Carbon and N isotopic compositions of approximately 300 nm areas of carbonaceous regions of five of the IDPs analyzed. We did not include IDP Plin because, owing to its high porosity and fine grain size, δ15N values obtained from 250 nm bins in the N isotopic image were strongly influenced by background. The approximate ranges (dashed lines) of C and N isotopic compositions observed among terrestrial rocks and organic matter (δ13C = −65‰ to +34‰; δ15N = −50‰ to +50‰) are shown for comparison (Deuser 1970; Wada et al. 1981; Altabet and McCarthy 1985; Hofmann and Bernasconi 1998).

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Table 2 and Fig. 19 summarize the C and N isotopic compositions of 47 morphologically distinct carbonaceous regions observed in the IDPs. Table 3 shows the number of isotopic measurements made for each type of morphology. Almost all of these regions were found to be 15N-rich relative to terrestrial materials. In one case, a dirty carbon area showed a δ15N depletion of −135‰. In addition, two regions (spongy and dirty) were found to be δ13C-poor (∼−120‰) (Figs. 16 and 17). All of the carbonaceous morphologies displayed wide ranges of δ15N values and there are no clear discriminators among their isotopic distributions. The dirty, spongy, and vesicular morphologies show the largest δ15N enrichments (≥1000‰), yet the dirty and vesicular morphologies also show terrestrial N isotopic compositions in some cases. Only the spongy and globular morphologies had consistently anomalous N isotopic compositions.

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Figure 19.  Carbon and N isotopic compositions of all the carbonaceous morphologies analyzed. The approximate ranges (dashed lines) of C and N isotopic compositions observed among terrestrial rocks and organic matter (δ13C = −65 to +34‰; δ15N = −50 to +50‰) are shown for comparison (Deuser 1970; Wada et al. 1981; Altabet and McCarthy 1985; Hofmann and Bernasconi 1998).

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Table 3.   Number of isotopic measurements for each type of morphology.
Type of morphologyNumber of measurementsParticles where measurements were performed
Spongy14Chocha, GS-4, Tetouille
Globular7Plin, Nayeli, Chuki, Tetouille
Vesicular10Chocha, Plin, Chuki, GS-4, Tetouille
Smooth9Chocha, Chuki, GS-4, Tetouille
Dirty11GS-4, Tetouille

We performed H isotopic imaging on microtome sections of Chocha and Chuki. The Chuki section was thick enough that it was possible to obtain H isotopic images after the C and N images were acquired. Chocha has a significant bulk D-enrichment, with a δD value of 490‰. Regions in the Chocha section measuring 500 nm have δD values that range from −150‰ to approximately 1500‰, with errors ranging from 100‰ to 200‰ (Fig. 14). The H isotopic images of Chocha show several distinct D-rich and D-poor regions that are clearly correlated with the morphology of the carbonaceous material (Fig. 14). This sample mostly contains vesicular and spongy carbonaceous material. The spongy material is consistently D-rich, whereas the vesicular carbonaceous material is found to have both D-rich and D-poor regions. One D-poor coldspot was also identified, with δD = −600 ± 100‰. Chuki has a normal bulk H isotopic composition (δD = −210‰) and only a few small distinct carbonaceous regions. However, one of these regions (an organic globule) displayed enrichments in both δ15N (∼900‰) and δD (∼900‰) (spot #1 in Fig. 15). The δD values of this organic globule and of the D-rich material in Chocha significantly exceed the fractionation effects we observed in most of the test samples exposed to electron beam irradiation in the TEM (∼200‰), indicating that these D enrichments are real.

Most of the isotopic images contained small regions that show considerably higher isotopic anomalies than the immediately surrounding material. Such “hotspots” (strongly enriched in the minor isotope) or “coldspots” (depleted in the minor isotope) are frequently observed in IDPs and primitive meteorites. By reference to the previously obtained TEM maps, we were able to clearly identify the type of carbonaceous material associated with all but two of the hotspots/coldspots we observed (Table 2). In fact, the size and shape of the hotspots very closely correlated with that of the associated carbonaceous inclusions. In these samples, hotspots occurred with similar frequency as each of the morphological types of carbonaceous material.

Heating Experiments

Figures 20 and 21 show examples of two areas, spongy and dirty, before and after heating. No obvious changes or alterations were observed in any carbonaceous areas of this sample after heating. It could be argued that air should be used in the experiments instead of just N2, because all the particles are exposed to air during atmospheric entry. However, the interior of the particles where the carbonaceous phases are found were not exposed to air during their passage through the atmosphere. Heating experiments using air instead of N2 were not attempted because: (1) the oxygen in the air could burn the carbon film used to support the microtomed section on top of the grid; (2) the oxygen in the air would possibly oxidize some of the phases of the sample, including the carbonaceous ones; and (3) the goal of this experiment was to decipher the role of temperature, not the role of the air components, over the fate of texture and morphology of the carbonaceous phases.

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Figure 20.  Bright field micrographs of a spongy area before and after being heated at different temperatures. A) Unheated sample. B) Heated at 620 °C. C) Heated at 660 °C. D) Heated at 750 °C. E) Heated at 850 °C.

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Figure 21.  Bright field micrographs of a dirty area before and after being heated at different temperatures. A) Unheated sample. B) Heated at 620 °C. C) Heated at 660 °C. D) Heated at 750 °C. E) Heated at 850 °C.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Sample Preparation
  6. Analytical Techniques
  7. Results
  8. Discussion
  9. Conclusions
  10. References

Comparison with the Morphology of Other Extraterrestrial Materials

Carbon in all the samples studied is heterogeneously distributed throughout the particle: it occurs as coatings on mineral grains, as discrete areas, and as large continuous regions. In fact, carbonaceous material in anhydrous IDPs is often described as the matrix that binds the submicrometer mineral grains together. Most of the carbonaceous morphologies described in the Results section have been observed within each of the IDPs. None of the morphologies appear to be dominant in the IDPs we studied. High-resolution TEM imaging shows that all the carbonaceous phases found in our study are made of amorphous carbon. This is consistent with previous studies of carbon-rich nanoglobules observed in carbonaceous chondrites (Garvie and Buseck 2004; Garvie 2006; Messenger et al. 2008). Carbon studied previously in anhydrous IDPs (Keller et al. 1996, 2004; Messenger et al. 2008) was also found to be mostly amorphous, although small amounts of well-crystallized graphite and PGC were observed in two particles. PGC was observed surrounding sulfides in Tagish Lake (Nakamura et al. 2002) and Murchison (Brearley 2002) meteorites. The presence of PGC indicates thermal processing that requires temperatures higher than could be reached in the parent bodies of these meteorites, therefore suggesting that the relationship of sulfides with PGC occurred in the solar nebula (Brearley 2002). In our study, we did not find PGC. The carbon matrix in which sulfides are embedded is completely amorphous, indicating that this material has not been subjected to high temperatures after its formation.

Globular Morphology

Carbon-rich nanoglobules were first reported in some carbonaceous chondrites (Claus and Nagy 1961) and the acid-residue of the Orgueil meteorite (Rossignol-Strick and Barghoorn 1971). However, given that these studies were performed on the acid-residue of the meteorites and not in situ, they have long been viewed with skepticism owing to the possibility that these objects either formed during the residue preparation or were simply contaminants. Roundish carbonaceous structures were also observed in AMMs (Maurette et al. 1995; Matrajt 2001), but because the AMMs samples were embedded in epoxy for study, it was believed that those structures were artifacts produced during the polymerization of the epoxy. The first report of in situ observations of carbon-rich nanoglobules was made on the Tagish Lake meteorite (Nakamura et al. 2002). That study benefited from the fact that Tagish Lake was a freshly fallen (and thus very clean) meteorite, and the samples were prepared without the use of epoxy. Similar carbon-rich globules were later reported in other carbonaceous chondrites (Garvie and Buseck 2004), both in the crushed meteorites and in their acid residues (Garvie 2006). Coordinated isotopic and TEM studies of Tagish Lake organic globules later established definitively that the organic globules were not only indigenous to the meteorite but probably predated the formation of the planets (Nakamura-Messenger et al. 2006). Isotopically anomalous organic globules have since been identified in a number of meteorites, IDPs, and comet Wild 2 samples returned by the Stardust spacecraft (Busemann et al. 2004; Matrajt et al. 2008; Messenger et al. 2008; Floss and Stadermann 2009a, 2009b; Nittler et al. 2009a, 2009b; Stroud et al. 2009; DeGregorio et al. 2010a, 2010b, 2010c; Floss et al. 2011; Herd et al. 2011).

The nanoglobules found in the Tagish Lake meteorite (Nakamura et al. 2002) have apparent diameters ranging from 140 to 1700 nm, comparable to the sizes we observed (30–500 nm). However, many of the globules in Tagish Lake display a concentric layered structure and contain bubble-like inclusions. None of these features were observed in the globules of our study. In addition, with one exception, all the globules observed in Tagish Lake were hollow and the authors did not observe cylindrical or tapered forms (Nakamura et al. 2002). By contrast, we observed several forms of globules (i.e., hollow, filled, round, elongated, etc.) on more than one occasion (Fig. 7). Interestingly, the isotopically anomalous organic globules observed in the Bells CM2 meteorite also display a wider range of forms, including some riddled with numerous tiny vesicles (Messenger et al. 2008) similar to the spongy morphology we identified in these IDPs. The organic nanoglobules observed in CM meteorites (Garvie and Buseck 2004) occur individually and in clusters; some are hollow and some are filled and they exhibit a range of sizes. These nanoglobules are free of mineral grains and for the most part are amorphous. The globules in our IDPs are also amorphous and free of mineral grains. They also exhibit a range of sizes. One main difference between the globules in our IDPs and the globules observed in CMs is the size distribution: globules <50 nm are common and larger globules and clusters are rare in CMs (Garvie and Buseck 2004). By contrast, most globules in IDPs are approximately 150 nm in size and smaller globules and clusters are rare. Hollow nanotubes were reported only in the Tagish Lake meteorite (Garvie and Buseck 2004). We did not observe any tubes in the IDPs studied. It is possible that the spongy morphology originated from a network of carbonaceous nanotubes except that in IDPs most of these appeared filled, not hollow.

The chemical composition of the globules in our IDPs has not yet been determined. However, the NanoSIMS images show that all the carbonaceous materials in these IDPs contain abundant N and, where measured, also contain abundant H. The presence of abundant H in these materials indicates the presence of organic materials. Moreover, these globules are morphologically similar to those observed in Tagish Lake and CM meteorites, which have been shown to be organic in nature (Garvie and Buseck 2006a; Nakamura-Messenger et al. 2006; DeGregorio et al. 2010a, 2010b; Herd et al. 2011).

Smooth Morphology

The smooth morphology is made up of regions of uniform, featureless carbon. This type of morphology was reported previously in anhydrous CP IDPs (Keller et al. 1996, 2004). Amorphous carbonaceous regions with “compact” or “blocky” morphologies were also reported in CR3 chondrites (Floss et al. 2011). These textures probably correspond to the smooth morphology described in our study.

Dirty Morphology

The dirty morphology is made of mineral grains embedded in the carbonaceous material. These mineral grains are mostly sulfides. Sulfides rimmed by carbon-rich material were also observed in the Tagish Lake (Nakamura et al. 2002) and Murchison (Brearley and Abreu 2001) meteorites and the Wild 2 cometary particle Febo (Matrajt et al. 2008). In Murchison, the sulfides are ubiquitously associated with C-rich material (Brearley 2002). This carbonaceous material appears in two degrees of order: either as PGC, as observed in Tagish Lake (Nakamura et al. 2002), or as an N- and S-bearing amorphous carbon. The amorphous carbon may be related to meteoritic macromolecular insoluble organic material that can be extracted by acid-residue (Brearley 2002). In our study, sulfides are not just surrounded by carbon, they are embedded in it. In addition, the carbon is always amorphous. Fe-Ni sulfide grains of <100 nm occurring as discrete grains embedded in amorphous carbonaceous material were also reported in an anhydrous CP IDP (Keller et al. 2004).

Vesicular Morphology

The vesicular morphology is made up of chunks of amorphous carbon that have tiny bubble-like inclusions that we call vesicles. These types of inclusions have also been observed in many of the carbonaceous globules found in the Tagish Lake meteorite (Nakamura et al. 2002). Organic globules described as “aggregates riddled with tiny holes” were observed in the CM Bells (Messenger et al. 2008). This phase probably corresponds to our description of vesicular morphology. Vesicular carbon was also observed in anhydrous CP IDPs (Keller et al. 1996, 2004). The carbonaceous material in these samples was also found to be amorphous. Vesicular carbon in the IDPs studied by Keller et al. (1996, 2004) showed an apparent correlation with magnetite rims, which was interpreted by the authors as evidence of loss of volatile components during atmospheric entry. In our study, however, we did not find such correlation. In fact, we often observed the vesicular texture together with other morphologies but magnetite rims were rarely observed. Similarly, vesicular carbon-rich areas reported in the meteorite Bells did not seem to be related to high temperature processing given that no graphite-like domains or magnetite rims were found (Messenger et al. 2008).

Spongy Morphology

The spongy morphology consists of a lace or network of tiny voids which looks like a sponge. If the voids are densely packed, then the texture looks fluffy. This type of morphology was observed in the Orgueil residue (Garvie and Buseck 2006a) where it was then called fluffy texture, similar to crumpled tissue paper. In a later study on a CR2 type meteorite, Garvie and Buseck (2006b) reported the presence of what they described as “nanospheres with complex and vesicular hollow cores.” The micrograph of such a morphology looks identical to the spongy areas we have seen in our IDPs (fig. 1c in Garvie and Buseck 2006b). Therefore, what they called nanospheres with vesicular hollow cores appear to correspond to a spongy morphology, so we consider this as an observation of a material with spongy texture in a CR2 meteorite. In our study of the Orgueil residue, we noticed that this spongy type of texture is dominant over the other morphologies, followed by the globular morphology. Nanodiamonds have been reported to occur in the spongy morphology of the Orgueil residue but not in the globules (Garvie and Buseck 2006a). We did not search for nanodiamonds. Given their somewhat similar textures, there may be a connection between spongy and vesicular morphologies. If there were a single process to form these morphologies, perhaps the spongy texture represents the end state of the vesicular texture under that process. In this case, these morphologies would have a genetic relationship. But at present we do not know whether this is the case.

Other morphologies of carbonaceous material have also been reported in several meteorites. For example, in Murchison the C-rich areas occur as either distinct irregular-shaped, nanometer-sized clumps or as diffuse, sometimes elongated regions <20 nm in size (Brearley 2002). Carbonaceous flakes and tubes were reported in Tagish Lake, Bells, Cold Bokkeveld, Mighei, Murchison and Murray CM meteorites (Garvie and Buseck 2004), and Orgueil (Garvie 2006). Compact amorphous carbon grains were reported in CR3 chondrites (Floss et al. 2011).

Origin of Morphologies

Here, we consider the potential influence of the sample processing and heating during atmospheric entry on the morphology of the carbonaceous material.

Chemical Processing

These IDPs were embedded in acrylic for ultramicrotomy and the acrylic was later removed by the use of chloroform vapors. We evaluated how this sample preparation procedure may have modified the texture and morphology of the carbon-rich material in the IDPs by subjecting samples of Orgueil acid residue to the same procedures. After embedding samples of Orgueil acid residue in acrylic, sectioning these samples, and then removing the acrylic we observed the same range of morphologies as those observed in Orgueil residue just deposited over a TEM grid (Garvie 2006; Garvie and Buseck 2006a). This indicates that our sample preparation procedures do not substantially affect the morphology of the carbonaceous material. In addition, we observed the same range of morphologies of carbonaceous materials in the Orgueil residue as those observed in fresh meteorite samples that were just crushed over a TEM grid (Garvie 2006; Garvie and Buseck 2006a) indicating that the acid extraction methods are not responsible for producing the range of observed morphologies of the carbonaceous material. The texture and morphology of these carbonaceous materials appear to be resistant to chemical dissolutions because they are not appreciably altered after either acid or chloroform treatments.

Thermal Processing

It is also important to evaluate the effect of thermal processing during atmospheric entry on the carbonaceous material in these particles. First, we note that all of the morphologies were found in the same particles, often separated by a few nanometers or even in direct contact (Fig. 9). Consequently, except for small interior thermal gradients, these phases all experienced the same thermal history during atmospheric entry. Therefore, it is clear that these particles contained chemically and physically distinct types of carbonaceous materials prior to atmospheric entry. Second, we did not observe any significant change in the morphology of carbonaceous materials in the IDP subjected to controlled heating in the laboratory. The organic matter making up these morphologies is apparently refractory, as its texture is not sensitive to elevated temperatures. In addition, the carbon in all the morphologies is amorphous. The lack of crystalline structure suggests that the carbonaceous materials have not been appreciably affected by the temperature during atmospheric entry, perhaps because these organic compounds resist graphitization (Buseck and Bo-Jun 1985; Cody et al. 2008). Finally, most of these morphologies have been observed in primitive meteorites like Tagish Lake (Nakamura et al. 2002; Garvie and Buseck 2004) and in other carbonaceous chondrites (Garvie and Buseck 2004; Messenger et al. 2008; this study). The interiors of these meteorites were not significantly heated during atmospheric entry. Even the vesicular morphology that has often been associated with heating effects (Keller et al. 2004) was observed in the interior of the meteorite Bells where its texture (see fig. 2a in Messenger et al. 2008) is identical to some of the vesicular areas we found in our IDPs (Fig. 10). Thus, these carbonaceous materials appear to be refractory because their textures and morphologies were not significantly changed by exposure to high temperatures in our heating experiments, indicating that the morphologies found in our samples are pre-atmospheric entry.

Isotopic Constraints on the Origins of Carbonaceous Components

Origins of H, C, and N Isotopic Anomalies

The H and N isotopic compositions of the carbonaceous materials in these IDPs span a wide range of values, as is typically observed in anhydrous CP IDPs and organic matter in meteorites (Messenger 2000; Aléon et al. 2003; Busemann et al.2006; Floss et al. 2004, 2006; Keller et al. 2004). The δ15N values we measured (∼−135‰ to 1400‰) largely cover the range observed in IDPs. On the other hand, the δD values we measured (−600‰ to 1500‰) do not reach the highest enrichments observed in IDPs (δD = 20,000–50,000‰; Messenger 2000). The C isotopic compositions we measured mostly fell between −50‰ and +50‰, the typical range of values measured in IDPs and insoluble meteoritic organic matter (Alexander et al. 1998, 2007; Messenger et al. 2002). The two C isotopic anomalies we measured (Figs. 16, 17, and 19) are of similar magnitude to the moderate anomalies rarely observed in IDPs and meteoritic organic matter (Busemann et al. 2004; Floss et al. 2004; DeGregorio et al. 2010a). Below we summarize the constraints on the origins of these isotopic anomalies.

It has long been argued that the highly elevated D/H ratios observed in meteorites and IDPs originated by low-temperature isotopic fractionation in a cold molecular cloud environment (Geiss and Reeves 1981; Zinner 1988; Messenger and Walker 1998). In these environments, the difference in binding energy between H and D may greatly exceed the thermal energy, leading to enormous isotopic fractionation during chemical reactions. Despite the low T, cold molecular clouds maintain an active chemical environment, enabled by ion molecule chemistry. Simple molecules in cold molecular clouds are observed to have very large enrichments in D, with D/H ratios reaching thousands of times the local average (Millar et al. 1989). However, recent theoretical models have raised the possibility that similar chemical fractionation effects could occur in the outermost regions of the solar nebula (Aikawa and Herbst 1999) where conditions are similar to cold molecular clouds.

Earlier studies reported rare IDPs with very low D/H ratios reaching δD = −510‰ (Messenger et al. 2002). We also found a D-poor component in the IDP Chocha with δD = −600 ± 100‰. Although deuterium is rapidly destroyed in stars by hydrogen burning, a nucleosynthetic origin is highly unlikely because these D-poor components are not accompanied by large C isotopic anomalies. On the other hand, these D-poor components nevertheless have higher D/H ratios (∼8 × 10−5) than the bulk solar system (∼2.5 × 10−5; Geiss and Gloecker 1998). It remains unclear why these materials are isotopically distinct from most other H-bearing materials in meteorites and IDPs.

The origins of the N isotopic anomalies are less certain. Unlike H, anomalous N has not been observed in the interstellar medium. In part, this is attributable to the difficulty of spectrally resolving the isotopomers of N-bearing molecules. In principle, nucleosynthetic processes may generate highly anomalous N isotopic ratios (Clayton 2003). However, in this case, N isotopic anomalies should be accompanied by equally large C isotopic anomalies, which greatly exceed the anomalies we measured in these IDPs. Therefore, a nucleosynthetic origin is unlikely. A spallation origin is also unlikely, given the magnitudes of the anomalies observed (Geiss and Bochsler 1982). Finally, there has been speculation that photochemical self-shielding of N2 in the solar nebula could lead to substantial N isotopic fractionation. However, recent modeling indicates that the magnitude of this effect is well below the magnitude of the anomalies observed in primitive solar system organics (Lyons 2010).

By process of elimination and by the association of N isotopic anomalies with D-rich materials (as we and others have observed), low-T chemical fractionation is implicated for the origin of 15N enrichments. However, owing to the small relative difference in the masses of the N isotopes, substantial isotopic fractionation may only be possible at very low temperatures (<10 K; Charnley and Rodgers 2004). The only plausible locations where chemical fractionation could occur at such low temperatures are the cores of cold molecular clouds or near the midplane of the outermost reaches (>100 AU) of the protoplanetary disk.

The origins of 15N depleted materials are uncertain. There have been only a few reports of 15N depletions, including a cluster IDP (Messenger 2000) with a δ15N = −93‰, one case in Floss et al.’s (2006) study with bulk δ15N = −108‰, a few cases in CR3 chondrites (Floss and Stadermann 2009b) and only one case in our study with δ15N = −135‰, corresponding to a dirty morphology. It has been suggested that 15N depletions could have a nucleosynthetic origin, provided they are accompanied by a comparably large C isotopic anomaly (Messenger 2000). However, given that we did not observe large C isotopic anomalies, the nucleosynthetic origin is unlikely. On the other hand, measurements of the Jovian atmosphere and of the solar wind have shown that the solar system nitrogen isotopic composition is highly depleted in 15N (δ15N ∼ −400‰, Abbas et al. 2004; Marty et al. 2010, 2011a, 2011b). Thus, analogous to H isotopes, even the most 15N depleted components of IDPs (relative to terrestrial values) are enriched in 15N relative to the average solar system. The 15N-poor materials may somehow have partially equilibrated with protosolar or interstellar N2 gas or avoided significant 15N enrichment.

Considering how common and how large N isotopic anomalies are in primitive organic matter, it is surprising that even larger anomalies are not observed in C isotopes, where the relative difference in isotopic mass is greater than that of N isotopes. This conundrum has not been resolved, but it has been suggested that the answer lies in the differing volatilities of CO and N2, the major reservoirs of C and N in the interstellar medium. Although the specific mechanism of N isotopic fractionation remains unclear, organic molecules condensed into ices or refractory grains may retain their 15N enrichments because the isotopically normal (or light) N2 gas does not condense even at such low temperatures. By contrast, molecules with C isotopic anomalies may isotopically equilibrate with isotopically normal CO, which does condense onto grains. Whether this occurs is likely to be sensitive to the temperature and density of the local environment.

Both of the C anomalous spots we identified are associated with 15N enrichments. Interestingly, one of them (δ13C = −122 ± 26‰) also shows the greatest N isotopic anomaly we measured (δ15N = 1440 ± 100‰). A spongy area having the second highest δ15N enrichment we measured (1360 ± 32‰) also has a light C isotopic composition (δ13C = −60 ± 12‰). These grains have a very similar composition to an unusual anomalous inclusion identified in an IDP by Floss et al. (2004) and to a region in IOM of a CR2 chondrite (Busemann et al. 2006). This apparent correlation of the light C and heavy N component strongly suggests the C anomaly originated by isotopic fractionation. Organic grains in IDPs with this light C and heavy N isotopic composition may be related by either formation process or location.

Isotopic Heterogeneity and Hotspots

The IDPs we analyzed were isotopically heterogeneous at micrometer spatial scales (Figs. 13–17 and 22). For example, we found within IDP Chocha (Fig. 14) both a very low D/H ratio (δD = −600‰) along with a high D/H ratio (δD = 1500‰). Large H isotopic variations were also found in the cluster IDP Tetouille. We also observed a 15N-depleted area within 1 μm of a 15N-rich region of the particle Tetouille (Fig. 22). Such spatial variations in H and N isotopic compositions are commonly observed in IDPs (Messenger 2000; Floss et al. 2006). This isotopic heterogeneity is an indicator of the primitive nature of these samples.

image

Figure 22.  Isotopic map of one fragment of particle Tetouille, a cluster IDP. A) NanoSIMS elemental N map, showing N concentrations throughout the particle. B) δ15N isotopic map of section shown in (A). C) TEM bright field image of section shown in (A). D) EFTEM carbon map of section shown in (A). The yellow arrows point to the area where the 15N depletion is located. It corresponds to a dirty morphology. Scale bar = 2 μm.

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We found that the spatial variations in H, C, and N isotopes are directly attributable to morphologically distinct carbonaceous inclusions in the IDPs. Yet, there is no clear systematic correlation between morphology and isotopic composition (Fig. 19). Nevertheless, essentially all of the isotopically distinct hotspots are clearly associated with morphologically distinct carbonaceous inclusions. Even immediately adjacent morphologically distinct regions are consistently observed to be isotopically distinct from each other, as exemplified in the H isotopic image of Chocha (Fig. 14). Although hotspots may not necessarily express the largest anomalies, they do appear to have escaped isotopic equilibration with their surroundings. These observations suggest that the isotopically and morphologically distinct inclusions in these IDPs were not appreciably changed after being incorporated into the IDP parent body.

Based on their N isotopic compositions, IDPs have been divided into two groups (Floss et al. 2006): the “isotopically primitive” and the “isotopically normal” groups. In the first group, one can find IDPs that have an anomalous bulk N composition along with abundant 15N-rich hotspots, occasional C isotopic anomalies, and abundant presolar silicate grains. The second group consists of IDPs that have normal bulk N, with occasional 15N-rich hotspots but no C isotopic anomalies and lower abundances of presolar silicate grains. Our results are very consistent with this classification: the three particles (Chocha, GS-4, and Tetouille) where we found anomalous bulk N are also the only particles where we found 13C depletions (Table 2). In addition, particle Chocha has also anomalous bulk H isotopic compositions and particle Tetouille has occasional 15N-depleted “coldspots.” The rest of the particles have normal bulk N isotopic compositions with several 15N-rich hotspots and no C isotopic anomalies.

Origin of the Different Morphologies in IDPs

The variations in both the morphology and isotopic signatures of the carbonaceous materials in these IDPs suggest that the organic materials formed in different environments and/or by different chemical and physical processes. Organic phases in CM and Tagish Lake meteorites also display a range of compositions, morphologies, and sizes for which different origins have been suggested (Garvie and Buseck 2004). Garvie and Buseck (2006a) reached a similar conclusion for the two morphologies observed in the residue of Orgueil, which differ in structure, order, and chemical composition.

Organic globules are perhaps the most well-recognized morphologically distinct carbonaceous phase in primitive solar system materials. The first organic globules measured for their isotopic compositions were found in Tagish Lake (Messenger et al. 2004; Ashley et al. 2005; Nakamura-Messenger et al. 2006). In all these studies, it was found that all the globules have elevated 15N/14N ratios with δ15N ranging from 200‰ to 1000‰. Later studies on C-rich globules found in the meteorite Bells (Messenger et al. 2008) showed an even wider range of δ15N values from 500‰ to 2000‰. A C-rich globule measured in an anhydrous IDP had a δ15N = 700‰ (Messenger et al. 2008). The organic globules we measured had δ15N values ranging from approximately 200‰ to 900‰ (Fig. 19). Many of the past studies also reported H isotopic measurements for some of the globules (Messenger et al. 2004, 2008; Nakamura-Messenger et al. 2006) where all the globules showed elevated δD values ranging from 1800‰ to 8100‰. The one globule for which we measured H isotopic compositions had a moderate enrichment of δD = 960‰ (Table 2). Organic globules in meteorites show a narrow range of C isotopic compositions, with δ13C varying from −50‰ to 100‰ (Ashley et al. 2005; Nakamura-Messenger et al. 2006). All the globules measured in our study have δ13C values that fall within this range.

Tiny flakes of the hydrated mineral saponite were observed within some of the Tagish Lake organic globules (Nakamura et al. 2002), which the authors interpreted as evidence of the globules’ formation on the asteroidal parent body through aqueous alteration. However, pronounced isotopic anomalies clearly argue against such an origin. We have identified organic globules in both hydrated and anhydrous IDPs, providing further evidence that their formation is not linked to aqueous alteration. Indeed, given the isotopic anomalies of these globules, their formation did not happen within the parent body but before they assembled into the parent body. These isotopic signatures indicate origins in a very cold environment, where condensation of volatile organic ices occurs. The organic globules are postulated to have formed by the radiation processing of interstellar organic ice grains (Nakamura-Messenger et al. 2006). The commonly observed hollow cores may have originally hosted a water ice grain that served as the nucleation site.

The various other forms of carbonaceous materials we have identified may have been equally widespread in the early solar system, but apparently formed in somewhat different conditions. As all of the carbonaceous morphologies express similar ranges of isotopic anomalies, it is likely that they originated in a similar environment as the organic globules. The broad range in isotopic and chemical compositions of these materials may reflect the sensitivity of interstellar and nebular chemistry to the temperature, density, and ionization rate. These properties also strongly influence the size and composition of condensed species. The morphology of the carbonaceous materials provides some clues to their formation conditions. For example, the vesicular and spongy morphologies may have obtained their characteristic foamy texture by sublimation of nanometer-sized volatile ice grains entrained within refractory organic material formed by radiation processing. A similar formation process has been proposed for the organic globules observed in the Tagish Lake meteorite (Nakamura-Messenger et al. 2006). One interesting observation from the study performed by Keller et al. (2004) is that they found a region with δD = 10,000‰ described as a poorly ordered carbonaceous material with embedded GEMS and fine-grained sulfides, hence corresponding to a dirty morphology. Unfortunately, in our study none of the sections analyzed by NanoSIMS for H isotopic compositions contained dirty morphologies. The highest δD values measured in our study correspond to a spongy morphology and they are one order of magnitude lower than the ones measured in the dirty morphology by Keller et al. (2004) and in the carbonaceous areas measured by Busemann et al. (2004). As with the organic globules, each of the differing types of isotopically anomalous organic grains probably formed as free-floating icy condensates in the outermost portions of the solar nebula or a preceding cold molecular cloud.

The carbonaceous phases observed in IDPs are refractory, given that they survived pulse heating up to T = 850 °C. It is believed that refractory carbonaceous material is a component of interstellar dust (Gibb and Whittet 2002). In fact, carbonaceous nanoparticles have been observed in interstellar clouds (Duley 2001). The isotopically anomalous carbonaceous phases we observe in IDPs and primitive meteorites may be closely related to such cold molecular cloud grains, and some may in fact have originated in those environments. If so, their refractory nature allowed them to survive intact since their genesis and until they reached the Earth’s surface.

Organic globules seem to be ubiquitous among primitive solar system materials and share similar ranges of H, C, and N isotopic compositions. Organic globules have now been reported in carbonaceous chondrites (Garvie and Buseck 2004, 2006), Tagish Lake (Nakamura et al. 2002; Garvie and Buseck 2004; Nakamura-Messenger et al. 2006), AMMs (Maurette et al. 1995; Matrajt 2001), IDPs (Messenger et al. 2008; Busemann et al. 2004; this study), and Stardust comet Wild 2 samples (Matrajt et al. 2008; DeGregorio et al. 2010). Organic globules were a fundamental component of the solar system starting materials, apparently predating the formation of the asteroids, comets, and planets. The other distinct morphologies of carbonaceous materials we have identified here may represent new types of primordial organic building blocks. It is remarkable that these carbonaceous materials may have been transported across vast stretches of the solar system largely intact.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Sample Preparation
  6. Analytical Techniques
  7. Results
  8. Discussion
  9. Conclusions
  10. References

We identified five distinct types of carbonaceous morphologies in IDPs: globular, spongy, vesicular, smooth, and dirty. All the morphologies were found to be isotopically anomalous to varying extents. A number of isotopically distinct hot spots and cold spots were also found and their corresponding carbonaceous morphologies were identified. The comparison of these morphologies with similar ones reported in carbonaceous chondrites and Tagish Lake shows that some of these morphologies, particularly the carbonaceous globules, are ubiquitous. The ubiquitous character of the globules and their isotopic compositions indicate that these morphologies are pristine, and predate the formation of the IDP parent body. The diversity in the isotopic composition and morphology of these materials shows that they originated from multiple reservoirs. Those carbonaceous phases showing 15N and D enrichments probably formed in a cold environment such as the presolar molecular cloud or at the edge of the forming solar system. Those showing D and 15N depletions might have formed through other processes and in other environments. Whatever their origins, these are among the most ancient, pristine, fundamental building blocks of solar system organic matter ever recognized. Future detailed investigations of these materials are expected to provide valuable insight into the earliest evolutionary stages of solar system organic matter.

Acknowledgments–– The authors wish to thank C. Floss and L. Nittler for their helpful comments and reviews. The IDP samples were provided by the cosmic dust curation facility at NASA Johnson Space Center. The Orgueil residue was kindly provided by the Muséum d’Histoire Naturelle in Paris, France. This work was supported by NASA grants NNX10AI89GS01 and NNG06GG00GS05. G. M. wants to thank Nayeli Pagès-Matrajt who inspired and supported her work.

Editorial Handling–– Dr. Scott Sandford

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