Origins of H, C, and N 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.
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.