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Wieler et al.: Trapping and Modification of Noble Gases and Nitrogen in Meteorites 499

499

Trapping and Modification Processes of Noble Gasesand Nitrogen in Meteorites and Their Parent Bodies

Rainer WielerEidgenössische Technische Hochschule (ETH) Zürich

Henner BusemannUniversity of Bern (now at Carnegie Institution of Washington)

Ian A. FranchiOpen University

We review the inventories of primordial noble gases and nitrogen in meteorites, their car-rier phases, how and where they may have been incorporated, as well as processes modifyingtheir abundances on meteorite parent bodies. Some of the many distinct noble gas and nitro-gen components have an isotopic composition very different from that in the Sun. These com-ponents reside in presolar grains. “Anomalous” noble gas and nitrogen components thus areused to infer parent stars of presolar grains as well as theories of stellar nucleosynthesis. Othernoble gas components have an isotopic signature roughly similar to the solar composition. Someof these “normal” components are also carried by presolar grains and probably approximatelyrepresent the average isotopic composition of their parent stars. Carriers of other normal com-ponents remain ill-defined and their origin unclear. Their isotopic identity was possibly estab-lished in the solar nebula, but it appears increasingly likely that this often also happened earliersomewhere in the presolar molecular cloud. Apart from allowing us to study meteorite forma-tion, primordial noble gases and nitrogen also are important tracers to constrain the metamor-phic history of meteorite parent bodies.

1. INTRODUCTION

Noble gases are extremely rare in meteorites. A remark-able consequence of this scarcity is the impressive variety ofdifferent noble gas “components” that can be distinguishedin meteorites, i.e., reservoirs with a relatively well-definedelemental and/or isotopic composition of a more or lesswell-understood origin. Podosek (2003) gives an insightfulexplanation of this basic feature of noble gas cosmochemis-try. Some components were produced in situ, e.g., the cos-mogenic and the radiogenic noble gases discussed else-where in this book (Eugster et al., 2006; Krot et al., 2006).Other components were trapped by the meteoritic materi-als or their precursors. These trapped noble gases — in par-ticular the primordial noble gases trapped very early in solarsystem history — are the focus of this chapter.

Among all elements, N is most similar to the noble gasesin cosmochemical behavior. Although in a few minerals Nbelongs to the crystal structure, more often than not it ispresent as an ultratrace element only. Often N has also beentrapped (or produced in situ) by similar processes as noblegases, and its variability in isotopic composition in mete-orites is among the largest of all elements. Furthermore, Nand noble gases are often analyzed in parallel on the same

sample. For all these reasons, it is logical to include someaspects of N in the discussion here.

The multitude of trapped gas components allows one tostudy a wide variety of processes throughout most of theepochs discussed in this book, and some longstanding prob-lems in noble gas cosmochemistry have their roots in thefact that it is not always clear in what particular epoch a cer-tain component received its identity. It is nevertheless appro-priate that this chapter is found in Part VI of this volume,because the “accretionary epoch” certainly has left manyimprints on the record of trapped noble gases and N.

The multitude of distinct noble gas and N componentsmakes the subject notoriously difficult to follow. In thewords of Ozima and Podosek (2002), “noble gas geochem-istry often seems to non-practitioners to have much the airof the secret society and its dark art.” This certainly holdsno less for noble gas cosmochemistry. We try to mitigate thisproblem by focussing on processes that may have shapedthe noble gas and N record in meteorites rather than on thedetails of the various components. Readers are referred to arecent review by Ott (2002) for a comprehensive presenta-tion of the component structure of trapped noble gases inmeteorites and details of their composition. We will adoptOtt’s nomenclature, which is an attempt to structure the

500 Meteorites and the Early Solar System II

noble gas “alphabet.” Another comprehensive recent reviewon noble gases in meteorites is by Podosek (2003), whileGrady and Wright (2003) review the available N data. Sev-eral review papers in books (Porcelli et al., 2002; Kallen-bach et al., 2003) also discuss various aspects of noble gasand N cosmochemistry.

In the following we first define the relevant terminologybefore we discuss carriers and inferred trapping processesfor various components and constraints from primordial no-ble gases and N on the accretionary history of meteoritesas well as the metamorphic history on parent bodies. In thefinal section we give an outlook.

2. COMPONENTS

Trapped noble gases in meteorites have traditionally beensubdivided into “solar” and “planetary” varieties (Signerand Suess, 1963). Signer and Suess recognized that in somemeteorites the elemental abundances are similar to those ex-pected for the Sun, whereas in other meteorites the lighternoble gases are heavily depleted relative to Xe and solarabundances, with abundance patterns resembling that ofEarth’s atmosphere. Later it was realized that the isotopiccomposition of the solar noble gases in meteorites indeed isvery similar to that in the solar wind (Geiss, 1973) and thatelemental abundances of the “planetary” gases in meteoritesalso resembled those of Venus and Mars.

Early workers called both the solar and the planetaryvarieties “primordial.” This term implies that the gases hadbeen incorporated very early, either in precursor solids ofmeteoritic materials or during formation of meteorites andtheir parent bodies. However, it was soon recognized that thesolar noble gases had been trapped from the solar wind inregoliths on asteroids (Eberhardt et al., 1965), similar to thelarge amounts of solar-wind noble gases implanted into thedust grains on the lunar surface (e.g., Pepin et al., 1970).While at least for some meteorites this trapping may haveoccurred early in solar system history, the solar componentnevertheless lost the status of being “primordial,” since it isnot thought that the solar noble gases in asteroidal regolithsgenerally reflect the accretionary or preaccretionary historyof meteorites. Wieler (1998, 2002) and Podosek (2003) pro-vide recent reviews on regolithic solar noble gases. We willuse solar gases as a baseline composition, but not discussthem otherwise. However, we will address recent work indi-cating that some meteorites contain solar-like gases thatappear to deserve again the attribute “primordial,” as theyseemingly were trapped during the formation of the solarsystem. We will also review attempts to derive the solar N-isotopic composition from lunar dust samples. Note alsothat the true abundances and isotopic compositions of noblegases and N in the Sun (more precisely, its outer convec-tive zone) might be slightly different from measured valuesin the solar wind (Wieler, 2002). However, for the purposesof this paper these differences are small and will hardly beof concern.

Unlike the solar component, the origin of Signer andSuess’ planetary noble gases has resisted a straightforward

understanding, and partly remains mysterious despite de-cades of effort. What has become clear is that they repre-sent a complex mixture of components of diverse origin,residing in different carrier phases, only some of which havebeen identified. It has also become clear that many parts ofthis mixture — if not all — were trapped into their carriersprior to accretion, either in the solar nebula or in presolarenvironments, and it is highly questionable whether thesegases are the major source of the noble gases in the terres-trial planets. In order not to perpetuate such a misleadinglink, many workers thus discontinued to use the term “plan-etary” for noble gases that actually are found in meteoritesonly. This is also the position taken here. Note, however,that in the view of others the term is still useful to denote theensemble of meteoritic components strongly depleted in thelight elements relative to solar abundances (e.g., Podosek,2003), in part because no well-accepted alternative term hasemerged. We will sometimes use the term “primordial noblegases” instead, which, however, would also encompass pri-mordially trapped noble gases of solar-like composition.

Table 1 (modified from Ott, 2002) lists the most impor-tant primordial noble gas components. A few additional im-portant terms should be introduced here (cf. Podosek, 2003).Some primordial noble gas components have isotopic com-positions not too different from solar. These are termed “iso-topically normal.” Normal components may be of “local”origin, i.e., derived from a solar-like composition after thesolar nebula had been isolated from the surrounding mo-lecular cloud, or they may represent a presolar mixture fromvarious nucleosynthetic sources that resembles the solarmix. The counterpart to “normal” is “anomalous.” An anom-alous component has an isotopic composition sufficientlydifferent from solar in at least one noble gas as to excludea possible derivation from a reservoir of solar (or approxi-mately solar) composition. Anomalous components thusindicate (possibly diluted) contributions from specific nu-cleosynthetic processes in stars. Anomalous componentsare carried by presolar grains (which may also carry normalcomponents). Some of these grains formed around otherstars, and their components are also often called “exotic.”We will avoid using this term, because such componentscan at the same time be normal. Note, however, that someauthors use “exotic” as a synonym for “anomalous.”

Figure 1 shows several isotopic ratios of one anomalous(HL) and two normal (P3 and Q) components, normalizedto solar composition. Details are given in the figure cap-tion and below in the sections on presolar diamonds andphase Q. The figure illustrates that anomalous componentsoften have isotopic compositions very distinct from solar(e.g., the factor of 2 enrichment of both lightest and heavi-est isotopes in Xe-HL). However, for the light noble gasesHe and Ne the characterization as normal or anomalous isoften not based primarily on isotopic composition but ratheron relations with the respective components in the heaviergases. An example is again the HL component.

Nitrogen, with only two stable isotopes, is much moreambiguously assigned to such headings as normal or anom-alous. The willingness of N to participate in chemical reac-


tions means that large isotopic fractionations associated withreactions at low temperature are difficult to resolve unam-biguously from the contribution of anomalous componentsunless clearly associated isotope signatures from other ele-ments are present.

Figure 2 displays the elemental abundances of majorcomponents, normalized to solar composition. It should benoted that it is not always possible to quantify or even iden-tify specific N components associated with all of the noblegas components, although some success has been achievedin identifying N-isotopic signatures associated with some ofthe noble gas components. Details about the elemental and

TABLE 1. Major primordial noble gas components.

Component Alternative Names* Carriers Remarks

Q gases P1, OC-Xe† phase Q dominates Ar, Kr, Xeδ15N = –50‰ to –15‰

Ureilite gases Kenna type† diamond, graphite very similar to Q gasesδ15N = ≈–120‰

P3 gases Ne-A1 presolar diamond isotopically normalP6 gases presolar diamond isotopically normal?HL gases Ne-A2‡ presolar diamond nuclear component (r-process?)N component SiC and presolar graphite§ isotopically normalG component Ne-E(H), Kr-S, Xe-S SiC and presolar graphite§ nuclear component (s-process)R component (Ne only) Ne-E(L)¶ presolar graphite radiogenic 22Ne from 22NaPrimordial solar bulk/silicates**

* Alternative names often refer to one noble gas only, although the component may be defined for all five noble gases.† As Q gases are ubiquitous in all primitive meteorite classes, we discourage the use of host-specific names for this component.‡ Ne-A2 = Ne-HL + Ne-P6.§ SiC and graphite probably carry discrete N and G components respectively.¶ Ne-E(L) = Ne-R + Ne-G (Amari et al., 1995a).

** “Subsolar” gases, mainly found in enstatite chondrites, are a mixture containing a small fraction of solar noble gases (section 3.3.5).

Table adapted from Ott (2002), where the isotopic compositions of these and other components are also compiled. Most componentsoccur in all primitive meteorite classes, some in achondrites as well.

Fig. 1. Selected isotopic ratios of one isotopically “anomalous”and two “normal” noble gas components in meteorites (data fromOtt, 2002). For Xe compare also with Fig. 4. The anomalous HL aswell as the normal P3 component are both carried by presolar dia-monds; the Q component resides in the ill-defined phase Q. Iso-topic ratios are normalized to the protosolar composition for He[assumed to be identical to that in Jupiter (Mahaffy et al., 1998)],to best estimates for the bulk solar composition for Ne and Ar(Wieler, 2002) and to solar-wind values for Kr and Xe (Wieler,2002). Protosolar He rather than solar wind He is used for nor-malization because the latter has become enriched in the Sun byD burning. In the three heavy noble gases, the anomalous HL com-ponent is much less close to solar composition than the normal Qand P3 gases. Most diagnostic is the factor of ~2 enrichment inboth the lightest and the heaviest isotopes in Xe-HL. However, inHe and Ne no such clear distinction is seen. The figure thereforealso shows that for the light gases the attributes “normal” andanomalous” are not based on isotopic composition but on affinitieswith the respective heavy gases, such as similar release character-istics.

Fig. 2. Elemental abundances of major primordial noble gascomponents in meteorites (Ott, 2002), normalized to abundancesestimated for the Sun (Wieler, 2002). The abundance pattern of theterrestrial atmosphere is also shown (except for He, which escapesfrom the atmosphere).


isotopic composition of the various noble gas and N compo-nents in meteorites are given by Ott (2002) and Grady andWright (2003).

3. ORIGIN OF CARRIERS AND TRAPPINGPROCESSES OF PRIMORDIAL NITROGEN

AND NOBLE GASES

3.1. Component Separation

Early workers separated various noble gas componentsby stepwise heating or physical separation of bulk meteor-ite samples (Reynolds and Turner, 1964; Black and Pepin,1969; Eberhardt, 1974). Such work provided clear-cut evi-dence that presolar solids had survived in meteorites. In-vestigating these, Lewis et al. (1975) found that most of theprimordial noble gases of the Allende carbonaceous chon-drite reside in the residue that remains after dissolving thebulk meteorite in HF and HCl. Since this landmark paper,work on primordial noble gases largely concentrated onacid-resistant residues and much of the subsequent discus-sion will be focused on such data. Notwithstanding the hugesuccess of this approach, it also has its potential pitfalls;however, since primordial gas components residing in acid-soluble phases might go largely undetected, just as the“burning the haystack to find the needle” approach to iso-late presolar grains will fail to recognize less-resistant typesof such grains. Indeed, we will discuss also recent in vacuoetch studies of bulk meteorite samples that have allowedcharacterization of primordial noble gas components in away never before possible.

In the following, we first discuss the noble gas and Ncomponents in bona fide presolar grains, before treating thecomponents whose origin or carriers are less clear.

3.2. Presolar Grains and Their NobleGases and Nitrogen

Arguably, the most remarkable fact about noble gasesin meteorites is that preserved presolar grains contain a size-able fraction of the bulk noble gas budget. Therefore, noblegases are ideal to trace enrichments or depletions of presolargrains in separated phases. Nitrogen shows similarly ex-treme variations, but in most cases the analyses are per-formed independently of the noble gases, often benefitingfrom analyses of individual grains with corresponding iso-topic measurements of other elements in the same grains,although hardly ever noble gases. Therefore, it is not alwaysstraightforward or even possible to link the N informationto that obtained from the noble gases.

All types of presolar grains studied for noble gases (SiC,graphite, and nanodiamonds) contain both anomalous andisotopically normal components. The anomalous compo-nents can be linked to specific nucleosynthetic sources, e.g.,the s-process [for a discussion of nucleosynthetic processessee Meyer and Zinner (2006)], whereas the normal com-ponents presumably represent a mixture of various sources

representative of the various parent stars of the grain as-semblage, just as the solar noble gas composition representsthe average of all sources having contributed to the Sun. Wenow present first an overview of the noble gas and N inven-tories in the various types of presolar grains and then discusstrapping and modification processes.

3.2.1. Noble gas and nitrogen components in presolarsilicon carbide, graphite, and diamonds. 3.2.1.1. Siliconcarbide. A normal (N) and an anomalous (G) noble gascomponent are present in SiC separates from the MurchisonCM2 chondrite (Lewis et al., 1990, 1994; cf. Ott, 2002) (Todistinguish the N component from the elemental symbol Nfor nitrogen, the N component, and hence all other acro-nyms for noble gas components, are italicized.) Silicon car-bide grains show isotopic anomalies in many elements asexpected for contributions from s-process nucleosynthesis(Meyer and Zinner, 2006), strongly indicating an origin inasymptotic giant branch (AGB) stars for the large majorityof the SiC grains. Indeed, both the N and the G componentare remarkably close in composition to values expected forthe envelope and the s-process region of AGB stars respec-tively (Gallino et al., 1990, Lewis et al., 1994) (see Fig. 3).

The N component is thus a mixture of the noble gasesthat made up the bulk of the many AGB stars that contrib-uted SiC grains to the Murchison meteorite. Although iso-topically normal, the N component is enriched in the twoheaviest noble gases Kr and Xe by more than 3 orders ofmagnitude relative to He and Ne and solar composition.

In contrast, the G component (“G” stands for “giant”)represents products of nucleosynthesis in the s-process re-gion in C-rich red giant stars in their AGB phase, as is dis-cussed next. Neon very highly enriched in 22Ne was amongthe first anomalous components recognized (Black andPepin, 1969; Eberhardt, 1974). Known as Ne-E, it was orig-inally thought to derive from the decay of very short-lived22Na (half-life = 2.6 yr). This remains the accepted interpre-tation for the major fraction of the 22Ne in presolar graph-ite (see below). However, the theoretical and experimentalwork on SiC has shown that the subcomponent known asNe-E(H) actually also contains minor amounts of 20,21Neand was produced by stellar nucleosynthesis in the samezone as the s-process-enriched Kr and Xe in SiC grains(Lewis et al., 1990, 1994; Gallino et al., 1990; Straniero etal., 1997). This has been confirmed by He and Ne analy-ses on single SiC grains by Nichols (1992), Nichols et al.(1992a, 1994), and Heck et al. (2005). Only SiC grains richin 22Ne also contain 4He above background, with 4He/22Neratios indicating an origin of this component in the regionof s-process nucleosynthesis (Straniero et al., 1997) in (low-mass) AGB stars. Ne-E(H) has thus been rebaptized Ne-G.

Kr-G and Xe-G are both strongly enriched in the isotopesproduced only by the s-process (relative to the N componentor solar composition). Xenon-G as well as several other im-portant Xe components discussed later are shown in Fig. 4.Kr-G is particularly interesting, because its composition de-pends on the neutron density and temperature during the s-process (Ott et al., 1988, Gallino et al., 1990). Lewis et al.


(1990, 1994) showed that the sensitive Kr-isotopic ratiosvary with the grain size of the SiC. The Kr-isotopic compo-sition correlates with variations in the elemental abundances,especially the Ne/Xe ratio. Gallino et al. (1990) and Pigna-tari et al. (2003) showed that the measured Kr-G and Xe-G compositions are strikingly similar to values expected forAGB stars of ~1.5–3 M and metallicities slightly less thansolar. The Ne and Ar data support this conclusion (Lewiset al., 1994).

Stepped heating or combustion extractions of SiC haverevealed a bulk N-isotopic composition enriched in 14N withδ15N values down to –650‰ (e.g., Ash et al., 1989) andcorresponding 13C enrichments (δ13C ≈ 1430‰ for bulkresidue) (e.g., Russell et al., 1997). While no evidence formultiple components has been uncovered by stepped com-bustion, ion microprobe measurements of individual SiC

grains have revealed a number of different grain types.About 90% of the grains (the mainstream grains) have iso-topic signatures consistent with those identified by thestepped-heating experiments, with elevated 14N/15N ratios upto 5000 (δ15N ≈ –950‰) coupled with 12C/13C ratios from≈40 to 100 (δ13C ≈ +1250‰ to –100‰) (for details see Zin-ner, 1998; Hoppe and Zinner, 2000; Meyer and Zinner,2006). This indicates an input from H burning via the CNOcycle in red giant carbon stars. The additional presence ofs-process noble gases indicates an origin in the thermallypulsing asymptotic giant branch (TP-AGB) phase.

Apart from the mainstream grains, several further groupsof SiC grains, labeled A, B, Y, and Z, also contain N withelevated 14N/15N ratios. They formed in similar types of nu-cleosynthetic environments as the mainstream grains. Vari-ations in the isotopic and elemental abundances of thesegrains reveal much about the metallicity or evolutionary his-tory of the source star (for details see Hoppe et al., 1997;Amari et al., 2001a,b; Ott, 2002; Meyer and Zinner, 2006).Approximately 1% of the SiC grains are so-called X grains,with 14N/15N ratios of 13–180, much less than the solar esti-mate of 435 (section 5.3) and 12C/13C ratios generally higherthan solar — indicative of He burning. Most probably they

Fig. 3. Comparison of elemental abundances (left) and isotopicratios (right) of noble gases in SiC from the Murchison meteoritewith values calculated for production in s-process region in AGBstars. Isotopic ratios are normalized to solar ratios, elemental abun-dances to solar abundances and 4He. Boxes encompass range ofvalues calculated for the s-process zone (Busso et al., 1990; Gal-lino et al., 1990). Measured data and figure are from Lewis et al.(1994).

Fig. 4. Isotopic compositions of some major Xe componentsdiscussed in the text. Data are from Ott (2002), who also listsfurther Xe components not shown here. Ratios are normalized to132Xe and solar-wind composition as given by Wieler (2002).


originate in Type II supernovae, as indicated by large over-abundances of 28Si among other isotopes (Hoppe and Zin-ner, 2000; Ott, 2002; Meyer and Zinner, 2006).

3.2.1.2. Graphite. Unlike SiC, which is dominated bygrains from AGB stars, presolar graphite has sizeable con-tributions from several sources (e.g., Zinner, 1998; Meyerand Zinner, 2006). The noble gases are again mixtures ofnormal and anomalous components. As in SiC, Kr and Xeare both dominated by a G component from s-process nu-cleosynthesis. Hence, noble gases are in line with conclu-sions based on other elements that AGB stars also supply alarge part of the presolar graphite grains. Unlike SiC, the Nebudget in graphite is dominated by Ne-R, however (Amariet al., 1995a), with only 5–50% being Ne-G. Ne-R [alsoknown as Ne-E(L)] is monoisotopic 22Ne from the decay of22Na (hence R = “radiogenic”). About 30–40% of the singlegraphite grains studied by Nichols et al. (1992a, 1994) con-tain 22Ne above background levels, but not even the most22Ne-rich grains contain detectable amounts of 4He or 20Ne.This implies that the bulk of the 22Ne in graphite separatesdoes not come from AGB stars, in agreement with Amari et al.(1995a). Strictly speaking, however, only for the most 22Ne-rich grains is an origin from 22Na decay definitely needed,implying explosive nucleosynthesis from novae or super-novae. Additional single grain analyses would be highly de-sirable to further constrain the actual fraction of graphitegrains containing Ne from explosive nucleosynthesis.

Ion microprobe analysis of individual graphite particlesreveals that almost 50% contain N and C with isotopic sig-natures similar to the SiC X grains and indicative of an ex-plosive nucleosynthetic origin (e.g., Hoppe et al., 1995;Amari et al., 1995b; Zinner et al., 1995; Ott, 2002). Othergraphite grains have isotopic signatures indicative of theAGB stars identified as the source of the mainstream andthe A and B SiC grains. A final group has solar-like 12C/13Cratios and N-isotopic compositions that concentrate stronglyaround the terrestrial value, with only a few examples ex-tending to more 15N-rich values. This has been taken to indi-cate that these grains condensed from the molecular cloudfrom which the solar system formed (Hoppe et al., 1995;Zinner et al., 1995). However, it should be noted that ac-cording to the most recent estimate the solar N-isotopiccomposition is more 14N-enriched than terrestrial N (seesection 5.3). Therefore, the graphite grains with terrestrial-like N may have more in common with the source of theterrestrial atmosphere than the bulk solar system value.

3.2.1.3. Nanodiamonds. This presolar carrier contrib-utes typically some 90% to the primordial He and Ne inven-tory and still some 10% to the heavier primordial noble gasesin bulk primitive meteorites. The isotopic compositions ofthe various noble gas components carried by diamonds havebeen compiled elsewhere (e.g., Ott, 2002). The most diag-nostic gas is the anomalous Xe-HL, which is enriched inboth the heaviest (H) and the lightest (L) isotopes (Fig. 1).Xenon-HL was first detected by Reynolds and Turner (1964)and has been instrumental in separating the diamond carri-ers. It is accompanied by a distinct component in all other

noble gases, which have thus also been assigned the HL la-bel, although the isotopic composition of He-HL and Ne-HL is not obviously anomalous. Also, Ar-HL and Kr-HL areconsiderably less different from solar composition than isXe-HL. To complicate matters further, the diamonds appearto contain two additional primordial components of appar-ently “normal” (but clearly distinct) composition in all fivenoble gases, called P3 (Fig. 1) and P6 (Huss and Lewis,1994) (see section 3.3.4).

Xenon-HL is of particular interest because it containslarge enrichments of isotopes produced only in the p-proc-ess (124,126Xe) and large enrichments of isotopes made inthe r-process only (134,136Xe). At least those few diamondscarrying Xe-HL are thus of likely supernova origin (Meyerand Zinner, 2006). It is a major question whether diamondscarrying Xe-L differ in some property from those carryingXe-H. Attempts to partially separate the two subcomponentsby selective absorption of laser light have remained incon-clusive so far (Meshik et al., 2001), although Verchovsky et al.(1998) reported variations in Xe-isotopic composition withgrain size that they argue indicate different Xe componentswere implanted into the diamonds at different energies. Notethat on average only 1 out of 1,000,000 nanodiamonds con-tains an atom of Xe-HL, and hence in all likelihood is pre-solar. This is often stressed because the isotopic compositionof C and probably also N (see next paragraph) of nanodia-mond samples are solar-like, suggesting the possibility thatmost of the nanodiamonds have a solar system or molecularcloud origin (Meyer and Zinner, 2006). However, it shouldnot be forgotten that possibly 1 out of 10 nanodiamondscontains an atom of He-HL (the extreme alternative that 1out of 1,000,000 diamonds contains 100,000 He-HL atomsis unrealistic, given that an individual nanodiamond consistsof a mere 1000 or so C atoms). Since He-HL is closely tiedto the exotic Xe-HL, this may thus indicate that the preso-lar fraction of the nanodiamonds is considerably larger thanthe minimum implied by the Xe-HL concentration.

Nitrogen is abundant in the nanodiamonds, with con-centrations of up to 1.3 wt%. There is no evidence for anysignificant variation in the 14N/15N ratio of the diamonds,although there may be a N-rich and N-poor population(Russell et al., 1996). The distinctive δ15N value of about–350‰ (14N/15N ≈ 420) was initially taken as outside nor-mal solar system values and therefore indicated that many,if not all, of the nanodiamonds were presolar (Lewis et al.,1983; Russell et al., 1996). The measured δ15N value in thediamonds is, however, remarkably similar to several recentlymeasured values for ammonia in the atmosphere of Jupiterby the Infrared Space Observatory (ISO) (Fouchet et al.,2000), Galileo (Atreya et al., 2003), and Cassini (Fouchet etal., 2004): –480–280

+240‰, –374 ± 82‰, and –395 ± 140‰,respectively. The jovian value is generally interpreted as agood proxy for the solar value. Therefore the similarity withthe value for the nanodiamonds does at least call into ques-tion whether the N-rich component deserves the label pre-solar. The near-solar 12C/13C ratio (≈92) of the nanodia-monds (e.g., Lewis et al., 1983) also suggests a local origin


for the vast bulk of the nanodiamonds. However, the verylarge range in δ15N values displayed by materials generallyaccepted as forming in the solar nebula (e.g., many meteor-itic components, Jupiter, the Sun — see section 5.3) or latermeans that it is difficult to determine unequivocally a preso-lar origin for the N-containing nanodiamonds. On the otherhand, the inferred 12C/13C ratio possibly as high as 120 inthe N-rich nanodiamond fraction postulated by Russell et al.(1996) does offer some support to the argument for a pre-solar origin for the N-rich nanodiamonds.

3.2.2. Trapping and modification processes of noblegases in presolar grains. Ion implantation from a stellarwind is the most likely trapping mechanism for most — ifnot all — noble gas components in the various types of pre-solar grains. For the G component in SiC the main argumentfor this inference is that the measured elemental abundancesagree remarkably well with predictions for He shell produc-tion (Lewis et al., 1994), hence a mechanism that does notstrongly fractionate elemental abundances is required. Sub-sequent diffusive losses of the lighter gases apparently didnot occur and SiC therefore seems to contain a pristine sam-ple of matter from the s-process zone in AGB-stars. It ap-pears likely that also graphite trapped the G component byion implantation (Ott, 2002), but Ne possibly was lost later,as its abundance is lower than expected by 1–2 orders ofmagnitude (Amari et al., 1995a).

The relatively high Kr and Xe abundances in the N compo-nent in SiC (Fig. 3) indicate a fractionation process. Lewis etal. (1990) proposed a scenario in which SiC grains trappedions from a stellar wind during condensation in the enve-lopes of AGB stars. The variable elemental ratios in N and Gcomponents appear to require two wind components, onefrom a fully ionized region and one from a cooler, partly ion-ized zone. The latter would contribute most of the Kr and Xeof the N component. Support for this scenario comes frommodel calculations by Verchovsky et al. (2004), who suggestthat the N component was implanted with low energy (windspeed 10–30 km/s) during the main AGB stage, whereas theG component was implanted with a few thousand kilome-ters per second at the very end of the AGB phase.

It is remarkable that the isotopic composition of Kr-G,which is established in the hot interior of a star, correlateswith the size of the carrier grains, which depends on condi-tions in the cool atmosphere. Lewis et al. (1994) point outthat both properties ultimately depend on mass and metallic-ity of the star and might be coupled this way. Nuth et al.(2003, and personal communication) indeed predict largegrains (those mainly analyzed so far) to come from low-mass stars (1–3 M ), whereas larger stars would producesmaller (0.1 µm) grains.

Ion implantation is also the most viable trapping processfor both the anomalous and the normal noble gas compo-nents in diamonds. Arguments for this are summarized byOtt (2002) and include decreasing gas content with grainsize (Verchovsky et al., 1998) and similar release patternsfrom artificially implanted and meteoritic nanodiamonds(Koscheev et al., 2001). Artificial ion implantation experi-

ments into terrestrial nanodiamonds call for caution wheninterpreting noble gas data from meteoritic diamonds, be-cause their very small size may lead to isotopic fractiona-tions of the implanted portion and also to a bimodal tem-perature release pattern of a single implanted component(Koscheev et al., 2001). One possible consequence of this isthat the true Xe-HL composition may actually be free of anys-process contribution. Another consequence of this work isthat the isotopic fractionation may compromise our under-standing of the relationship between Xe-HL on the one handand its accompanying He-HL to Kr-HL on the other, becausethe isotopic compositions of the implanted gases may bedifferent from what we measure (Huss et al., 2000). In fact,Huss and Lewis (1994) suggested previously that the HLcomponent has a normal and an anomalous part, the anoma-lous one becoming more prominent in the heavy gases.

The Ne-R very likely was incorporated into presolargraphite by condensation of 22Na during grain growth ratherthan by implantation as 22Ne already, as otherwise substan-tial amounts of 20Ne would also be expected (Nichols et al.,1992a).

In the case of N there is no systematic variation in theabundance levels with nanodiamond grain size, indicatingthat ion implantation is not the primary trapping mechanism(Verchovsky et al., 1998). Rather, the N content is thoughtto be determined by the partial pressure of N or N-bearingspecies (e.g., HCN) at the time and place where the dia-monds formed (Verchovsky et al., 1998), probably by chem-ical vapor deposition (section 3.3.4). High, and sometimesvariable, concentrations of N in the SiC and graphite grains(e.g., Ash et al., 1989; Smith et al., 2004) also show thatthe N is trapped chemically and that implantation plays aminor role at most.

As described above, the noble gas component structure innanodiamonds is even more complicated than those of SiCor graphite, which is one reason why the former is less wellunderstood than the latter two. It therefore remains openwhether the various components may reside in different sub-populations produced in different circumstellar environ-ments. Huss and Lewis (1994) conclude that the HL compo-nent resides in bulk diamonds rather than trace phases andthat the carriers of HL and P6 gases are probably different.

In summary, grains having entered the solar nebula assolids and having remained solid ever since contribute aremarkably large fraction of the primordial noble gases, butnot N, in meteorites. Some of these gas components appearto have remained unfractionated, testifying to the excellentstate of preservation of some presolar grains. On the otherhand, a large fraction of the presolar grains may have losttheir primordial noble gases (Nichols et al., 1992a).

3.3. Components of Less-Certain — Presolaror Early Solar System — Origin

In addition to noble gas and N components that can di-rectly be attributed to nucleosynthesis in stars, meteoritescontain components whose origin is not unequivocally pre-


solar or whose identity most likely was established in theearly solar system. These are the “Q gases” and the relatedN residing in the enigmatic “phase Q” in chondrites andtheir close relatives found in ureilites, as well as the “P3”and “P6” noble gases residing in nanodiamonds. Further-more, primordial noble gases of solar composition (not to beconfused with solar wind noble gases incorporated more re-cently in a planetary regolith) are being found in an increas-ing number of meteorites of various classes.

3.3.1. Noble gases in phase Q. Lewis et al. (1975) ob-served that a large portion of the primordial noble gases inthe carbonaceous HF/HCl-resistant residues of the CV3chondrite Allende were lost upon oxidation. These authorsdubbed the oxidizable, almost mass-less gas-rich carrier“phase Q” (for “quintessence”). Numerous subsequent analy-ses yielded relatively uniform Ne-Xe element and normalisotope abundances of the Q gases in all primitive meteoriteclasses, implying that phase Q was widespread in the earlysolar system (e.g., Srinivasan et al., 1977, 1978; Reynoldset al., 1978; Alaerts et al., 1979a,b; Matsuda et al., 1980;Moniot, 1980; Huss et al., 1996). Comparing the noble gasesin unoxidized and oxidized residues does not allow a goodcharacterization of He and Ne in phase Q due to their lowabundance relative to the presolar gases present in oxidizedresidues. A precise determination of the He- and Ne-isotopiccompositions in phase Q became possible with in vacuo etchexperiments releasing essentially the pure Q gases (Wieleret al., 1991, 1992; Busemann et al., 2000, 2001).

Q gases are heavily fractionated relative to solar compo-sition favoring the heavier elements and isotopes (Figs. 1,2, and 4) (Ott, 2002). Up to 10% of the primordial He and90% of the primordial Xe in primitive meteorites originatefrom phase Q. Solely Ne and He appear to show isotopicvariations, whereas Ar, Kr, and Xe in phase Q are isotopi-cally uniform. The Q component encompasses “OC-Xe” inordinary chondrites (Lavielle and Marti, 1992). Average car-bonaceous chondrite (AVCC) composition represents thesum of all primordial components in carbonaceous chon-drites (Eugster et al., 1967). Despite many efforts, the car-rier phase Q and its origin remain unknown. In the follow-ing, we will discuss proposed trapping mechanisms, startingwith models that assume a presolar origin of phase Q, fol-lowed by hypotheses for a “local” origin, in the vicinity ofthe early Sun or on parent bodies.

Much remains to be determined regarding the origin ofphase Q. The most critical issues to be accounted for are(1) the fractionated isotope and element abundances of theQ gases relative to solar, especially the isotopic composi-tion of He-Q (3He/4He = (1.23 ± 0.02) × 10–4) (Busemann etal., 2001) and variations in Ne-Q (Busemann et al., 2000);(2) the presence of small but significant amounts of He andNe (Busemann et al., 2000; Wieler et al., 1991); (3) the highQ gas release temperatures of ~1200°–1600°C (Huss et al.,1996), (4) the chemical resistance to HF and HCl combinedwith the strong susceptibility to oxidizing agents (Moniot,1980; Ott et al., 1981; Huss et al., 1996); and finally (5) thegas concentration in phase Q, which is extremely high irre-spective of the ill-constrained extent to which the carrier

phase is actually destroyed during noble gas release. In addi-tion, physical conditions of the suggested trapping environ-ments (e.g., partial pressure of the noble gases in the nebula)must be considered.

Various workers favor a presolar origin of phase Q. Thepresence of Q gases in all primitive meteorite classes im-plies complete mixing and widespread distribution of phaseQ in the solar system. This appears easier to achieve in thepresolar cloud than in planetary environments. Huss andAlexander (1987) pointed out that the noble gases in theSun’s parent molecular cloud, mixed from different stellarsources, were already isotopically normal. They suggestedthat at the low dust temperatures of 10–30 K Ar-Xe wouldhave been trapped in icy mantles around presolar grains.Exposure to UV photons or energetic ions would form radi-cals and other organic molecules. Evaporation of the icewould leave carbonaceous mantles that keep preferentiallythe heavy noble gases. While this model explains the enrich-ment in the heavy noble gases, the trapping efficiency forHe and Ne remains uncertain as well as the general isoto-pic fractionation of the Q gases relative to solar abundances.Sandford et al. (1998) simulated trapping in the Sun’s parentmolecular cloud (10–25 K, ionizing UV irradiation). Theysucceeded at producing carbonaceous residues that carryheavy noble gases in similar concentrations as acid-resistantresidues and with Q-like fractionation. However, the majorgas release from the residues occurred already by 250°C.Enormous amounts of Xe were also trapped in vapor-de-posited silicate condensates by anomalous chemical adsorp-tion. These grains may subsequently have been coated bystable organic mantles under UV irradiation in the cold pre-solar molecular cloud (Nichols et al., 1992b). The trappedXe was indeed released at relatively high temperatures andshowed the Q-typical isotope fractionation. However, Neand Ar were not significantly captured and the presence ofnoble gases in coated silicates in meteorites is speculative.

Hohenberg et al. (2002) suggested active capture andanomalous adsorption to explain the large Kr and Xe con-centrations of phase Q. Anomalous adsorption on growingsurfaces involves chemical bonds, in contrast to the Van derWaals forces of normal adsorption. When chemically ac-tive species are insufficiently available, Kr and Xe can formtransient chemical bonds on fresh surfaces. Subsequent dep-osition of material enhances capture efficiency and gas re-tentivity. Active capture experiments reproduced both thelarge Xe concentrations in phase Q as well as the fractiona-tion of the Xe isotopes relative to solar composition. Possi-bly, an enhanced energetic proton flux from the pre-main-sequence Sun formed radicals on the surface of presolardiamonds, which adsorbed Kr and Xe, although it is unclearwhether the process can replicate the isotopic fractionationof Kr-Q relative to solar, or is capable of accounting for thelighter Q gases.

The correlation of the abundances of both Q gas and pre-solar grains with petrologic type in various meteorite classesimplies an intimate mixing of phase Q and presolar dust inthe Sun’s parent molecular cloud and thus a presolar originof phase Q (Huss, 1997). The putative formation of phase Q


in the presolar cloud as carbonaceous grain mantles requirespresolar grains as “condensation nuclei.” Busemann et al.(2000) observed a correlated release of Ne-R/G (most likelyfrom presolar graphite) and a subfraction of the Q gasesupon mild etching of the CM2 chondrite Cold Bokkeveld. Aclose relationship between phase Q and presolar diamondswas suggested by Amari et al. (2003), based on the similarQ/diamond ratios in various separates of Allende. However,Q has higher gas-release temperatures and a better resistanceto metamorphism than presolar diamonds, which leads tovariable Q/diamond ratios (Huss, 1997). This argues againsta close physical relationship between the two carriers.

“Local” models aim to derive Q gases from solar noblegases after formation of the Sun. However, most of thesemodels have not identified adequate regions within the solarsystem for the required fractionation and the formation ofthe chemically resistant, carbonaceous carrier. Furthermore,phase Q must have been well mixed into the meteorite form-ing regions together with presolar grains and dust. Ozima etal. (1998) suggested that Rayleigh fractionation of initiallysolar noble gases would explain the isotopic differences be-tween Q- and solar gases. An additional fractionation mech-anism would be responsible for the elemental compositionof the Q gases, presumably somewhere in the gaseous neb-ula. In this scenario, the starting He is isotopically similar topresent-day solar wind, i.e., the Q gases originate from theSun after pre-main-sequence D burning (but see also Pepinand Porcelli, 2002).

Based on experiments with laboratory diamonds, Mat-suda and Yoshida (2001) proposed that Q gases may havebeen trapped on the surfaces of presolar diamonds in a plas-ma (see section 3.3.3 for similar mechanisms suggested forthe Q gases in ureilitic diamonds). However, as pointed outabove, a close physical relationship between Q and presolardiamonds appears to be unlikely.

Two models suggested by Pepin (1991, 2003) explore thepossibility that the Q gas pattern was generated by hydro-dynamic escape. The first model invokes isotopic fractiona-tion during the blow-off of transient atmospheres of earlyplanetesimals. Primordial solar Ne-Xe accreted in ice-man-tles of presolar grains later became concentrated below sur-face ice layers on these bodies. Impacts caused hydrody-namic blow-off of the noble gases, driven by H2 or — morelikely — CH4. The residual gas is enriched in the heavierspecies and reproduces the isotopic patterns observed in theQ gases. Subsequent impacts induced the formation of phaseQ. Among other constraints on the dynamics of the earlysolar nebula, this model requires that the Q gas carriers fromvarious planetesimals must have been thoroughly mixed be-fore having been incorporated into present-day meteoriteparent bodies.

The second model involves fractionation during the dis-sipation of the solar accretion disk. Ultraviolet irradiationfrom a nearby massive star initiated photo dissociation andstrong H outflows from the disk. The consequence is hydro-dynamic escape of the noble gases that are assumed to beinitially solar in composition (Pepin, 2003). However, thenoble gases were sufficiently fractionated to resemble Q ele-

ment patterns only at the accretion disk’s boundary, whilethe isotopic compositions observed for Q were not repro-duced at all. Furthermore, the hot environment would ham-per retentive trapping of the gas.

The origin of phase Q is closely related to its physicalproperties and will most likely only be resolved by establish-ing correlations of Q gas release with compositional prop-erties of the releasing phase(s), such as the D/H or 15N/14Nratios, C contents, or changes in their carrier structures upongas release. A number of attempts have been made to iden-tify and characterize the N associated with phase Q. Theresults indicate variable δ15N values, from ≈–50‰ in CO3meteorites to –15‰ in ordinary chondrites (Murty, 1996;Hashizume and Nakaoka, 1998), while a component in ureil-ites most similar to Q ( see below) has a δ15N value of –21‰(Rai et al., 2003a). However, the significance of these val-ues is unclear. Nitrogen is present in a wide range of phaseswhereas high-resolution stepped combustion of HF/HClresidues of a suite of enstatite chondrites reveals that Q no-ble gases appear to be highly concentrated in a very minorpart of the acid-resistant C inventory (<5%). The N in theseresidues does not appear to be related to either the maincarbon or noble gas releases (Verchovsky et al., 2002).

3.3.2. The nature of phase Q. Substantial work has alsofocused on revealing the nature of phase Q. The earliest sug-gestion was chromite or an Fe-Cr-Ni-rich sulfide (Anders et al., 1975; Gros and Anders, 1977), but both were ex-cluded by Frick and Chang (1978). Subsequently, trappingexperiments on synthesized carbonaceous matter as well asdensity separation and combustion of meteoritic residueshave shown that phase Q is mainly carbonaceous (Frick andChang, 1978; Frick and Pepin, 1981; Ott et al., 1981). Later,Wacker et al. (1985) and Wacker (1989) suggested that theQ gases might have been physically adsorbed at low pres-sures in the solar nebula in micropore labyrinths of amor-phous carbon near grain surfaces. Adsorption experimentsat low pressures by Marrocchi et al. (2005) showed that car-bon blacks can trap gas amounts larger than those found inphase Q, with relative Ar and Kr (less so Xe) abundances inQ being reproduced. Best matches were achieved for trap-ping temperatures of 75–100 K, which the authors arguemight be realistic in a later-stage solar nebula in the meteor-ite-forming region. It remains to be seen whether it is alsopossible to reproduce the release temperatures of phase Qof ~1000°C and to trap the small amounts of He and Ne.Verchovsky et al. (2002) concluded from stepped-combus-tion experiments that phase Q is presolar, volume ratherthan surface correlated, and a minor component of the totalmacromolecular C in chondrites (see also Schelhaas et al.,1990). Phase Q appears to be more resistant to thermal meta-morphism than the bulk macromolecular C.

Fullerenes (C60) can enclose He-Xe atoms, if producedunder high gas pressure (Saunders et al., 1996). Becker andcoworkers (e.g., Becker et al., 2000) suggested fullerenesas carriers of the Q gases, following an earlier proposal byHeymann (1986) but withdrawn by Heymann and Vis (1998).However, convincing evidence for a correlation of Q gas andfullerene content in meteorites is lacking. Ott and Herrmann


(2003) showed that the noble gas trapping efficiency of arti-ficial fullerenes decreases as mass increases from He to Xe,inverse to the trend required to explain the Q gas elemen-tal patterns (Fig. 2). Moreover, most workers could not con-firm the presence of C60 in carbonaceous chondrites at all(Buseck, 2002). Heymann and Vis (1998) and Vis and Hey-mann (1999) suggested that carbon nanotubes could be thecarrier of the Q gases. However, nanotubes, as well as car-bynes, suggested by Whittaker et al. (1980) to carry Q gas,have not been found in Allende, Leoville, and Vigarano (Viset al., 2002). Nevertheless, these authors suggested variousthree-dimensional closed carbon structures, such as nano-tubes, carbon onions, or adsorption sites in labyrinths ofpores in amorphous carbon, but not fullerenes, as possibleQ gas carriers. A similar suggestion has been made by Gar-vie and Buseck (2004), who found hollow nanospheres andnanotubes in Tagish Lake and various CM chondrites.

Matsuda et al. (1999) and Amari et al. (2003) succeededin significantly enriching phase Q and presolar diamonds inthe Allende meteorite by physical separation techniques. Thematerial most enriched in phase Q (the carrier itself?) ap-pears to have a low density of (1.65 ± 0.04) g/cm3, consistentwith earlier observations by Ott et al. (1981).

There are hints on the existence of subfractions of phaseQ. Gros and Anders (1977) defined phase Q1 and Q2 basedon the susceptibility to nitric acid. While the identificationof metal-sulfide and chromite as gas carriers turned out tobe wrong (see above), the presence of two different sub-phases with different resistance to nitric acid and differentnoble gas compositions has been confirmed (Busemann etal., 2000). Distinct release systematics of Ar and Xe alsosuggest different Q subcarriers (Verchovsky et al., 2002).

3.3.3. Primordial noble gases and nitrogen in ureilites.Ureilites typically contain concentrations of C and primor-dial noble gases comparable to those in the most primitivecarbonaceous chondrites. It appears difficult to reconcilethis with the achondritic, igneous nature of the ureilites(Goodrich, 1992). A major carrier is diamond (not identi-cal to the presolar nanodiamonds). The much more abun-dant graphite appears to be mostly gas-free (Weber et al.,1976; Göbel et al., 1978); gas-rich graphite has only beenreported by Nakamura et al. (2000). A third carrier, evenmore gas-rich than diamond (Göbel et al., 1978), was iden-tified as amorphous fine-grained C (Wacker, 1986). Ott et al.(1985, 1986) and Rai et al. (2002) postulated a close affin-ity of this poorly characterized carrier to phase Q, which issupported by similar release characteristics of residues fromenstatite chondrites and ureilites (Verchovsky et al., 2002).The overall elemental fractionation pattern and the Ne-Xe-isotopic compositions in ureilites resemble those found inphase Q (Marti, 1967; Wilkening and Marti, 1976; Ott et al.,1985; Busemann et al., 2000).

Two widely different scenarios for the origin of theunique, gas-rich diamonds are discussed, according to whichthey either formed in the solar nebula or by shock on theureilite parent body. The shock hypothesis (e.g., Lipschutz,1964; Vdovykin, 1970; Bischoff et al., 1999; Nakamuta andAoki, 2000) may account for the igneous origin of the ureil-

ites. It can explain the absence of diamonds in the least-shocked ureilites, the presence of compressed graphite, andalso the good retention of noble gases. However, the char-acter of the carbonaceous precursor and the reason for itshigh abundance remain uncertain. Graphite may be a po-tential precursor, but it is then difficult to explain why mostgraphite is gas-free (Göbel et al., 1978). Possibly a moreviable candidate is the amorphous fine-grained C (Wacker,1986; Goodrich et al., 1987).

The correlation between diamond concentration andshock stage in ureilites is subject to debate. Furthermore,N-isotopic data may argue against shock conversion ofgraphite or amorphous C into diamonds (Rai et al., 2002,2003a; see below). These observations may rather speak infavor of a scenario where ureilitic diamonds and the amor-phous C have been formed by chemical vapor deposition(CVD) in the nebula (Arrhenius and Alfvén, 1971; Fuku-naga and Matsuda, 1997; Fukunaga et al., 1987).

Most noble gas trapping mechanisms suggested for theureilites resemble those described for the Q gases (section3.3.1). The elemental abundances relative to solar compo-sition correlate well with the first ionization potential of thenoble gases, which might indicate that the abundances wereestablished at high nebular plasma temperatures (Weber etal., 1971; Göbel et al., 1978; Rai et al. 2003b).

Plasma-induced fractionation in the nebula agrees wellwith a CVD origin of the ureilitic diamonds, because syn-thetic CVD diamonds can trap significant noble gas amountswith a fractionation pattern similar to that observed in ureil-ites (Matsuda et al., 1991). However, it is unclear how therequired plasma conditions could have been established,especially as the presence of solar nebula diamonds exclu-sively in ureilites would require explanation. Furthermore,the (much smaller) presolar nanodiamonds (also probablyproduced by CVD, see next section) would have been de-stroyed in ureilites but at the same time CVD-produced dia-monds from the nebula would have survived processing onthe ureilite parent body.

Ozima et al. (1998) suggested mass as the main param-eter governing noble gas fractionation in ureilites and inphase Q (section 3.1.1). However, the mass of the noblegases correlates less perfectly than the first ionization po-tential with the element depletion in ureilites (Weber et al.,1971), and the isotope ratios are not fractionated to the ex-tent expected from a mass-dependent element-depletionprocess (Rai et al., 2003b).

As noted previously, there are a number of different Ncomponents in ureilites, the origins of which are not alwaysclear. Some of the N does appear to be associated with thenoble gases in the diamond phase but most of it is containedin amorphous or graphitic C (with and without noble gases,respectively) and more importantly associated with metaland silicate phases (without noble gases) (Yamamoto et al.,1998; Rai et al., 2003a). The N with the most extreme iso-topic composition resides in the diamond phase (≈–120‰or less) and in a poorly characterized phase in the polymictureilites (540‰) (Grady and Pillinger, 1988). The N in thediamond phase is quite distinct from that in the graphitic or


amorphous C (>+50‰ and ~–20‰ respectively). ThereforeRai et al. (2003a) concluded that the diamond could not havebeen produced in situ by shock from one of these phasesbut rather prefer a nebula origin. In contrast, Yamamoto et al.(1998) argue that the heavy N seen previously in the poly-mict ureilites is also present in some of the monomict ureil-ites and therefore argue for late-stage injection of severalvolatile-rich objects into ureilitic silicates.

3.3.4. P3 and P6 noble gases in diamonds. Two isotop-ically apparently normal components, “P3” and the minor“P6,” have been found in presolar diamonds in addition tothe anomalous “HL” noble gases (Huss and Lewis, 1994)(see also section 3.2.1). Both P3 and P6 are elementally andisotopically distinguishable from the Q gases (Figs. 1, 2, and4) (Ott, 2002). Despite their “normal” compositions, P3 andP6 are considered to be presolar by Huss and Lewis (1994).While P3 is released by pyrolysis below 800°C, which pos-sibly reflects superficial trapping, P6 is released at highertemperatures than the HL gases, implying distinct sites forthese components.

The most popular formation scenario for presolar nano-diamonds is chemical vapor deposition in the vicinity ofsupernovae or carbon stars (e.g., Saslaw and Gaustad, 1969;Jørgensen, 1988; Lewis et al., 1989; Daulton et al., 1996),but annealing of hydrocarbons (Nuth and Allen, 1992) andthe transformation of C by shockwaves in the ISM (Tielenset al., 1987) have also been suggested. The P3 and P6 noblegases could have been enclosed during the formation of thediamonds or trapped later into existing diamonds, e.g., byion implantation in stellar environments or interstellar space.Ion implantation is supported by experiments that succeededin separating diamond grain size fractions (Verchovsky et al.,1998) and trapping noble gases in synthetic diamonds inconcentrations even exceeding those in presolar diamonds(Koscheev et al., 2001). Moreover, the gas release patternof the irradiated artificial diamonds closely resembles thatof natural diamonds. This suggests that the components HLand P3 may be located in the same fraction of diamonds,implanted by distinct events. Most of the Ne, Ar, and 130Xeaccompanying the anomalous Xe-HL may actually be frac-tionated P3 (Huss et al., 2000; Ott, 2003).

3.3.5. Primordial solar-like noble gases in non-carbona-ceous meteoritic matter. The noble gas and N componentsdiscussed in the previous sections reside in minor carbon-aceous phases, which remain after demineralization of thehost meteorites. In chondrites, this carbonaceous matter car-ries most of the primordial noble gases, whereas primordialgases in major phases have hardly received attention. Thisappears unjustified, because there is increasing evidence thatsilicates also carry primordial noble gases and that these areelementally and isotopically considerably closer to solarcomposition than the gases in carbonaceous matter. Notethat we consider here primordial solar-like noble gases, incontrast to solar-wind noble gases trapped later in an aster-oidal regolith.

The most prominent example for primordial noble gasesin meteoritic silicates is the so-called subsolar or Ar-rich“component” in enstatite chondrites (Crabb and Anders,

1981; Patzer and Schultz, 2002). Gas concentrations maybe substantial. For example, the trapped 36Ar in the silicatesof South Oman, which is commonly used as reference sam-ple for the subsolar gases, is higher than typical contribu-tions of Q gases in carbonaceous chondrites. Hints of sub-solar gases have also been found in other meteorite classes(e.g., Alaerts et al., 1979b; Matsuda et al., 1980; Schelhaaset al., 1990). High-resolution gas release experiments re-vealed that the subsolar noble gases in the E chondrite St.Mark’s represent a mixture of probably primordially trappedsolar gases, Q gases, and terrestrial contamination (Buse-mann et al., 2003a,b). Hence, the subsolar component ap-pears to be a mixture and does not represent a well-definednoble gas composition.

High concentrations of solar-like heavy noble gases (Ar-Xe on the order of values in South Oman) have been foundin chondrules of the enstatite chondrite Y 791790 (Okazakiet al., 2001). This is surprising in view of the general scar-city of trapped noble gases in high-temperature objects likechondrules (Vogel et al., 2004a,b) (section 4). Okazaki andco-workers (Okazaki et al., 2001) concluded that chondrulescarry the “subsolar” noble gas component and proposed thatsolar gases were implanted into chondrule precursors byenergetic particles from the early Sun. The primordial solarHe and Ne would subsequently have been lost. This inter-pretation is not easy to reconcile with the observation byBusemann et al. (2003a) that the “subsolar” component inanother enstatite chondrite actually contains a fraction ofelementally unfractionated solar gases. A survey of noblegas concentrations in chondrules of other enstatite chon-drites is thus highly desirable.

Particularly noble-gas-rich, acid-susceptible rims havebeen observed in a primitive dark inclusion in the C3 chon-drite Ningqiang (Nakamura et al., 2003b). Again, the con-centrations are comparable to those found in the bulk ofSouth Oman. The element composition resembles that of the“subsolar” mixture in E chondrites and ureilitic diamonds,implying a common origin, e.g., by implantation of nebu-lar noble gases that were incompletely ionized in a plasma(Nakamura et al., 2003b).

There are also indications of primordial noble gases ofsolar-like composition in differentiated meteorites. Mathewand Begemann (1997) and Busemann et al. (2004) foundsolar-like Ne in silicates of the Brenham pallasite and theD’Orbigny angrite, respectively. Busemann and co-work-ers suggest that solar gases were transported from the inte-rior of the angrite parent body by volcanic CO2 and trappedin the quenched D’Orbigny glass near the parent-body sur-face, analogous to the trapping of terrestrial mantle gasesin ocean basalts.

Although many details remain unclear, the observationsof primordial noble gases of solar composition in some me-teorite parent bodies may indicate that meteoritic precur-sor material trapped solar-wind noble gases during an earlyirradiation prior to accretion to large parent bodies (Podoseket al., 2000; Okazaki et al., 2001; Busemann et al., 2003b).However, most of the dust accreted to larger objects whilebeing efficiently shielded from solar wind (Nakamura et al.,


1999b, 2003a). This may indicate that some early solar sys-tem material has been irradiated, e.g., off-disk or at the inneredge of the solar system (Wetherill, 1981).

3.4. Primordial Noble Gases in InterplanetaryDust Particles and Micrometeorites

Noble gases in interplanetary dust particles (IDPs) andmicrometeorites are mostly dominated by the solar windtrapped while the particles were on their way to Earth(Olinger et al., 1990; Nier and Schlutter, 1992, 1993; Pepinet al., 2000, 2001; Osawa et al., 2003). The solar gaseshinder the assessment of the primordial noble gas inven-tory, which is regrettable since many IDPs probably arefragments of parent bodies so far unsampled otherwise. Yetin some cases primordial noble gases have been detected.Osawa et al. (2003) report that Kr and Xe in micrometeor-ites from Antarctic ice are dominated by a primordial com-ponent. It has not yet been possible, however, to establishwhether the signatures of the heavy primordial gases in mi-crometeorites differ in any respect from those of “normal”meteorites.

Remarkably high 3He/4He ratios of up to 40× the solar-wind value have been reported in some IDPs (Nier andSchlutter, 1993; Pepin et al., 2000, 2001). Production of therequired amounts of 3He in parent-body regoliths by cosmic-ray interactions would imply exposure times of up to morethan 109 yr (Pepin et al., 2001). Although this might not beunreasonable for the regoliths of Kuiper belt comets, Pepinet al. (2001) also consider the intriguing possibility that the3He excesses represent a 3He-rich primordial componentfrom an unknown source.

3.5. Nitrogen in Organic MacromolecularMaterial and Interplanetary Dust Particles

While phase Q is most likely associated with some minorC-rich phase, this represents only a tiny part of the total Cfound in the carbonaceous chondrites. Most of the C in thesemeteorites exists as organic material and probably containsno distinct noble gas components. It does contain consider-able amounts of N, however, contributing a few thousandparts per million N to the most primitive chondritic meteor-ites (e.g., CI, CM, and CR). The organics consist of an insol-uble macromolecular material plus lesser amounts of a widevariety of soluble compounds, which can be extracted withorganic solvents.

The N-isotopic composition varies considerably betweenand even within the different organic components. It is be-yond the scope of this chapter to detail the extensive bodyof research that covers the organic material (see recent re-views by Kerridge, 1999; Botta and Bada, 2002; Sephton,2002; Gilmour, 2003; Pizzarello et al., 2006). The macro-molecular material can be subdivided into two operation-ally defined components, refractory organic matter that isstable under harsh conditions with δ15N around –25‰ anda more labile component with measured δ15N values up to90‰ (Sephton et al., 2003). The free organic compounds

include amino acids with δ15N values of up to 184‰ (e.g.,Engel et al., 1990; Engel and Macko, 1997) and other N-rich fractions up to 103‰ (see review by Botta and Bada,2002). In many cases, elevated δ15N values are associatedwith elevated D/H ratios (up to 3400‰), which has beentaken to infer formation of these compounds in the ISMwhere gas-phase ion-molecule reactions at low temperatureare believed capable of producing large isotopic fractiona-tions (e.g., Adams and Smith, 1981).

Recent calculations have shown that ammonia and or-ganic compounds can be enriched in 15N by such interstellarchemistry by almost a factor of 2 (e.g., Terzieva and Herbst,2000; Charnley and Rodgers, 2002). If the solar δ15N valueof –374‰ (section 5.3) is taken as representative of thelocal ISM value at the time of solar system formation, thenthese calculations indicate that condensed forms of N couldhave δ15N values as high as 250‰. As reviewed by Gradyand Wright (2003), and shown in Fig. 5, most meteoritescontain N with isotopic compositions between these twovalues. Mixing of these two components therefore couldgenerate much of the wide range of values observed.

A number of meteorites such as bencubbinites and someordinary chondrites contain 15N enrichments well beyondthe limits calculated for the ISM, with δ15N values of up to1500‰ (e.g., Prombo and Clayton, 1985; Franchi et al.,1986; Mostefaoui et al., 2000, 2002). The original carrier ofsuch a N component remains unknown, as this 15N-rich Nnow appears to reside in secondary, or very modified carbo-naceous phases (e.g., Mostefaoui et al., 2000, 2002). There-fore, it may be that an additional source of heavy N is re-quired to account for these components. Alternatively, ourunderstanding of the isotopic enrichments that can be gen-erated in the ISM is incomplete.

Further evidence of very 15N-enriched N in associationwith organic C and elevated D/H ratios is observed in IDPs.These tiny particles contain abundant carbonaceous, largelyorganic matter. Significant amounts of N are also present,primarily in macromolecular-like material but also as freecompounds such as amino acids (e.g., Brinton et al., 1998;Maurette et al., 2000; Aléon et al., 2003). The range ofisotopic compositions of this N is almost equal to that seenin meteorites with δ15N values ranging from –373‰ to+1250‰ (see compilation of data by Messenger et al., 2003;Aléon et al., 2003). Some of the 15N-rich areas (δ15N >400‰) are intimately associated with elevated D/H ratios(>5000‰) and have C/H ratios similar to chondritic insolu-ble macromolecular material but with N contents up to anorder of magnitude greater (Aléon et al., 2003). Similarlyhigh N/C ratios have been measured in CHON grains fromComet Halley (e.g., Jessberger et al., 1988). Coupled withobserved 15N excesses in CN radicals from two comets, thissuggests a cometary origin for organic matter in IDPs (Ar-pigny et al., 2003). However, HCN in Comet Hale-Bopp hasbeen reported to be depleted in 15N (Jewitt et al., 1997), in-dicating a very heterogeneous N-isotopic distribution incomets at the molecular level and making it difficult to gen-eralize about the relative abundance of the N isotopes incometary material.


4. NOBLE GAS CONSTRAINTS ONTHE ACCRETIONARY HISTORY OF

METEORITE PARENT BODIES

In the previous section we discussed gas trapping froma “microscopic” point of view, e.g., when, where, and howprimordial gases were incorporated into their carriers suchas presolar grains or phase Q. In this section, we summarizeinferences about the accretionary history of meteoritic ma-terial from studies of trapped noble gases in acid-resistantresidues of bulk meteorites but also in “bulk samples” sepa-rated from mineralogically or texturally well-defined entitiessuch as chondrules, chondrule rims, or matrix.

Comprehensive studies of the abundances of the variousprimordial noble gas components in acid-resistant residuesfrom a large number of chondrites of different types havebeen performed by Huss and Lewis (1995) and Huss et al.(1996, 2003). These data were used to determine abundancesand characteristics of the different carriers in the differentmeteorite classes, with the goal of studying the metamor-phic history of meteorite parent bodies (Huss and Lewis,1995) as well as pre-parent-body modifications of precur-sor materials of carbonaceous chondrites (Huss et al., 2003).The former topic is discussed in section 5, the latter here.

CI and CM chondrites have the highest abundances ofpresolar noble gas carriers, in particular, the most fragiletypes. Other meteorite classes show considerably differentnoble gas abundances after accounting for gas loss inducedby parent-body thermal metamorphism. Meteorites rela-tively poor in volatile elements are also depleted in presolarnoble gas carriers, and the most fragile carriers are mostefficiently lost. An example is shown in Fig. 6 (adapted fromHuss et al., 2003). If bulk meteorite compositions and abun-dances of noble gas carriers indeed reflect the same proc-ess, volatile-element and noble gas signatures are the resultof different levels of heating of presolar materials ratherthan variably efficient condensation, because the presolargrains would not have survived in a completely vaporizednebula (Huss et al., 2003).

Primordial noble gases predominantly reside in fine-grained constituents (“matrix”), whereas high-temperaturephases such as chondrules and CAIs are gas-poor (Göbelet al., 1982; Nakamura et al., 1999a; Vogel et al., 2004a,b).The one intriguing exception, the high concentrations oftrapped heavy noble gases in the chondrules of an enstatitechondrite (Okazaki et al., 2001), has been discussed in sec-tion 3.3.5. The most detailed study of noble gases in chon-drules is by Vogel et al. (2004a). At least about 20% of allstudied chondrules contain primordial Ne that cannot beexplained by possible contamination with gas-rich matrix.Even most chondrules contain primordial Ar, on levels ofup to a few percent of the respective matrix values. Primor-dial noble gases were thus not quantitatively expelled fromall chondrules, although these were once at least partly mol-ten. Vogel et al. (2004a) conclude that chondrules retain HLgases better than Q gases. The latter and their carrier appearto have been removed from chondrules by metal-sulfides.

Fig. 5. Nitrogen-isotopic variation of the main meteorite group-ings. Solid bars show range of δ15N values for whole-rock or well-characterized significant components. Hashed bars show the rangeof δ15N values for minor components (other than known presolargrains and nanodiamonds) — e.g., organic fractions, peaks instepped heating extractions. Developed from reviews by Botta andBada (2002), Grady and Wright (2003), and Messenger et al.(2003), as well as other papers referenced in the text. ISM andsolar values inferred — see sections 3.5 and 5.3 and Fig. 7.

Fig. 6. Abundances of primordial noble gas components or car-riers in three different carbonaceous chondrites normalized toabundances in the CI chondrite Orgueil. The components are ar-ranged in order of increasing resistance to chemical and thermaldestruction of the carriers from left to right. The figure illustratesthat a correlation exists between volatile-element depletion in bulkmeteorites and a depletion of fragile noble gas carriers. Figureadapted from Huss et al. (2003).


In contrast to chondrules, CAIs do not contain measur-able amounts of primordial Ne (Göbel et al., 1982; Vogelet al., 2004b). Occasional reports of the presence of 22Ne-rich Ne-G or Ne-R should be viewed with caution, becausecosmogenic Ne from Na-rich minerals (abundantly presentin CAIs) mimicks additions of pure 22Ne. Presumably, CAIsare also devoid of primordial Ar (Göbel et al., 1982; Vogelet al., 2004b). Unless CAI precursors were much poorer inprimordial noble gases than chondrule precursors, the noblegas data thus imply a more efficient degassing of the CAIprecursors, in agreement with evidence that CAIs once werehotter and cooled more slowly than chondrules (Jones etal., 2000).

A few studies attempted to elucidate the accretionary his-tory of primitive meteorites based on the distribution of no-ble gases in fine-grained components on a submillimeterscale (Nakamura et al., 1999a; Vogel et al., 2003). In mostcases fine-grained rims around chondrules show the highestconcentrations of primordial gases. This supports rim accre-tion around chondrules in the nebula and argues against rimformation by aqueous alteration on parent bodies. The noblegas carriers phase Q and presolar diamonds are well mixedon a scale of a few micrograms (Nakamura et al., 1999a).The decrease of noble gas concentrations from rim to ma-trix appears to reflect a progressive dilution of noble-gas-rich nebular dust with gas-poor material (Vogel et al., 2003).However, in parts of the nebula, an opposite effect seemsto have happened, since, e.g., in the LL chondrite Krymkathe chondrule rims are less gas-rich than the matrix. Macro-scopic assemblages of the presumed noble-gas-rich dustadded to the nebula in the Krymka region are present as darkinclusions in this meteorite (Vogel et al., 2003).

5. NOBLE GAS AND NITROGENCONSTRAINTS ON THE METAMORPHIC

HISTORY OF METEORITE PARENT BODIES

In this section we will discuss the evolution of primordialnoble gas and N inventories during parent-body differentia-tion and alteration. Ureilites are discussed in section 3, how-ever, because they show primitive features such as large Cand noble gas contents that are incompatible with a moreevolved, igneous origin.

5.1. Primordial Noble Gases andNitrogen in Chondrites

The trapped noble gas content in chondrites is one crite-rion for their metamorphic classification (Heymann andMazor, 1968; Sears et al., 1980; Anders and Zadnik, 1985).This presumes that all members of a certain class had thesame original primordial gas inventories and that the ma-jor gas carriers react similarly to the various planetary proc-esses. Indeed, within a meteorite class, the concentrations ofQ, HL, P3, and P6 are all strongly inversely correlated withthe metamorphic grade (Huss, 1997). Phase Q is more re-sistant than presolar grains to thermal and aqueous altera-

tion (Huss and Lewis, 1995; Huss, 1997; Nakasyo et al.,2000), whereas diamonds are more resistant to shock thanphase Q (Nakamura et al., 1997). Phase Q may consist oftwo subpopulations that are distinctly susceptible to ther-mal and aqueous alteration and carry Q gases of slightlydifferent elemental composition (Busemann et al., 2000).Hence, subclassifications based on planetary alteration suchas those suggested for CV or CM chondrites (Browning etal., 1996; Guimon et al., 1995) are reflected in the elementalcomposition of the Q gases (Busemann et al., 2000).

Only a few studies on the N-isotopic signatures of ordi-nary chondrites have been reported, possibly because analy-ses of the low N contents encountered are challenging. Themost unequilibrated ordinary chondrites (type 3) containsignificant amounts of organic material. Alexander et al.(1998) argued that this was initially similar to the 15N-richmacromolecular material present in CR carbonaceous chon-drites. This organic material shows decreasing δ15N withincreasing C/N ratio as graphitization progresses as the re-sult of thermal metamorphism. This shows that the 15N-richcomponent is more labile (Alexander et al. (1998) and ac-companies an overall decrease in N abundance with increas-ing metamorphic grade (Hashizume and Sugiura, 1995). Athigher petrologic types (4–6) the measured N abundancereaches a plateau as it approaches the detection limit of theanalytical systems (Hashizume and Sugiura, 1995). Whilelarge isotopic heterogeneity exists (δ15N from –200‰ to+750‰) between different ordinary chondrites (Sugiura andHashizume, 1992), some homogenization of internal isoto-pic variation would be expected during the metamorphicevent, potentially diluting the isotopic extremes measured(Hashizume and Sugiura, 1995). Taking this further, nu-merical simulation of the redistribution of reactive volatileelements during thermal metamorphism of an OC parentbody indicates that the observed N concentration in taenitecan only be achieved with a parent-body interior with lowpermeability and that the isotopic heterogeneity observedcan be retained on a single parent body provided that the sizeof isotopically distinct components is large, i.e., >0.1 of thebody (Hashizume and Sugiura, 1998).

5.2. Primordial Noble Gases and Nitrogenin Differentiated Meteorites

We noted in the previous section that primordial noblegas concentrations in chondrites strongly decrease with in-creasing metamorphic processing. One might thus expectthat differentiated meteorites contain even less primordialgases than highly metamorphosed chondrites, reflectinglosses during igneous activity. Indeed, many differentiatedmeteorites contain negligible amounts of primordial Ne andAr, but only very few are largely devoid of any primordialXe (see Busemann and Eugster, 2002, for compilation).Nevertheless, the usually small amounts of trapped heavynoble gases are often overprinted by terrestrial contamina-tion, since atmospheric Xe in particular can be adsorbedvery tightly (e.g., Niedermann and Eugster, 1992). In cases


where contamination is small, the primordial Xe-isotopicsignature resembles mostly Q-Xe (Busemann and Eugster,2002), supporting the idea that the achondrites were formedfrom chondritic material.

Many acapulcoites and lodranites, primitive achondritesbeing residues of partial melting of chondritic precursors,show remarkably high concentrations of primordial noblegases. Values are much higher than those of more evolvedachondrites and sometimes almost reach those of primitivechondrites (Busemann and Eugster, 2002). This indicatesthat the degree of igneous planetary processing of originallyprimitive matter may be one parameter that controls the sur-vival of primitive noble gases. Metal- and troilite-rich phasesof Lodran show the largest concentrations of heavy Q gases,indicating that carbonaceous phase Q might have partitionedinto the metal during partial melting (Busemann and Eug-ster, 2002). The acapulcoite Y 74063 contains much higherXe concentrations, approaching those of the most gas-richureilites. These gases might reside in gas bubbles formedduring partial melting and oxidation of originally carbona-ceous carriers (Takaoka et al., 1994). This is in line with theobservation that much of the Xe in Acapulco is lost uponcrushing (Kim and Marti, 1994).

In the case of N, the acapulcoites show minimum δ15Nvalues comparable or even lower than those in ureilites withvalues of –120‰ to –154‰ (El Goresy et al., 1995; Kimet al., 1995). The light N is present in a number of phasesincluding graphitic particles associated with, but most likelynot exsolved from, metal (El Goresy et al., 1995), or in themetal itself (Kim et al., 1995). However, other phases suchas the chromites and silicates have much higher δ15N values,around 15‰ (Kim and Marti, 1994). This indicates that iso-topic homogeneity was not achieved from what was clearlya very heterogeneous body with respect to N, despite tem-peratures reaching the melting point of the rock. The lodran-ites, which have experienced hotter or longer heating, con-tain little N, with a uniform isotopic composition around10‰ (Kim and Marti, 1994), indicating that the more ex-tensive heating resulted in major loss of volatiles and ho-mogenization of what remained.

Observations of trapped noble gas components in ironand stony-iron meteorites are rare, in agreement with thehigh temperatures probably experienced by these meteor-ites. Trapped gases were mainly found in silicate inclusionsof IAB iron meteorites, often associated with C (e.g., Bo-gard et al., 1971; Crabb, 1983; Mathew and Begemann,1995), and in silicates of the pallasite Brenham. The lattercontains solar Ne and possibly Ar (Mathew and Begemann,1997) and solar or possibly U-Xe, a hypothetical componentthat will be discussed below (Nagao and Miura, 1994), im-plying an early incorporation of the gases, prior to the for-mation of the pallasite parent body.

The Xe-isotopic composition in iron meteorites appearssurprisingly inhomogeneous. Primordial Xe in most silicateinclusions is similar to that in chondrites (Bogard et al.,1971). Nonmagnetic acid residues from the Campo del Cielo(“El Taco”) IAB iron meteorite may show a Xe composi-

tion similar to terrestrial Xe, but depleted in the light iso-topes (Murty et al., 1983). However, these anomalies couldnot be found in acid-resistant graphite residues from thesame meteorite (Mathew and Begemann, 1995). Those sam-ples contain Xe that appears distinct from all known Xecomponents (“El-Taco-Xe”), which could be fractionatedU-Xe plus 244Pu fission Xe or fractionated solar Xe plus afission component of unknown origin. Otherwise, Xe in thesilicates is common Q-Xe (Mathew and Begemann, 1995).Xenon components similar to El-Taco-Xe have also beenreported in graphite from the IAB iron meteorite Bohumilitz(Maruoka et al., 2001).

Xenon-HL from presolar diamonds has been found inthe IAB iron meteorite Mugura (Maruoka et al., 1998) andmay also be present in the Campo del Cielo residues ofMathew and Begemann (Maruoka, 1999). Similarly, Ne-Qand Ne-HL may be present in carbonaceous residues of theCanyon Diablo iron meteorite, implying that their carrierswere present when the carbonaceous material in the ironmeteorites was incorporated (Namba et al., 2000). The ob-servation of terrestrial Ar, Kr, and Xe in iron meteorites iscommonly attributed to terrestrial weathering, although anextraterrestrial origin has also been postulated (Hwaung andManuel, 1982). It has been suggested that these inhomo-geneities imply a low-temperature formation of iron meteor-ites directly from the nebula (Kurat, 2003). However, abun-dant presolar grains and phase Q would be expected inprimitive nebular condensates. Their absence argues againstsuch a hypothesis. In any case, the results discussed aboverequire further confirmation.

The presence of U-Xe or fractionated U-Xe has beenreported for some differentiated meteorites (Michel andEugster, 1994; Nagao and Miura, 1994; Weigel and Eugster,1994; Mathew and Begemann, 1995). U-Xe is a theoreti-cally derived component, supposed to represent the mostprimitive solar system Xe. It is similar to solar Xe exceptfor lower abundances in the two heaviest isotopes (Pepin,2000) (Fig. 4) and is essential to account for the Xe com-position in the present terrestrial atmosphere. However,most of the experimental evidence for U-Xe could not beconfirmed. Xenon in the silicates of Brenham can be ex-plained as a mixture of the known Xe components solar, air,fission, and spallogenic (Mathew and Begemann, 1997) andXe in Tatahouine and Lodran appears to be Xe-Q plus air(Busemann and Eugster, 2002). Thus, the alleged presenceof the most primitive U-Xe in differentiated meteorites can-not be used to imply their direct formation in the nebula asproposed by Kurat (2003).

The iron meteorite groups show a wide range of δ15Nvalues from –95‰ to +155‰ although quite limited withinindividual groups (Franchi et al., 1993; Prombo and Clay-ton, 1993). Given that measurable quantities of O are rarelypresent in these highly reduced meteorites, the N-isotopicsignature is a useful tool in determining genetic links be-tween some stony and iron meteorite groups — e.g., IIEirons and H chondrites have δ15N values around 20‰ andIVA irons and L chondrites around 0‰ (e.g., Prombo and


Clayton, 1993). Some isotopic fractionation of the N mayoccur during slow degassing of the molten or solid metalas it cools (Franchi et al., 1993) — this has been taken toextreme levels in the case of the pallasites, which, althoughstrongly related to the IIIAB irons (Scott, 1977), containmuch less N and δ15N values up to 100‰ higher than thelatter (Prombo and Clayton, 1993). Correlation of the Ncontent with δ15N for the pallasites indicates that extensiveloss of molecular N has occurred during their formation(Franchi et al., 1993), the extent of which may offer infor-mation on the location of the pallasites within the IIIABparent body.

The origin of the N in the iron meteorites is unclear. TheN content of metal condensing from the solar nebula shouldbe very low — around 0.1 ppm or less for reasonable pres-sure-temperature (PT) conditions (Fegley, 1983), yet theiron meteorites contain several orders of magnitude higherconcentrations of up to 85 ppm (e.g., Prombo and Clayton,1993; Franchi et al., 1993). This indicates that the N musthave been acquired by the metal at a later stage, most prob-ably by a redistribution of organic N during the heating/melting event on the parent body (Fegley, 1983). As out-lined above, the δ15N values of organic matter in meteoritesand IDPs are generally much larger than most of the valuesfound in the irons, indicating that an additional source ofaccretable 14N-rich N may be required.

The distribution of N within the iron meteorites is quiteheterogeneous, with γ-Fe containing up to 1 wt% N whilethe α-Fe was recorded as containing 200 ppm — most ofwhich was deemed to be contamination (Sugiura, 1998).This diffusion-controlled redistribution of N during coolingand subsolidus crystallization of the metal phase generatesdistinctive and variable N-distribution curves within the γ-Fe that appear related to the cooling rate of the meteorites(Sugiura, 1998). Calibration of such profiles may offer val-uable insight into the thermal history of these parent bodies.

5.3. Solar Nitrogen in the Lunar Regolith

Although this chapter does not review noble gasestrapped from the solar wind in dust on the surfaces of aster-oids and the Moon (see Wieler, 1998, 2002; Podosek, 2003),we address here the controversial origin of N in the lunarregolith, which is relevant for the inferred isotopic composi-tion of N in the Sun and the solar nebula. The most recentcomprehensive reviews of the topic are by Becker et al.(2003) and Marty et al. (2003).

At the center of the controversy is the observation that theN-isotopic composition in lunar soils varies dramatically —by up to ~35% — in samples of different antiquity as well asin different extraction steps of individual samples (Kerridge,1975; Clayton and Thiemens, 1980). For a long time theprevailing interpretation has been that this implies a secu-lar increase of the 15N/14N ratio in the solar wind, with thepresent-day value being higher than the atmospheric ratioby perhaps ~100‰ (Kerridge, 1993). The alternative inter-pretation is that only a minor part of the N in lunar soils is

of solar wind origin and that the isotopic variability is due tovariable admixture of nonsolar sources, presumably meteor-ites, micrometeorites, or comets (Wieler et al., 1999; Hashi-zume et al., 2000, 2002; Marty et al., 2003). Hashizume etal. (2000) derived an upper limit of the δ15N value in thesolar wind of –240‰ relative to the terrestrial atmosphere,close to the value inferred earlier for ancient solar wind(Clayton and Thiemens, 1980). The isotopically light com-position of solar-wind N advocated by Hashizume et al.(2000) is in agreement with measurements in Jupiter’s at-mosphere (Fouchet et al., 2000; Atreya et al., 2003). Withinits large limits of uncertainty, the reevaluated solar-wind15N/14N ratio measured by the Solar and Heliospheric Ob-servatory (SOHO) mission (Kallenbach, 2003) is consistentwith either of the values (δ15N ~+100‰, <–240‰) inferredfor the solar wind from lunar samples. Figure 7 shows asummary of these main isotopic compositions. Here weadopt the Jupiter value 15N/14N = (2.3 ± 0.3) × 10–3 or (δ15N =–374 ± 82)‰ measured by the Galileo probe mass spec-trometer (Atreya et al., 2003) as the isotopic composition ofprotosolar N.

6. OUTLOOK

Much progress has been made since the publication ofMeteorites and the Early Solar System (Kerridge and Mat-thews, 1988) toward a more detailed characterization of theprimordial noble gas and N record in meteorites, as has been

Fig. 7. Nitrogen-isotopic composition of major components inthe solar system. Jupiter NH3 values from Fouchet et al. (2000),Owen et al. (2001), and Fouchet et al. (2004). Cometary data fromComet Hale-Bopp from Jewitt et al. (1997) and Arpigny et al.(2003). Solar-wind data from Kallenbach (2003) and Hashizumeet al. (2000). The shaded box shows the range of whole-rock me-teoritic values with the exception of the CH meteorites, which maybe strongly influenced by late-stage additional components. The“Solar?” and “ISM?” values are inferred from the other reservoirsand calculated fractionation effects respectively (see sections 5.3and 3.5).


summarized in this chapter. Nevertheless, some of the majorquestions asked in 1988 remain unanswered today. Here weattempt to sketch out top-priority issues that we hope mayhave come closer to a solution at the time Meteorites and theEarly Solar System III will go to press.

Presolar grains will remain one of the hot issues in me-teoritics in the foreseable future. Parallel to the continuousimprovements of microanalytical techniques to study singlepresolar grains (Nittler, 2003), noble gas studies on individ-ual grains as pioneered by Nichols and co-workers (Nicholset al., 1992a, 1994) should be extended to include smallergrains and, if possible, the heavy noble gases. Ideally, noblegas studies should be combined with prior isotope analysesof other elements on the ion probe. In particular, combiningthe extensive dataset of single-grain N data from secondaryion mass spectrometry (SIMS) analyses with noble gas dataand more conventional step-heating data would help in theidentification of populations and the ability to track themthrough secondary processes. For nanodiamonds individualgrain analyses are not feasible, but it will be important tocontinue efforts to separate the H and the L branches of Xe-HL in diamond separates (Verchovsky et al., 1998; Meshiket al., 2001), as this would allow the recognition of differentdiamond populations.

A better understanding of carriers and trapping mecha-nisms of the multitude of primordial noble gas componentsnot unambiguously residing in presolar carriers remainsessential as well. The nature of the elusive noble gas carrierphase Q needs to be studied further, presumably by devel-oping increasingly sophisticated physical separation meth-ods (e.g., Amari et al., 2003) combined with modern toolsof C-compound characterization. Trapping and release ofnoble gases by laboratory simulations also needs further in-vestigation (Hohenberg et al., 2002, Koscheev et al., 2001).The presence of primordial solar noble gases needs confir-mation, e.g., by further in vacuo etch analyses of bulk sam-ples from meteorites known to contain “subsolar” noblegases. Further miniaturization may allow combination of the“semimicroscopic” and the “microscopic” approach to studythe siting of primordial noble gases, by analyzing acid-re-sistant residues of well-characterized phases instead of bulkmeteorites.

A related topic of high priority is the improved deter-mination of the isotopic composition of the solar wind by theGenesis mission. Apart from O, most urgent in this contextare N and the heavy noble gases Kr and Xe. In the case ofN such a result is very important, given the large range ofisotopic ratios observed in meteorites and IDPs. Furthermeasurements of the N-isotopic variation in comets and theISM are required in order to better constrain the range of val-ues present. Also, more refined modeling or simulation ofthe possible reactions capable of producing large isotopicfractionations may help close the gap with the measuredrange in δ15N.

A central open question in noble gas cosmochemistry isthe origin and evolution of noble gases in planets. This topichas recently been reviewed by Pepin and Porcelli (2002)

and Porcelli and Pepin (2003). It would have been far be-yond the scope of this contribution to address it in detailhere, but it is clear that elucidating the role of meteoriticnoble gases and N in this context — if there is one — iscrucial. The fact that isotopic patterns of noble gases in plan-etary interiors often are “solar-like” (Becker et al., 2003)led many workers to favor mechanisms to incorporate noblegases in planets directly from a solar nebula source, avoid-ing the need to invoke large contributions of fractionatedmeteoritic gases [of the “planetary type” in the terminologyof Signer and Suess (1963)]. However, recent observationsof solar-like primordial noble gases in meteorites (e.g.,Busemann et al., 2003a, 2004) and suggestions that solar-wind implantation on planetesimals might be a source ofterrestrial noble gases (Podosek et al., 2000) may require areassessment of the importance of planet building blocks assuppliers of terrestrial noble gases. Since the isotopic com-position of noble gases in planetary atmospheres is distinctlydifferent from solar composition, the present atmosphericnoble gas compositions must be the result of fractionatingprocesses, either on the planets themselves or their buildingblocks. This is comprehensively reviewed by Pepin and Por-celli (2002) and Porcelli and Pepin (2003).

Acknowledgments. This work has been supported by grantsto the Swiss National Science Foundation and PPARC. We appre-ciate comments by N. Vogel; discussion with A. Verchovsky; thor-ough and constructive reviews by G. Huss, U. Ott, and R. Pepin;and editorial handling by F. Podosek. H.B. thanks the CarnegieInstitution for enabling him to finish this manuscript.

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