Pristine extraterrestrial material with unprecedentednitrogen isotopic variationGiacomo Briania,b,1, Matthieu Gounellea, Yves Marrocchia, Smail Mostefaouia, Hugues Lerouxc, Eric Quiricod,and Anders Meiboma

aLaboratoire de Mineralogie et Cosmochimie du Museum, Museum National d’Histoire Naturelle, Centre National de la Recherche Scientifique, Unite Mixtede Recherche 7202, Case 52, 57 Rue Cuvier, 75005 Paris, France; bDipartimento di Astronomia e Scienza dello Spazio, Universita degli Studi di Firenze, LargoE. Fermi 2, 50125, Firenze, Italy; cLaboratoire de Structure et Proprietes de l’Etat Solide, Unite Mixte de Recherche 8008, Universite des Sciences etTechnologies de Lille, F-59655 Villeneuve d’Ascq, France; and dUniversite Joseph Fourier, Centre National de la Recherche Scientifique/Institut National deSciences de l’Univers, Laboratoire de Planetologie de Grenoble Unite Mixte de Recherche 5109, BP 53, 38041, Grenoble, Cedex 9, France

Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved May 6, 2009 (received for review February 11, 2009)

Pristine meteoritic materials carry light element isotopic fraction-ations that constrain physiochemical conditions during solar sys-tem formation. Here we report the discovery of a unique xenolithin the metal-rich chondrite Isheyevo. Its fine-grained, highly pris-tine mineralogy has similarity with interplanetary dust particles(IDPs), but the volume of the xenolith is more than 30,000 timesthat of a typical IDP. Furthermore, an extreme continuum of Nisotopic variation is present in this xenolith: from very light Nisotopic composition (�15NAIR � �310 � 20‰), similar to thatinferred for the solar nebula, to the heaviest ratios measured in anysolar system material (�15NAIR � 4,900 � 300‰). At the same time,its hydrogen and carbon isotopic compositions exhibit very littlevariation. This object poses serious challenges for existing modelsfor the origin of light element isotopic anomalies.

isotopic fractionation � pristine meteorites � Solar System

Physical and chemical conditions during the earliest stages ofsolar system evolution can be studied in chondritic meteor-

ites and interplanetary dust particles (IDPs), believed to beamong the most primordial objects left over from the formationof the solar system some 4.567 billion years ago (1). In this workwe describe the unaltered mineralogy and light element (i.e.,hydrogen, carbon, and nitrogen) isotopic composition of aprimordial xenolith in the chondrite Isheyevo. Isheyevo is aFe-Ni metal rich (60 vol% on average) chondrite with 2 domi-nant lithologies that resemble CB and CH chondritic material(2). Apart from Fe-Ni metal grains, this rock is composed ofchondrules, rare Ca-Al rich inclusions (CAIs) and hydrated lithicclasts but does not otherwise contain fine-grained matrix, whichseparates high-temperature components in most other types ofchondritic materials (2–4).

In a survey of 2 polished sections of Isheyevo (the CHlithology), representing a total area of about 400 mm2, weidentified more than 100 lithic clasts. Their mineralogy has beenstudied by energy dispersive X-ray (EDX) spectroscopy, analyt-ical electron microprobe (AEM), scanning electron microscopy(SEM), and transmission electron microscopy (TEM). In gen-eral, these lithic clasts show a high degree of aqueous alteration,consistent with previous reports (3, 4). This indicates that theyare not genetically related to the high-temperature componentsin Isheyevo, which do not show effects of in situ aqueousalteration (3, 4).

Lithic clasts found in our 2 sections range in size between 50and 750 �m. Their mineralogical properties are broadly similarto matrix material of CI, CM, or CR carbonaceous chondrites,that is, dominated by a fine-grained matrix composed of sub-�m–size phases, mainly phyllosilicates with variable amount ofFe-Ni sulfides and magnetite. Matrix AEM point analyses showhigh contents of S (between 2 and 15 wt%), due to the presenceof either tochilinite and/or Fe-Ni sulfides. Anhydrous silicatessuch as olivine and pyroxene, present either as isolated crystalsin the matrix or as microchondrules (�30–60 �m in diameter),

are less abundant. Olivine is mainly Mg-rich, with compositionsranging from Fo98 and Fo84, with a mean value of Fo89 [Fo isdefined as the mol% ratio Mg/(Mg�Fe)]. Pyroxene ranges incomposition between En63–98 Wo0 – 4 Fs1–36 [En � Mg/(Mg�Fe�Ca) mol%; Wo � Ca/(Mg�Fe�Ca) mol%; Fs �Fe/(Mg�Fe�Ca)]. Magnetite is very abundant in some lithicclasts and absent in others. Carbonates are rare but have beenidentified in some cases. Sulfide grains are mostly pyrrhotite,with some rare grains of pentlandite. Metal is present as small,�m-sized grains associated with sulfides.

Among the approximate 100 lithic clasts studied here, 1 stoodout by its distinct textural and mineralogical properties and forits extreme variation in N isotopic composition. We describehere the discovery of this primordial xenolith (hereafter namedPX-18) and the implications concerning models for light elementfractionation.

PX-18: MineralogyPX-18 is a dark xenolith (�380 � 470 �m2), dominated by a veryfine-grained matrix, mainly composed of anhydrous Mg-richsilicates with tiny Fe-Ni sulfides grains and magnetite (Fig. 1A).High-resolution SEM images and EDX spectral analyses re-vealed a matrix composed of sub-�m-sized crystals, includingMg-rich olivine and pyroxene crystals (Fo84–89; En89–95, Wo0–4)with sizes of about 1–2 �m, Fe-Ni metal grains, magnetite, andvery rare carbonates grains.

TEM examination confirmed that the PX-18 matrix is primar-ily composed of crystalline, anhydrous silicate grains, mainlyMg-rich pyroxene, with a few, �m-sized forsterite (Fo97–99)grains (Fig. 1B). Low-Ca pyroxene grains show a wide compo-sitional range (En63–100, Wo0–5) but enstatite clearly dominates(36% of analyzed grains has En99–100). The most fine-grainedphase (labeled ‘‘mx’’ in Fig. 1B) contains abundant Fa-richolivine (grain size 20–100 nm) with compositions ranging fromFo24 to Fo70, and an average around Fo40. Small Fe-Ni metal andFe-oxide grains are present as minor components betweensilicates. Importantly, high-resolution TEM observations did notdid not yield any occurrence of layered phyllosilicates, whichseem to be absent from PX-18. These observations indicate thatPX-18 is mineralogically similar to other primitive material, suchas chondritic porous IDPs (5) and comet 81P/Wild2 samplesreturned by the Stardust mission (6). Clearly, PX-18 has avoidedthe extensive aqueous alteration that affected the other xenoliths

Author contributions: G.B., M.G., and A.M. designed research; G.B., Y.M., S.M., H.L., E.Q.,and A.M. performed research; G.B., M.G., H.L., and E.Q. analyzed data; and G.B., M.G., Y.M.,S.M., H.L., E.Q., and A.M. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

1To whom correspondence should be addressed. E-mail: briani@mnhn.fr.

This article contains supporting information online at www.pnas.org/cgi/content/full/0901546106/DCSupplemental.

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in Isheyevo (2) and therefore, represents a more pristine sample,which might have better preserved isotopic anomalies of lightelements.

PX-18: Isotopic CompositionWe performed isotopic composition analyses of PX-18 and 4other xenoliths, covering a total area of 30,400 �m2. The isotopicratios D/H, 13C/12C, and 15N/14N were imaged by multicollectionsecondary ion mass spectrometry, using a CAMECA NanoSIMSN50. We report our results in the �-notation, which expresses thedeviation in parts per thousand from terrestrial standards (i.e.,the SMOW value for the D/H ratio, the atmospheric 15N/14Nratio for N, and the PDB value for 13C/12C).

NanoSIMS imaging of the H isotopic composition in the 5xenoliths yielded homogeneous distributions with average�DSMOW values ranging between �70 � 20‰ and 420 � 50‰.In other words, the bulk D/H ratios obtained are generally higherthan the SMOW value, but no extreme D/H ratios were ob-served, neither in bulk nor as individual hotspots, compared withprevious studies of IDPs (7) and insoluble organic matter (IOM)extracted from primitive chondritic meteorites (8). Carbon iso-

topic compositions were also found to be relatively homoge-neous within individual xenoliths with values ranging from�13CPDB � �35 � 3‰ to �17 � 2.5‰, comparable to bulk Cisotopic composition of chondritic meteorites (9) and IOM (10).

The N isotopic compositions yielded a dramatically differentdata set. For all 5 xenoliths, systematic bulk enrichments in 15Nwere observed, with �15NAIR ranging from 110 � 10‰ to 318 �16‰. Even larger N isotopic anomalies were found in themineralogically primitive PX-18. The main characteristics of theN isotopic distribution in PX-18 are the following: 1) large areaswith a diffuse, but substantial enrichment in 15N over thesurrounding material (Fig. 2A); 2) extremely 15N-enriched, lo-calized anomalies (hereafter called hotspots, Fig. 2B); and 3)areas with negative �15NAIR values, comparable to those of thesolar nebula (11–13).

A total of 13 NanoSIMS images were obtained from PX-18,covering an area of approximately 12,800 �m2. These imageshave average �15NAIR values that range between 30 � 20‰ and700 � 20‰, indicating a highly heterogeneous distribution ofthe 15N/14N ratio in PX-18 (Table 1). In 3 images obtained onPX-18 (each 40 � 40 �m2), more than 90% of the surface ischaracterized by �15NAIR greater than or equal to 250‰ and

Fig. 1. PX-18 in the Isheyevo chondrite. (A) High-resolution SEM image. Theminimum and maximum dimensions of PX-18 are 380 �m and 470 �m,respectively. This xenolith is clearly dominated by matrix, but some largeolivine and pyroxene grains are also visible. White inclusions are metal grains.The black arrow indicates the point from which the TEM thin section showedin B has been extracted. (B) TEM bright-field micrograph of a thin sectionextracted by focused ion beam (FIB) technique from PX-18 in Isheyevo. Indi-vidual analyses made by TEM are reported in the figure. A large olivine grain(Fo100) is visible at the bottom of the section. ‘‘En’’ labels indicate pyroxenegrains with En98�100. The other pyroxene grains analyzed are: px1 (En87 - Wo1);px2 (En91 - Wo1); px3 (En63 - Wo1); px4 (En95 - Wo1); and px5 (En93 - Wo1). Fe-Nimetal grains (‘‘met’’) are visible in the left part of the section. Two grains ofpyrrhotite are also present (‘‘pyr’’). Labels ‘‘mx’’ indicate a very fine-grainedmatrix, composed of tiny crystals and minor amorphous material, with aver-age composition frequently close to olivine Fo60.

Fig. 2. Results of NanoSIMS measurements on PX-18: the �15NAIR distributionof two 40 � 40 �m2 regions (A and B) and hotspots with internal structures(C–E). Color scale has been changed for images C to E to better show internalstructures of hotspots. In all of the images, the black areas are regions in whichprecise isotopic ratios could not be determined. These pixels have thereforebeen removed from each image. (A) Region with average �15NAIR � 640 �11‰. About 90% of this image is characterized �15NAIR greater than or equalto 250‰ (Table 1). Several hotspots are also visible (indicated by white arrows).(B) Area with average �15NAIR � 700 � 17‰. Two hotspots, approaching�15NAIR � 4000‰, are indicated by arrows. (C) The hotspot is 0.598 �m2

(�773 � 773 nm2) in size and has �15NAIR � 3,200 � 150‰. The yellow internalregion has a surface of 0.251 �m2 (�500 � 500 nm2) and �15NAIR � 4,110 �200‰. 15N-richer, white areas are visible. They have �15NAIR � 4,900 � 300‰,and a total surface of 0.0967 �m2 (�310 � 310 nm2). This is the only case inwhich is possible to define 2 internal structures in the same hotspot. (D) For thelarger isotopic anomaly (1.29 �m2, �15NAIR � 2,600 � 170‰) an internalhotspot corresponding to the yellow region can be defined, with �15NAIR �3,700 � 300‰ and a surface of 0.467 �m2 (�683 � 683 nm2). For the smallerisotopic anomaly (1.1 �m2, �15NAIR � 1700 � 110‰), the internal hotspot hasan area of 0.124 �m2 (�352 � 352 nm2), with �15NAIR � 3,000 � 300‰. (E)Again, the yellow region represents the internal hotspot, corresponding to�15NAIR � 3,100 � 470‰ with a surface of 0.079 �m2 (�280 � 280 nm2) [for theentire hotspot the size is 0.896 �m2 (�946 � 946 nm2) and �15NAIR � 1,800 �160‰].

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very elevated average �15NAIR values (630 � 20‰, 640 � 11‰and 700 � 17‰). These high values are not due to the presenceof only a few small anomalously 15N-rich regions but rather to theoccurrence of a diffuse 15N-rich component, present in largefractions of the images (Fig. 2 A and Table 1). This is demon-strated by the frequency distribution of �15NAIR values obtainedfrom a grid of 600 � 600 nm2 squares laid over each �15NAIRimage. The resulting frequency distributions (Fig. 3) clearly showcontinuous and broad ranges of �15NAIR values, centered on the�15NAIR average value obtained for each image.

In addition to the diffuse distribution of 15N-enriched mate-rial, forty-six 15N hotspots with extremely high �15NAIR wereobserved in PX-18 (Fig. 2B). These hotspots, with areas ofapproximately 1 �m2, are distinct from the aforementioned,broad 15N-enriched zones, because their �15NAIR values minus a3� error bar (where � is the error on the hotspot �15NAIR value)are higher than the average �15NAIR value for the image plus 3�av(where �av is the error on the average �15NAIR value of the entireimage). These hotspots are highly heterogeneously distributed.For example, 1 NanoSIMS image (40 � 40 �m2) contains nohotspots, while other areas in PX-18 of similar size have between2 and 10 hotspots, and 1 image contains as much as 16 hotspots(Table 1). All of the 15N hotspots have sizes larger than 300 �300 nm2, up to approximately 2 � 2 �m2. The highest �15NAIRfor a single hotspot is 3,200 � 150‰. However, several hotspotsexhibit distinct internal structures, that is, small internal regionswith even higher values of �15NAIR. Four such cases have beenidentified (Fig. 2 C–E), in which the averages �15NAIR values foreach hotspot are 1,700 � 110‰, 1,800 � 160‰, 2,600 � 170‰,and 3,200 � 150‰, and in which there are subregions withhigher �15NAIR values of 3,000 � 300‰, 3,100 � 500‰, 3,700 �300‰, and 4,900 � 300‰, respectively. These hotspot subre-gions are the highest �15NAIR values ever measured in solarsystem material: 3,700 � 300‰ and 4,900 � 300‰ (Fig. 4). Westress here that these are results obtained by in situ measure-ments, and hence they represent an isotopic composition that isa mix of the 15N-rich phase with other, 15N-poor phases. Thisimplies that our highest �15NAIR values are a lower limit for the15N enrichment in the carrier phase.

Regions with negative �15NAIR values have been also identi-fied in PX-18. In some cases, they represent up to 52% of theanalyzed surface (Table 1). The global mean �15NAIR valueevaluated from all of the image fractions with negative �15NAIRis �150 � 30‰, and the minimum value measured for a singleimage is �15NAIR � �310 � 20‰ (Fig. 4). This latter value isconsistent with values inferred for the solar nebula (11–13) fromprevious measurements (see Fig. 4 for details).

The large range of �15NAIR values observed in PX-18 fromIsheyevo (Fig. 4) greatly expands the range of N isotopiccomposition for a single extraterrestrial object or type of mate-rial (e.g., IOM). On one hand, the heaviest N isotopic compo-

sitions observed (�15NAIR � 3,700 � 300‰ and 4,900 � 300‰)are enriched in 15N by factors of 4.7 and 5.7, respectively,compared with terrestrial atmospheric N and even more (factorsof 7.3 and 9.1, respectively) when compared with the bestestimate for the N isotopic composition of the solar nebula (11).On the other hand, negative �15NAIR values (�15NAIR � �310 �20‰) similar to those of the solar nebula and corresponding tothe lowest observed values, have also been measured. This,combined with PX-18 unique mineralogy, is consistent with thenotion that only the most pristine, unaltered material canpreserve primordial isotopic compositions in light elements.

The Carrier Phase of the N AnomaliesTo verify whether 15N hotspots in PX-18 are associated withspecific carriers, it is possible to exploit the high spatial resolu-tion of the NanoSIMS images (100–200 nm). By comparingNanoSIMS images with high resolution SEM images (see Fig.S1), the site of the 15N hotspots in PX-18 were located in thematrix and studied in more detail. Several important observa-tions were made: (i) hotspots have linear dimensions substan-tially larger than the size of typical matrix grains (see Fig. 5 foran example), (ii) typically, hotspot regions contain abundant,sub-�m-sized grains of Mg-rich pyroxene and magnetite, and(iii) EDX spectra of hotspot regions show a general enrichmentin C with respect to spectra obtained in other regions of PX-18(Fig. 5).

Presolar SiC grains or nanoglobules (14) can be excluded asthe carrier phase of the N isotopic anomalies because a) theygenerally have smaller size than the diffuse 15N enrichmentsobserved here, b) if 15N hotspots were due to presolar grains ornanoglobules, they would be identified in high resolution SEMimages at the hotspots locations (or in TEM observations), c)organic nanoglobules have D enrichments (14), not measured inPX-18, and d) most SiC grains carry with them large anomaliesin C, which are not observed (15). Together, these observationslead to the conclusion that 15N hotspots in PX-18 are due to thepresence of organic matter (OM).

The spatial extension of 15N-rich regions in PX-18 suggests thepresence of a diffuse component as carrier of these anomalies.Indeed, macromolecular OM has been identified in PX-18 byRaman spectroscopy. PX-18 was analyzed together with matrixfragments extracted from pristine chondrites that were used asstandards: Murchison (CM2), Cold Bokkeveld (CM2), Se-markona (LL/3.00), and Bishunpur (LL/3.15). The spectra ex-hibit the first-order carbon bands D and G, superimposed ontoa fluorescence background indicating that the macromolecularOM is widespread and abundant. Inspection of the combined setof spectra (Fig. S2) shows that in PX-18 these 2 bands are widerand more overlapping than in the type 3 ordinary chondritesSemarkona and Bishunpur. This suggests a lower degree ofthermal metamorphism in PX-18 macromolecular OM.

Table 1. Characteristics of isotopic measurements for eight 40- � 40-�m2 images from PX-18

Image Area, �m2 Average �15NAIR, ‰ �15NAIR � 250‰, % area �15NAIR � 0‰, % area No. of hotspots

g�31 394.6 140 � 13 34.26 27.42 3g�32 541.2 630 � 20 92.63 1.05 10g�33 507.6 700 � 17 95.76 0.79 2g�41 1218.9 220 � 9 35.89 29.95 7g�42 1082.1 640 � 11 89.67 1.45 16g�48 888.6 110 � 20 23.48 43.18 7g�49 601.4 30 � 22 28.78 52.63 0g�53 995.8 80 � 17 28.26 46.67 1

The column labeled ‘‘Area’’ reports the fraction of each ratio image that has a number of 14N counts at least equal to 5% of the 14N-image maximum. Average�15NAIR values are evaluated only on these image portions. These same regions also define for each image the basic area to which percentages of �15NAIR � 250‰ and �15NAIR � 0 ‰ refer.

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The Lorentz-Breit-Wigner-Fano (LBWF) fit analysis con-firmed the visual analysis of the spectra. The values of thespectral parameter FWHM-D (D band full width at half maxi-mum), which is a sensitive tracer of thermal metamorphism (16),rule out the presence of significant thermal metamorphism.Raman spectra from PX-18 have mean FWHM-D of 268 � 8cm�1 (Fig. S3), a value between those of the metamorphosedchondrite Bishunpur (170 � 20 cm�1) and Semarkona (250 � 3cm�1) and those of the little metamorphosed CM2 ColdBokkeveld (282 � 12 cm�1). To characterize highly disorderedcarbonaceous matter, the most suitable spectral parameters areFWHM-G and �G (G band peak position). A rough comparisoncan be made with data from IDPs and chondrites (17), keeping

in mind that different experimental conditions have been used.The G-diagram (Fig. 6) indicates that the macromolecular OMin PX-18 has a degree of thermal metamorphism lower than thatof the Bishunpur and Semarkona chondrites and comparable tothat of organic matter present in IDPs.

Nitrogen FractionationExcluding a stellar nucleosynthesis origin (i.e., related to preso-lar grains) for the observed N isotopic anomalies, processes suchas self-shielding and low-temperature ion-molecules reactionscan be considered. Self-shielding (18) is still being explored as anexplanation for O fractionation in early solar nebula (19–21) andit has been proposed to be effective also for N in the protosolaraccretion disk (20). However, quantitative models for N frac-tionation due to self-shielding have not been developed and theextent of N isotopic fractionation by this process is unknown.

Models for isotopic fractionation by low-temperature ion-molecule reactions can produce values of �15NAIR as high asthose observed in PX-18. In the most recent model (22), whichinvokes chemical reactions with N-containing molecules underdark molecular cloud conditions, values for �15NAIR of�9,000‰ for external layers of NH3 ice accreted on dust grainsare obtained. Transfer of fractionated N from NH3 ice to organicmatter is achieved by UV-induced transformations into polycy-clic aromatic hydrocarbons (PAHs). PAHs are abundant in thegas phase where N fractionation take place and they cancondense on dust grains and form ice (23). Experiments onvarious coronene-ice mixtures at low temperature (15 K) and low

Fig. 3. Frequency distribution for �15NAIR values of three 40 � 40 �m2 imagesfrom PX-18. These distributions are obtained from a grid of 600 � 600 nm2

regions of interest (ROIs) laid over each NanoSIMS image in areas for which12C14N signal was at least 5% of the image maximum. See Table 1 for averagevalues of each image. Vertical scale is the number of square ROIs in each �15Nbin. The right-hand tails of these distributions are due to hotspots, but thecorrespondence between ROIs and hotspots is only partial, because the ran-domly distributed, 600 � 600 nm2, ROIs do not exactly match hotspots. (A)Image g�32, the distribution is obtained from a total surface of 410.45 �m2. (B)Image g�33, the distribution is obtained from a total surface of 325.5 �m2. Thedistribution also contains a hotspot at �15NAIR � 3,900‰ (visible in the Inset).(C) Image g�42, the distribution is obtained from a total surface of 639.45 �m2.

Fig. 4. �15NAIR values measured in PX-18, compared with other data for solarsystem materials. Ranges reported in the figure comprise results from bulkmeasurements as well as from hotspots. The mean �15NAIR for the whole PX-18is 318 � 16‰. Bulk �15NAIR values for the other materials reported in the figurerange between �52.5‰ and 2,500‰ for CB/CH chondrites (31–33), between�108 � 9‰ and 500 � 20‰ for IDPs (7), between �54 ‰ and 452‰ for Wild2samples [more negative values are due to the presence of 15N-poor, 13C-richhotspots (34)], and between �66 � 3‰ and 415 � 2‰ for IOM extracted fromcarbonaceous chondrites (8, 10). The in situ measurements, and those on IOM,have been obtained by SIMS techniques. Stepped combustion has been per-formed for CB/CH chondrites. For HCN molecules in comets, astronomicalspectroscopy measurements (35) yielded values of �15NAIR � 330 � 450‰ (forcomet Hale-Bopp) and 960 � 370‰ (for comet 17P/Holmes). For CN moleculesin comets, �15NAIR values range between 650 � 400‰ and 1,100 � 450‰ (35,36). The values of the N isotopic composition for the solar nebula are obtainedfrom measures on: 1) ammonia in the Jupiter atmosphere analyzed by theGalileo spacecraft [�15NAIR � �370 � 80‰, (13)]; 2) solar wind N implanted inlunar regolith grains [�15NAIR � �240‰, (12)]; 3) an osbornite (TiN) inclusionin a CAI of Isheyevo [�15NAIR � �359 � 5‰, (11)].

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pressure have shown that -NH2 functional groups present in theice can be added to PAHs by the action of UV photolysis (23).

However, a fundamental problem is that low temperatureion-molecule reactions are also predicted to produce strongdeuterium enrichments in organic matter (24, 25), which are not

found in PX-18 or any other xenolith in Isheyevo. Other previousstudies (e.g., 2, 9, 10) have also found that H and N anomaliesare decoupled in extraterrestrial matter. These results call for anew theoretical and experimental approach, which must be ableto provide an explanation for the decoupling of these lightelement isotopic variations.

The primitive mineralogy of PX-18, in combination with theextreme range of N isotopic variation and the un-metamor-phosed organic matter it preserves, indicate that PX-18 repre-sents a class of extremely pristine extraterrestrial material, whichmight sample little processed outer solar system bodies. AKuiper Belt parent body is a likely possibility. The presence ofouter solar system material in an asteroidal body is a naturalconsequence of the idea that xenoliths represent samples ofprimordial matter scattered throughout the solar system duringthe Late Heavy Bombardment (26). In the context of globalscattering and mixing of bodies (27), objects composed of morefragile and/or unconsolidated material, such as primordial chon-dritic planetesimals, comets, and Kuiper Belt objects, are greatlyaffected by collisions (28), and can produce fragments that arescattered throughout the solar system and become embedded inmeteorite parent bodies.

Materials and MethodsTo search for primordial xenoliths, preliminary SEM and EDX spectral analysesof 2 Isheyevo polished sections have been performed at the Laboratoire deMineralogie et Cosmochimie du Museum of the Museum National d’HistoireNaturelle (Paris, France) by mean of a JEOL JSM 840-A SEM equipped with anEDAX Genesis X-rays detector, using a 3-nA primary beam accelerated by a15-kV potential difference. High-resolution SEM images have been acquiredat the University of Paris VI using a Zeiss Supra55 VP field emission SEM, witha 4-nA, 10-kV accelerated primary beam. By the same instrument, EDX spectralanalyses of sub-�m-sized grains in the matrix of PX-18 have been performed.

Fig. 5. High-resolution EDX analyses of PX-18 matrix.(A) High-resolution SEM image of a PX-18 matrix areacorresponding to 1 hotspot, indicated by the ellipse.Crosses labeled with white numbers are the pointsanalyzed by EDX spectroscopy. (B and C) EDX spectra ofpoints number 1 and 5, respectively. The first peak onthe left is that of carbon, clearly evident in these 2spectra. (D) EDX spectrum of point number 3. In thiscase, the C peak is smaller, corresponding to the back-ground signal due to the section carbon coating.

Fig. 6. G-diagram for results of Raman analyses on PX-18: the spectralparameter FWHM-G plotted against the peak position �G. This diagramreflects the degree of thermal metamorphism and structural order of theorganic matter. The solid and dashed lines show the range of values forchondritic meteorites and IDPs respectively (17). CM measurements wereobtained from matrix of Murchison (CM2) and Cold Bokkeveld (CM2).

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Quantitative analyses for the mineralogical composition of xenoliths havebeen obtained with a CAMECA SX-100 electron microprobe at the Universityof Paris VI. A 10-nA focalized beam, accelerated by a 15-kV potential differ-ence, is used for point analyses of xenoliths silicates, oxides, and metals.

For TEM analyses 2 thin sections have been extracted from PX-18 by focusedion beam technique at the Institut d’Electronique, de Microelectronique et deNanotechnologie (Lille, France). Individual matrix grains have been analyzed bya Philips CM 30 at the Laboratoire de Structure et Proprietes de l’Etat Solide of theUniversite des Sciences et Technologies (Lille, France). The accelerating voltagewas 300 kV. EDX microanalyses have been realized by a Noran detector.

Raman measurements have been performed at the Laboratoire de Sciencesde la Terre (ENS) with a Labram spectrometer (Horiba-Jobin-Yvon), equippedwith a Spectra Physics Argon ion laser using 514.5 nm excitation. The laserbeam was focused by a microscope equipped with a long distance workingobjective (X50), leading to a 2–3 �m spot. The reproducibility of Ramanmeasurements on poorly ordered macromolecular organic matter is con-trolled by atmospheric conditions, irradiation time, and power on the sample,due to heating and photo-oxidation effects during irradiation (29). Hencespectra have been acquired under strictly constant experimental conditions:power at the sample surface was 400 � 5 �W, exposure time of each mea-surement was 90 s and under an inert argon atmosphere. A 600 gr/mm gratinghas been used and provided with spectra in the spectral region 500–2,200cm�1, recovering the first- and second-order carbon bands. Raman spectrahave been quantitatively analyzed by fitting the first-order carbon bandsusing a Lorentzian profile for the D peak, and a Breit-Wigner-Fano one for theG peak [LBWF fit: see (30) for a discussion regarding the choice of theseprofiles]. The spectral parameters derived from the LBWF fit are the width ofthe bands FWHM-G and FWHM-D and their peak position �G and �D.

Secondary ion mass spectrometry analyses were performed with a Nano-SIMS CAMECA N50 at the Laboratoire de Mineralogie et Cosmochimie duMuseum of the Museum National d’Histoire Naturelle of Paris. We have useda Cs� primary beam in two sessions. The first session for detection of H� andD� (with a current on the sample of �40 pA) and the second one for 12C�, 13C�,12C14N�, and 12C15N� (current on the sample �4 pA). In each case an electronbeam has been used to compensate for charging effects. For H isotope

measurements, the experimental set up was such that the mass resolution wasM/M greater than or equal to 2,000. For C and N isotopes, a larger massresolution is needed, so the machine was set up to yield M/M between 7,500and 8,000. This is necessary to prevent possible interferences between differ-ent isotope masses, for example, 11B16O� on 12C15N�. To determine theinstrumental mass fractionation (IMF) we have used as standard reference atype III kerogen, with known isotopic compositions of H, C, and N in the samereference sample. The IMF was on the order of �315 � 5‰ for H, �50 � 1‰for C, and �6 � 4‰ for N. We selected for isotopic analysis a few 40 � 40 �m2

matrix-dominated regions from 5 xenoliths of Isheyevo. These regions wereexposed to high Cs� current (�1 nA) for 15–20 min before analyses, to reachsputtering equilibrium. Measurements were made in scanning imaging mode.For each selected region, between 7 and 35 plans composed of 256 � 256pixels were collected. Measurement time was 65.536 seconds per plan in thecase of H isotopes and 327.68 seconds per plan for C and N isotopes. Wherehotspots were found, close-up analyses on �10 � 10 �m2 areas have beenperformed. Details for data reduction can be found in the on-line SI Text. Herewe stress our criteria to define hotspots in 15N/14N ratio images. A selectedregion of interest (ROI) in 1 image is defined as a hotspot if: 1) (�15NAIR)hotspot

� 3�hotspot � (�15NAIR)av � 3�av, where �hotspot is the error on the (�15NAIR)hotspot

value of the selected ROI and (�15NAIR)av and �av are the average �15NAIR valueand error for the entire image; 2) the hotspot is larger than the image spatialresolution (all potential hotspots with size smaller than 250 � 250 nm2 havebeen discarded); and 3) each individual plan that composes the image has a15N/14N ratio that differs no more than 3�mean from the mean 15N/14N ratio(where �mean is the standard deviation calculated from the all set of isotopicratios relative to the same ROI).

The errors reported in the text express a 2� confidence level.

ACKNOWLEDGMENTS. We thank three anonymous reviewers that carefullywent through the manuscript and suggested corrections and improvements. Thisstudy was funded by the Program Nationale de Planetologie (PNP) and theEuropeangrantORIGINS[MRTN-ct-2006-035519].TheNationalNanoSIMSfacilityat the Museum National d’Histoire Naturelle was established by funds from theCentre National de la Recherche Scientifique, Region Ile de France, Ministeredelegue a l’Enseignement superieur et a la Recherche, and the Museum itself.

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