Nitrogen components in ureilites

Geochimica et Cosmochimica Acta, Vol. 67, No. 12, pp. 2213–2237, 2003 Copyright © 2003 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/03 $30.00 ⫹ .00

Pergamon

doi:10.1016/S0016-7037(02)01373-X

Nitrogen components in ureilites V. K. RAI,1 S. V. S. MURTY,1,* and U. OTT2 1

2

Physical Research Laboratory, Ahmedabad 380 009, India Max-Planck-Institut fu¨r Chemie, Becherweg 27, D-55128 Mainz, Germany (Received June 13, 2002; accepted in revised form November 20, 2002)

Abstract—Abundances and isotopic compositions of nitrogen and argon have been investigated in bulk samples as well as in acid-resistant C-rich residues of a suite of ureilites consisting of six monomict (Havero¨, Kenna, Lahrauli, ALH81101, ALH82130, LEW85328), three polymict (Nilpena, EET87720, EET83309), and the diamond-free ureilite ALH78019. Nitrogen in bulk ureilites varies from 6.3 ppm (in ALH 78019) to ⬃55 ppm (in ALH82130), whereas C-rich acid residues have ⬃65 to ⬃530 ppm N, showing approximately an order of magnitude enrichment, compared with the bulk ureilites, somewhat less than trapped noble gases. Unlike trapped noble gases that show uniform isotopic composition, nitrogen shows a wide variation in ␦15N values within a given ureilite as well as among different ureilites. The variations observed in ␦15N among the ureilites studied here suggest the presence of at least five nitrogen components. The characteristics of these five N components and their carrier phases have been identified through their release temperature during pyrolysis and combustion, their association with trapped noble gases, and their carbon (monitored as CO ⫹ CO2 generated during combustion). Carrier phases are as follows: 1) Amorphous C, as found in diamond-free ureilite ALH78019, combusting at ⱕ500°C, with ␦15N ⫽ –21‰ and accompanied by trapped noble gases. Amorphous C in all diamond-bearing ureilites has evolved from this primary component through almost complete loss of noble gases, but only partial N loss, leading to variable enrichments in 15N. 2) Amorphous C as found in EET83309, with similar release characteristics as component 1, ␦15N ⱖ 50‰ and associated with trapped noble gases. 3) Graphite, as clearly seen in ALH78019, combusting at ⱖ700°C, ␦15N ⱖ 19‰ and devoid of noble gases. 4) Diamond, combusting at 600 – 800°C, ␦15N ⱕ –100‰ and accompanied by trapped noble gases. 5) Acid-soluble phases (silicates and metal) as inferred from mass balance are expected to contain a large proportion of nitrogen (18 to 75%) with ␦15N in the range –25‰ to 600‰. Each of the ureilites contains at least three N components carried by acid-resistant C phases (amorphous C of type 1 or 2, graphite, and diamond) and one acid-soluble phase in different proportions, resulting in the observed heterogeneity in ␦15N. In addition to these five widespread components, EET83309 needs an additional sixth N component carried by a C phase, combusting at ⬍700°C, with ␦15N ⱖ 153‰ and accompanied by noble gases. It could be either noble gas– bearing graphite or more likely cohenite. Some excursions in the ␦15N release patterns of polymict ureilites are suggestive of contributions from foreign clasts that might be present in them. Nitrogen isotopic systematics of EET83309 clearly confirm the absence of diamond in this polymict ureilite, whereas the presence of diamond is clearly indicated for ALH82130. Amorphous C in ALH78019 exhibits close similarities to phase Q of chondrites. The uniform ␦15N value of ⫺113 ⫾ 13 ‰ for diamond from both monomict and polymict ureilites and its independence from bulk ureilite ␦15N, ⌬17O, and %Fo clearly suggest that the occurrence of diamond in ureilites is not a consequence of parent body–related process. The large differences between the ␦15N of diamond and other C phases among ureilites do not favor in situ shock conversion of graphite or amorphous C into diamond. A nebular origin for diamond as well as the other C phases is most favored by these data. Also the preservation of the nitrogen isotopic heterogeneity among the carbon phases and the silicates will be more consistent with ureilite formation models akin to “nebular sedimentation” than to “magmatic” type. Copyright © 2003 Elsevier Science Ltd Ureilites are known as an enigmatic group of achondrites because they show signs of both igneous and primitive signatures (Goodrich, 1992). In terms of mineralogy, texture, lithophile element chemistry, and some aspects of Sm-Nd systematics, they appear to be highly fractionated rocks: either magmatic cumulates (Berkley et al., 1976, 1980; Goodrich et al., 1987) or partial melt residues (Boynton et al., 1976; Scott et al., 1993) and thus the product of planetary differentiation processes. They also contain minor components, which have primitive characteristics that are unlikely to have survived extensive igneous processing on a parent body. These include high abundance of carbon that contains large amounts of fractionated primordial noble gases (Weber et al., 1976), and metal

1. INTRODUCTION

Ureilites are the second largest group among achondrites (after HED), consisting of more than 90 (unpaired) members. They are coarse-grained ultramafic rocks composed mainly of olivine and pigeonite in a carbonaceous matrix. They contain carbon comparable to, or sometimes even greater than, carbonaceous chondrites. Carbon is mainly present in the form of graphite and diamond, whereas in some cases lonsdalite has also been reported to be present (Mittlefehldt et al., 1998).

* Author to whom correspondence should be addressed (murty@ prl.ernet.in). 2213

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V. K. Rai, S. V. S. Murty, and U. Ott

with high abundance of trace siderophile elements (Spitz and Boynton, 1991), both of which are typical of undifferentiated chondritic material. In addition to these primitive signatures carried by minor components, they contain the oxygen isotope signature of primitive chondrites (Clayton and Mayeda, 1988). Oxygen is carried mostly by silicates, a major component of ureilites. In the three isotope plot, the oxygen isotopic data of ureilites fall along the “slope one” line defined by refractory inclusions of C2–C3 chondrites, whereas most of the differentiated bodies (e.g., Moon, Earth, Mars, other achondrites) form “slope half” mass dependant fractionation lines (Clayton and Mayeda, 1988). This has been interpreted to mean that the ureilite parent body did not undergo extensive melting, homogenization, and differentiation. Ureilites are expected to represent the products of melting of isotopically heterogeneous materials. The correlation between oxygen isotopic composition and the iron content of olivine and pyroxene implies that both the isotopic variation and the major chemical variations were inherited from nebular and not planetary processes (Clayton and Mayeda, 1988). Although ureilites are severely depleted in other highly volatile elements, they contain trapped noble gases in abundance comparable to those in carbonaceous chondrites. The relative abundance pattern of noble gases in ureilites is of the fractionated “planetary” type as in carbonaceous chondrites, specifically “phase Q”(Busemann et al., 2000). Among ureilites, gas contents vary considerably (e.g., Xe contents vary by a factor of ⬃100 in bulk samples), but elemental and isotopic ratios of trapped noble gases are similar (Go¨ bel et al., 1978). Analysis of separated fractions of the carbonaceous matrix or vein material showed the noble gases to be enriched in them at least 600-fold relative to the silicates and demonstrated that the gases are largely contained in carbon (Weber et al., 1971, 1976; Go¨ bel et al., 1978). In diamond-bearing ureilites, diamond is the principal gas carrier, and graphite is virtually free of trapped gases (Go¨ bel et al., 1978). The diamond-free ureilite ALH78019 has trapped noble gases, in abundance comparable to those of diamond-bearing ureilites. Wacker (1986) inferred that most of the noble gases in this ureilite are carried by fine-grained carbon, the structural state of which is unknown. His conclusion seems supported by recent data of Rai et al. (2002), which contradict the conclusion of Nakamura et al. (2000), who reported that graphite is the major carrier of noble gases. Ever since diamond has been found in ureilites, its origin has been a topic of great debate. Two mechanisms have been proposed for its origin: in situ shock conversion of graphite (Lipschultz, 1964; Vdovykin, 1970; Go¨ bel et al., 1978) and direct condensation from solar nebula (Matsuda et al., 1991). A major argument for the in situ conversion of graphite into diamond has been that almost all ureilites show the signature of shock. Further support comes from the absence of diamond in one of the least shocked ureilites, ALH78019. A major problem is the absence of noble gases in graphite, which is supposed to be precursor. Nevertheless, it is possible that graphite contained noble gases initially but lost them during shock heating, whereas diamond, being more refractory, was able to retain gases. Evidence for a nebular origin comes from, among other things, the correlation of noble gas abundances with ionization potential (Go¨ bel et al., 1978) and the remarkable similarity of elemental as well as isotopic composition of noble gases to

those of phase Q in primitive chondrites, which in all likelihood acquired noble gases in the nebula. Also, laboratory simulation of the diamond formation strongly suggests that ureilite diamond condensed in a low-pressure plasma by a process similar to chemical vapor deposition (Fukunaga et al., 1987; Matsuda et al., 1991). The emphasis of this study is to identify and understand the nitrogen components and their carriers as well as the origin of carbon phases in ureilites. Nitrogen abundances and isotopic compositions have been reported for several bulk samples and six acid residues (Grady et al., 1985; Grady and Pillinger, 1986, 1988; Yamamoto et al., 1998; Murty, 1994; Russell et al., 1993). Bulk samples contained a few to tens of ppm of nitrogen, whereas the acid residues were found to contain several tens of ppm to a few hundred ppm of nitrogen. Monomict ureilites, in general, showed the presence of a light nitrogen with ␦15N ⬍–100‰, whereas the polymict ureilites showed extremely heavy nitrogen with ␦15N up to 540‰ (Grady and Pillinger, 1988). Earlier work on ureilites has focused either on noble gases only or nitrogen and carbon (Go¨ bel et al., 1978; Grady et al., 1985). The present work is a comprehensive simultaneous study of nitrogen and noble gases in ureilites including chemically separated phases and has been aimed to address the following aspects: 1) to decipher the number of N components in ureilites and their relation if any, to the noble gases; 2) to decipher the N composition in different C phases and to infer the origin of the C phases; 3) implications of N isotopic systematics to the processes on the ureilite parent body (UPB); and 4) similarities between the amorphous C phase in ureilites and phase Q of chondrites. Ten ureilites (seven monomict, three polymict) have been analyzed in the present study. Details are given in Table 1. Bulk samples (of all 10) and acid-resistant, carbon-rich residues (of eight) have been analyzed for nitrogen and noble gases. Here we present only nitrogen and argon data. The other noble gas data will be discussed elsewhere (Rai et al., in preparation). 2. EXPERIMENTAL 2.1. Sample Preparation 2.1.1. Bulk samples Bulk samples were analyzed after ultrasonic cleaning by distilled water followed by alcohol and acetone. To remove atmospheric contamination, the samples were degassed after loading in the extraction line at ⬃150°C using infrared lamps before analysis. Some of the features (⌬17O, % Fo of olivine and shock state) of the ureilites analyzed are listed in Table 1. They cover a wide range of cosmic ray exposure ages as well as different degrees of shock ranging from very low (ALH78019) to highly shocked (Havero¨ ). 2.1.2. Residue preparation Acid-resistant residues were prepared from ALH78019, ALH81101, ALH82130, LEW85328, EET87720, and EET83309. Briefly, a few hundred milligrams of the bulk samples were treated alternately with 10N HF/6N HCl and 6N HCl. This procedure was repeated several times (four to six cycles in general). The residues were then washed with water, CS2, and alcohol several times before drying. The treatment with hydrochloric acid and hydrofluoric acid removes the major phases (silicates, metals) that carry most of cosmogenic (and solar, where present) gases, whereas the 6N HCl treatment removes precipitated fluorides and most sulfides and elemental sulfur is removed by

Nitrogen components in ureilites

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Table 1. List of samples studied and important features.

Sample Monomict ureilites Havero¨ (fall) ALH81101 Kenna (find) Lahrauli (fall) LEW85328 ALH82130 ALH78019 Polymict ureilites EET83309 EET87720 Nilpena (find)

Short Name$

Fo(%)‡ Olivine

⌬17O† (‰)

Shock state*

Starting weights

Treatment

Yield (mg)

Number of HClO4 treatments

Hav A01 Ken Lah L28 A30 A19

79.3 78.5 78.9 79 80 94.9 76.7

⫺0.59 ⫺0.23 ⫺1.02

1642 mg 252.2 — — 463 466.6 252.2

HF/H2SO4 HF/HCl — — HF/HCl HF/HCl HF/HCl

23.9 (1.5)# 5.1 (2.0)

2

⫺0.62 ⫺2.45 ⫺0.83

High High Medium Medium Medium Low Very low

13.8 (3.0) 9.9 (2.1) 8.4 (3.3)

1 2 4

E09 E20 Nil

62–98 79–87 77–81

⫺1.01 ⫺0.53 ⫺1.39

Breccia Breccia Breccia

HF/HCl HF/HCl HF/HCl

18.6 (3.8) 15.3 (2.5) (4.6)%

3 4

485.5 604.7 —

* Berkley and Jones (1982). Clayton and Mayeda (1988). See Goodrich (1992) and references therein. # Numbers given in brackets are % yield. $ Letters B, A, and O added with an hyphenation to the sample name mean respectively bulk, acid residue, and oxidized residues. † ‡

the CS2. At the end of the HF/HCl cycles, the residues consist primarily of carbon phases, with refractory oxides and chromites contributing a minor fraction. Acid residue from Havero¨ has been prepared earlier by treatment with HF/H2SO4, instead of the usual HF/HCl (Go¨ bel et al., 1978). Roughly one half of the residue obtained from HF/HCl dissolution was treated with perchloric acid (HClO4) several times. This treatment was intended to remove noncrystalline (amorphous) forms of carbon and fine-grained graphite (coarse-grained graphite can survive, see scanning electron microscope [SEM] photographs in Figure 1). For the meteorites Havero¨ and Nilpena, the acid residues used in this study are splits of those used for earlier noble gas studies (Go¨ bel et al., 1978; Ott et al., 1984). For brevity, the names of the meteorites have been abbreviated by using the first letter and the last two digits (for Antarctic ureilites) and the first three letters (for non-Antarctic ureilites), as listed in Table 1. Similarly, bulk samples and the acid residues have also been abbreviated by adding the letter B (for bulk), A (for HF/HCl or HF/H2SO4 residue in the case of Havero¨ ), and O (for HClO4 residue), respectively, to the meteorite name, with a hyphenation. Acid residues from some of the ureilites analyzed in this study have been characterized by other workers earlier. X-ray diffraction (XRD) studies have revealed that ALH78019 contains only graphite and is diamond-free (Ott et al., 1984; Wacker, 1986; Nakamuta and Aoki, 2000). Preliminary XRD studies of EET83309 and ALH82130 (Grady and Pillinger, 1987; Ott et al., 1986) have indicated them to be diamond-free and most of the carbon to be present as graphite. Havero¨ and Nilpena residues have been shown to contain diamond by XRD (Go¨ bel et al., 1978; Ott et al., 1984). In this study, we did not attempt XRD characterization of the acid residues mainly because of the scarcity of samples. Phase identification is mostly inferred through gas release temperature by pyrolysis and combustion, as well as the association with primordial noble gases and also through comparison with those ureilites for which the presence of diamond and graphite has been authenticated by XRD. We only investigated the morphological features of the residues by SEM. 2.2. Gas Extraction and Separation Depending on the nature of samples, nitrogen and noble gases were extracted either by stepwise combustion, pyrolysis, or a combination of these two. 2.2.1. Pyrolysis Because the bulk samples contain mostly silicates and metal (in addition to a small amount of carbon), for which combustion is not

effective for gas release, they were pyrolyzed. The samples, wrapped in Al foil, were dropped into a Mo crucible in a double-walled vacuum chamber and heated stepwise up to 1850°C by radio frequency (RF) heating. Samples were heated for 45 min at each temperature, and gases released were collected simultaneously on a liquid-nitrogen-cooled stainless steel mesh (SSM) finger. This protocol minimizes the possible loss of nitrogen due to gettering by metal, as demonstrated by Murty and Marti (1994). After gas extraction, the volume containing the crucible was isolated; the gases collected on the SSM finger were desorbed by warming to 150°C and were cleaned by exposure to pure oxygen generated by heating CuO wrapped in Pt foil up to 750°C. After 20 min of exposure to oxygen, the excess oxygen was taken back by the CuO by cooling it to 400°C. With this process, unwanted gases such as CO and H2 were oxidized to CO2 and H2O and were condensed on a cold trap at liquid nitrogen temperature (LNT) and isolated. A fraction of the cleaned extracted gas was isolated for nitrogen analysis, whereas the rest was exposed to a TiZr getter, collected on a charcoal finger at LNT and used for Ar, Kr, and Xe analysis.

2.2.2. Combustion The acid residues were mostly composed of carbon, and combustion was used to extract gases. The samples were packed in gold foil so that they could be combusted safely up to 1050°C, keeping the gold foil intact. Samples were heated stepwise in a double-walled quartz furnace in a pure oxygen atmosphere generated by heating CuO at 800°C in a double-walled quartz furnace. Once the required pressure of O2 (2 to 5 torr) was achieved, the volume containing the quartz furnace with the sample was isolated and heated for 45 min at the required temperature using a resistance heater. At the end of this heating, the evolved gases were exposed to CuO at 750°C for removal of CO and hydrocarbons through oxidation to CO2, H2O and trapping on a cold finger at LNT. Ar, Kr, Xe, and N2 were adsorbed onto SSM at LNT and isolated. Because large quantities of CO2 and CO were generated during combustion, the gases collected on SSM were cleaned on CuO for a second time to ensure complete removal of CO and hydrocarbons. A fraction of the extracted and cleaned gases was isolated for nitrogen isotopic analysis on a SSM finger adjacent to the mass spectrometer, and the rest of the gases was exposed to TiZr and SAES getters to remove reactive species and was used for noble gas analysis. Noble gases were split into two parts (roughly 30:70). The smaller part was introduced into the mass spectrometer and analyzed for the abundances of Ar, Kr, and Xe. The other was separated into Ar, Kr, and Xe fractions by differential adsorption on charcoal, and each of the gases was analyzed for its isotopic composition. For bulk samples (both in pyrolysis and com-

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V. K. Rai, S. V. S. Murty, and U. Ott

Fig. 1. Scanning electron microcsope photomicrographs of acid-resistant residues and oxidized residues of ureilites. A. HF/HCl residue of LEW85328, showing its fine-grained nature. Small area marked is shown at higher magnification in B, where small (ⱕ1 ␮m), regular structures (probably diamonds) are seen. C. HF/HCl residue of EET83309, showing coarse graphite crystals with a coating of finegrained amorphous carbon on its surface. The coating is removed by HClO4 treatment, revealing graphite crystals with clean surface as shown in D. E and F. HF/HCl residue of ALH82130 revealing fine graphite flakes. The surface of the crystals is pitted heavily by HClO4 treatment. G. Oxidised residue of ALH81101, showing its fine-grained nature and graphite crystals.

Nitrogen components in ureilites

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Table 2. Summary of the results of nitrogen study on bulk ureilites.

␦15N (‰) Sample

14

L28-B1 L28-B2 KNA-B HAV-B A01-B A30-B LAH-B A19-B E09-B E20-B Nil-B

N/36Ar

10114 13316 37334 43440 214408 426408 108046 9156 31970 155886 64196

N (ppm)

Total

max.

min.

Type

15.4 20.1 17.5 22.1 53.2 54.9 52.2 6.3 10.8 34.1 32.9

⫺40.2 ⫺10.0 ⫺70.4 ⫺4.4 ⫺8.6 ⫺10.7 ⫺2.3 ⫺13.5 54.1 130.4 ⫺37.6

96.0 27.8 24.5 54.3 6.4 6.7 13.0 ⫺3.5 107.3 194.5 31.2

⫺117.6 ⫺98.8 ⫺117.0 ⫺77.8 ⫺85.9 ⫺100.3 ⫺32.2 ⫺47.5 ⫺9.3 ⫺99.9 ⫺104.3

Monomict Monomict Monomict Monomict Monomict Monomict Monomict Diamond-free Polymict Polymict Polymict

bustion) He and Ne were also analyzed before Ar, Kr, and Xe analysis. In this paper, however, we only discuss argon data along with nitrogen. 2.2.3. Isotopic analyses Each of the separated fractions was introduced into the mass spectrometer and analyzed for its isotopic composition by scanning the peaks manually in a number of cycles. Noble gases were usually measured on the electron multiplier, except for 40Ar, which in some cases was measured using the Faraday cup. Nitrogen was measured in the molecular form on the Faraday cup at masses 28, 29, and 30. For smaller nitrogen amounts, after scanning on the Faraday cup, the peaks at the masses 29 and 30 were run on the multiplier. The signal output from the mass spectrometer was acquired and recorded on an online computer. Data processing including extrapolation to time of gas inlet and corrections for blank, interferences, and instrumental mass discrimination was done offline. 2.2.4. Blank correction Blank measurements were performed in identical fashion to the sample steps, both before and after samples. Blank contributions were mostly ⱕ5 % for nitrogen as well as noble gases, except for 40Ar, where blanks occasionally accounted for up to about 50% of the measured signal. Absolute amounts of nitrogen blanks varied between 30 pg (at lower temperatures) up to 100 pg (at the highest temperature). In a few temperature steps, the 40Ar blank is around 100% or more of the value of the measured signal. This is not because blanks are high at that particular temperature but because of a very low 40Ar signal. Only measured ratios are given in those steps in Tables A1 and A2 (Appendix). Isotopic compositions of the blanks were ␦15N ⫽ 5 ⫾ 2‰ for nitrogen and atmospheric within error for the noble gases. 2.2.5. CO interference corrections Small amounts of CO may survive the cleanup process and result in an apparent higher ␦15N value. This can be corrected by measuring the peak heights of mass 30 and 31 (for details see Murty, 1997a). The mass 30 peak can have contributions from 15N15N, 12C18O (accompanied by negligible 13C17O), and possibly hydrocarbons. Assuming that all the hydrocarbon contributions at masses 30 and 31 are comparable and considering the near absence of any signal at mass 31 peak, the hydrocarbon contribution at mass 30 appeared negligible. Therefore, the excess over 15N15N at mass 30 was assigned wholly to CO and mass 29 accordingly corrected.

long periods of time. Data reported here have been corrected for blanks, mass discrimination, and interferences, and the corresponding errors have been propagated. Errors in concentrations of N and Ar are ⫾10%, whereas errors in ␦15N and 40Ar/36Ar represent 95% confidence limits. 3. RESULTS 3.1 SEM Photographs

SEM photomicrographs of all acid residues were taken on a LEO440-i Scanning Electron Microscope with 25kV and 20kV EHT at various magnifications at the Institute of Plasma Research, Gandhinagar. Some are shown in Figure 1. Plate-like big crystals of graphites (20 –500 ␮m) are seen clearly in all the photographs, and graphite seems to be dominant in all the residues (Figs. 1C and 1D). Clear crystal structures are only seen for the HF-HCl residue of LEW85328 (Figure 1a and 1b).In some cases the graphite plates are deformed; most likely the reason is the shock stress that most of the ureilites suffered in their parent body(ies), most clearly seen in E09-A and O (see Figs. 1c and d). Other striking features of the acid-resistant residues are the rough grain surfaces; they became clean and smooth after perchloric acid treatment (see SEM pictures for the acid residues of ALH 78019, Rai et al., 2002). This is probably due to removal of amorphous (noncrystalline) carbon sticking on the large graphite crystals. In all residues except one (LEW85328, Fig. 1b), no clear diamond crystals have been seen even with a resolution at the micrometer level. This indicates that in most of the cases diamond size is submicron or even smaller. Most likely diamond is intergrown with graphite. Although the rough surface seen on top of graphite is likely to be an amorphous layer of carbon that is removed almost completely by perchloric acid treatment, coarse-grained graphite flakes are clearly seen in most of the oxidized residues. Obviously graphite is only partially removed (and coarse-grained graphite less affected) by perchloric acid treatment though the etching effect of the acid on the surfaces is clearly visible (E09-O, Fig. 1d). 3.2. Nitrogen in bulk ureilites

2.2.6. Calibration of the mass spectrometer Air standards were run periodically to calibrate the mass spectrometer for sensitivity and mass discrimination. Sensitivities determined on standards over several months varied by less than 5%. For a particular sample, the corresponding air standard sensitivity (with respect to time) was used. Mass discrimination (⬃2.5%/amu) was reproducible over

Most of the bulk samples were analyzed by pyrolysis following a few initial steps of combustion (mainly to remove surficial contamination). Data are compiled in Table A1 (Appendix) and a summary of the nitrogen data is given in Table 2. Release patterns and corresponding isotopic compositions of N

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V. K. Rai, S. V. S. Murty, and U. Ott

Fig. 2. Release patterns in pyrolysis of bulk ureilite samples of nitrogen and corresponding ␦15N profiles. For most samples, initial steps were combustion (usually up to 400°C). Lahrauli and Nilpena were combusted up to 1000°C followed by pyrolysis. For further details, see text.

and Ar as a function of release temperature are plotted in Figures 2 and 3. Two samples, Lahrauli and Nilpena, were analyzed by a combination of combustion (up to 1000°C) and subsequent pyrolysis. N concentrations in the bulk samples range from 6.3 ppm (for ALH78019) to 55 ppm (for ALH82130). Both monomict and polymict ureilites (excluding ALH78019 and EET83309) seem to have two major nitrogen components, as is discussed later. At least three additional minor components are required to explain the isotopic trends in detail. The major nitrogen component released at ⱖ1200°C in pyrolysis and between 600 and 900°C in combustion, is isotopically light with ␦15N of ⱕ–100‰, and is present ubiquitously in both monomict and polymict ureilites. The lowest ␦15N observed for any bulk ureilite sample so far is –126‰ in the 1500°C pyrolysis of Kenna. Though this light nitrogen seems to be present in all ureilites that have been analyzed (except for ALH78019 and EET83309), sometimes it is not resolved clearly because of mixing with the other major nitrogen component that is relatively heavy and also present in all the ureilites. Observed heavy composition shows large variations with maximum ␦15N ranging from 5‰ (for most monomict ureilites) to nearly 200‰ (for E20-B). Irrespective of “find” or “fall,” significant amounts of nitrogen with ␦15N of 5 to 30‰ (released at ⱕ500°C in combustion and ⱖ1000°C in pyrolysis) are observed in all the

monomict ureilites. Because atmospheric and organic contamination cannot survive to these temperatures, a significant proportion of this N seems to be indigenous. In some monomict ureilites, the highest ␦15N value of the heavy nitrogen reaches more than 50‰ (e.g., Havero¨ , 54‰, and LEW85328, 96‰; also ALH77257 and Asuka 881931 analyzed by Yamamoto et al. (1998). Heavy nitrogen in polymict ureilites is, in general, heavier than that in monomict ureilites, with ␦15N up to 540‰ reported for EET83309 (Grady and Pillinger, 1988). In our study, ␦15N in this meteorite reaches only up to 107‰, whereas in another polymict ureilite, EET87720, our maximum ␦15N is 196‰, much heavier than the 75‰, reported by Rooke et al. (1998). Most likely, these differences are due to inhomogeneous distribution of the carriers of heavy and light nitrogen i.e., sample heterogeneity. In three of the bulk samples, Havero¨ , ALH81101, and ALH82130, the proportion of heavy nitrogen released at low temperature is high. The latter two samples are Antarctic finds, and therefore a significant contribution from terrestrial contamination is possible (and indicated by ␦15N). In the case of Havero¨ , a significant fraction must be indigenous, and this is also reflected by the much heavier nitrogen signature (␦15N ⬃54‰ in 1000°C pyrolysis). This nitrogen is not accompanied by much 36Ar (primordial noble gas) release. In contrast, almost 20% of the total nitrogen released in the initial two steps

Nitrogen components in ureilites

Fig. 3. Stepwise release patterns in pyrolysis of 36Ar along with Nilpena were combusted up to 1000°C followed by pyrolysis.

of Ken-B is also heavy (with ␦15N ⱖ 24‰), but it is accompanied by a significant amount of indigenous 36Ar (ca. 23% of total 36Ar, Figs. 2C and 3C). Also, the release patterns of 36Ar and heavy nitrogen in the first four temperature steps (three combustion and one pyrolysis) of L28-B (which contains heavy N with ␦15N ⱖ 96‰) reveal that in this meteorite heavy N and primordial 36Ar occur in a readily combustible phase (Figs. 2b and 3b). It is imperative to discuss the approach adopted in the assignment of carrier phases for different N components before further analyzing the results. Carbon phases (diamond, graphite, and amorphous carbon) and silicates (and metal) are the possible hosts of N in ureilites. Complete identification of the host phases may not be possible from bulk sample analyses alone, although a combination of combustion (more effective for C phases) and pyrolysis does help. As for pure C phases, we rely on mostly characteristic release patterns in combustion and pyrolysis (Wright and Pillinger, 1989; Grady et al., 1985; Wacker, 1986; Go¨ bel et al., 1978), as well as the well-established fact that diamond is the major carrier of noble gases in ureilites whereas graphite is free of noble gas (Go¨ bel et al., 1978), to identify the major release temperature of a given carbon phase. On combustion, major release of gases from amorphous C occurs at ⱕ500°C, whereas gases from diamond and graphite

40

2219

Ar/36Ar for bulk samples of ureilites. Lahrauli and

are coreleased in the 600 to 800°C interval, unless graphite is coarse grained, in which case its gas release shifts to ⬎800°C (Wright and Pillinger, 1989; Grady et al., 1985). On the other hand, major gas release on pyrolysis occurs at ⱕ1000°C, ⱖ1200°C, and ⱖ1400°C for graphite, amorphous carbon, and diamond, respectively (Go¨ bel et al., 1978; Ott et al., 1985; Rai et al., 2002). Combustion is not effective for silicates (and metal), and on pyrolysis ⱖ1000°C is needed for appreciable gas release. A preliminary assignment of carriers is made from the bulk sample data, using the guidelines described earlier, and these assignments are further confirmed from the study of acid residues discussed later. 3.2.1. Polymict ureilites For Nilpena, combustion up to 500°C releases N with ␦15N up to 31‰, accompanied by small amounts of 36Ar (the slightly lower ␦15N at ⬍500°C could be due to adsorbed air contamination). This component is most likely from amorphous C. In the 700 to 1050°C combustion region, N with ␦15N ⱕ –93‰ (accompanied by 36Ar) is released, most likely from diamond. Slightly heavier N in the 700°C and 800°C combustion fractions indicates admixture of another heavy N component, possibly from coarse-grained graphite or from a foreign clast as might be expected in a polymict ureilite (Brearley and Prinz,

2220

V. K. Rai, S. V. S. Murty, and U. Ott Table 3. Summary of the results of nitrogen study for acid residues of ureilites

␦15N (‰) Sample

14

N/36Ar

N (ppm)

Acid-resistant (HF/HCl or HF/H2SO4) residues L28-A1 3622 423 Hav-A1 18102 257 A01-A 18210 140 A30-A 26966 108.3 A19-A 7392 82.3 Nil-A 11668 273.9 E20-A 19090 533.4 E09-A 3868 126.4 HClO4–treated residues L28-O1 5354 490 Hav-O1 9416 391 A01-O 27346 340.4 A30-O 48776 108.1 E20-O 6990 181.6 E09-O 30194 64.6

Total

max.

min.

Type

⫺83.55 ⫺20.9 ⫺21.4 ⫺37.7 ⫺7.0 ⫺42.5 21.5 51.8

25.48 2.6 27.1 87.3 18.9 77.2 107.9 152.7

⫺114.1 ⫺104.2 ⫺94.5 ⫺53.3 ⫺21.1 ⫺109.9 ⫺60.1 ⫺22.3

Monomict Monomict Monomict Monomict Diamond-free Polymict Polymict Polymict

⫺102.1 ⫺9.2 ⫺44.3 ⫺56.3 ⫺52.33 10.5

11.86 2.6 23.6 18.5 19.2 11.7

⫺112.7 67.9 ⫺107.7 ⫺103.3 ⫺99.3 ⫺0.7

Monomict Monomict Monomict Monomict Polymict Polymict

1992). Pyrolysis steps following 1050°C combustion yield only small amounts of N with ␦15N down to –70‰ (accompanied by negligible 36Ar). This indicates release of either cosmogenic N or N from foreign clasts superimposed on a small amount of light N from diamond (that was protected by silicates and did not combust). For EET87720, ␦15N signatures observed in combustion up to 500°C (␦15N ⫽ 155‰, with negligible trapped 36Ar), 800 to 1000°C pyrolysis (195‰, negligible 36 Ar), 1400°C pyrolysis (–55‰, with peak 36Ar release), and ⬎1400°C pyrolysis (⫺100‰, with low 36Ar) can be interpreted as due to release from amorphous C, graphite, diamond plus foreign clast, and diamond, respectively. The release from bulk EET83309 is different from that observed for the other two polymict ureilites. There are three ␦15N signatures, and all

are accompanied by 36Ar: combustion up to 500°C (␦15N ⫽ 70‰), 800 to 1200°C pyrolysis (107‰), and ⬎1200°C pyrolysis (–9‰). The low-temperature combustion component could be a pristine amorphous C phase, while the other two pyrolysis components seem to be either due to new components or may indicate admixtures from N in foreign clasts or cosmogenic N (at ⱖ1200°C steps). Study of acid residues does allow clarification of this (discussed later). 3.2.2. ALH78019 The release pattern of nitrogen from this diamond-free ureilite as well as the trends in isotopic composition are distinctly different from those of other ureilites and a detailed discussion

Fig. 4. Stepwise nitrogen release patterns as well as ␦15N of the HF/HCl (HF/H2SO4 in the case of Havero¨ )-resistant residues of ureilites. Gases were extracted by combustion.

Nitrogen components in ureilites

2221

Fig. 5. Stepwise release patterns of 36Ar, C, as well as 40Ar/36Ar ratios of HF/HCl (HF/H2SO4 in case of Havero¨ )-resistant residues of ureilites. Gases were extracted by combustion. Carbon release was monitored by measuring the CO ⫹ CO2 pressure using a convectron gauge.

is given by Murty et al. (1999) and Rai et al. (2002). Briefly, trapped noble gases and nitrogen (with ␦15N⫽ –21‰) are released at ⱕ500°C by combustion. In these steps, only a very small amount of carbon combusted. Most of the carbon has combusted at ⬎600°C and is devoid of noble gases, but has nitrogen with ␦15N ⱖ19‰. From the combustion temperature and the association with noble gases, the low and high temperature combusting carbon phases have been identified as amorphous carbon and graphite respectively. The N/C and 36Ar/C values of the amorphous carbon are several hundred times higher than the corresponding values for diamond (see Table 5), considered to be highly enriched in trapped noble gases.

3.2.3. Minor components In addition to the major nitrogen components (light and heavy nitrogen) discussed earlier, other minor nitrogen components are required to explain the observed isotopic trends. A component released predominantly at lower temperatures with ␦15N close to 0‰ (most prominently seen in Antarctic ureilites ALH81101 and ALH82130), most likely are organic contaminants or adsorbed atmosphere, as it is accompanied by argon with 40Ar/36Ar close to the air value (⬃295). Another minor nitrogen component is needed to explain the excursion in ␦15N and 40Ar/36Ar values at higher temperatures (40Ar/36Ar in most

Table 4. Mass balance for noble gases and nitrogen in acid resistant residues and ␦15N inferred for acid soluble phases. 36

Sample Monomict ureilites Havero¨ (HF/H2SO4) Havero¨ (HF/H2SO4) LEW85328 (HF/HCl) ALH81101 (HF/HCl) ALH82130 (HF/HCl) Dyalpur (HF/HClO4) Dingo Pop Donga (HF/HClO4) Diamond-free ureilite ALH78019 (HF/HCl) Polymict ureilites EET83309 (HF/HCl) EET83309 (HF/HCl) EET87720 (HF/HCl) Nilpena (HF/HCl)

Kr (%)

132

Xe (%)

N (%)

N (ppm)‡

58.9 191 227 63 66 595 598

113 181 81 95 89 208 858

354 165 149 95 92 214 1063

25

16.9, 18.6

⫺3

82 5 4

2.9

155 ⫺8 ⫺25

1 2 1 1 1 2 2

59

95

70

48

3.4

⫺19

1

146

132

137

6.2

323 210

171 157

158 114

45 25 40 38

56 600 201 ⫺35

1 3 1 1

Ar (%)

84

* 1 ⫽ This work; 2 ⫽ Go¨ bel et al., 1978; 3 ⫽ Grady and Pillinger, 1988. ‡ Of acid-soluble phase.

21.1 21.3

␦15N (‰)‡

References*

2222

V. K. Rai, S. V. S. Murty, and U. Ott

Fig. 6. Stepwise combustion release patterns as well as corresponding isotopic compositions of nitrogen for individual HClO4 treated residues of ureilites.

bulk ureilites and ␦15N in Havero¨ and ALH 82130). Superimposed on these N components, spallation nitrogen sometimes is seen (primarily in case of ureilites with high cosmic ray exposure age) as excursion in ␦15N. Polymict ureilites, as discussed earlier, may also show additional N components contained in foreign clasts. Further characterization of some of these N components and identification of their carriers is facilitated by the study of acid-resistant residues. 3.3. Nitrogen in Acid Residues 3.3.1. HF/HCl residues Out of the 10 ureilites for which bulk samples have been studied, seven HF/HCl–resistant residues and the HF/H2SO4 residue from Havero¨ (hereafter, acid residues) have been analyzed by combustion for abundance and isotopic composition of nitrogen and noble gases. The results of nitrogen and argon are compiled in Table A2 (Appendix), and a summary of the nitrogen data is given in Table 3. Nitrogen and argon abundances and isotopic compositions (␦15N and 40Ar/36Ar) along

with the percentage carbon release (measured as CO ⫹ CO2 pressure) have been plotted in Figures 4 and 5. Compared with the case of the bulk samples, the release patterns of acid residues show more differences both within monomict and polymict types as well as among them. Among the monomict ureilites, Havero¨ (HAV-A) and ALH81101 (A01-A) acid residues show a peak release of N at ⬃500°C with relatively heavy nitrogen, L28-A has the peak release at ⬃700°C with light nitrogen, and A30-A shows a bimodal release pattern. Polymict ureilites Nil-A and E20-A both show bimodal nitrogen release, low-temperature heavy nitrogen and high-temperature light nitrogen. Nitrogen release patterns as well as ␦15N profiles of the residues A19-A and E09-A are unique. The N content of the residues range from ⬃60 to 550 ppm with ␦15N ranging from 52‰ for E09-A down to –102‰ for L28-O. Although the acid residues account for almost all the noble gases contained in the bulk samples, mass balance calculations (Table 4) indicate that a significant amount (18 –95%) of nitrogen is carried by acid-soluble phases. The minimum ␦15N value obtained among all the acid residues is –114‰ for

Table 5. Characteristics of different nitrogen carriers in ureilites. Release temperature (°C) 15

36

N/C (⫻105)

Ar/C (⫻109)

Combustion

Pyrolysis

Carrier

␦ N (‰)

Diamond Graphite Amorphous Carbon in Diamond-free ureilite Polymict ureilites Monomict ureilites Acid-soluble Phase*

ⱕ⫺100 ⱖ⫹19

2,000–40,000 12,000–18,000

4–42 1.7

11–82 1.2

600–800 600–800

ⱖ1600 ⱖ1000

⫺21 ⫹50 to ⫹107 ⫹20 to ⫹95 ⫺35 to ⫹155

2000–4000 200–8000 160,000–106 —

5900 270–780 230–1200

14,000 22–28 17–440

ⱕ500 300–800 ⱕ500 —

1000–1400 1000–1400 1000–1400 —

* Estimated from mass balance.

14

N/36Ar

Nitrogen components in ureilites

the 800°C combustion step of L28-A. In some cases, there is no clear minimum because of admixing of a minor amount of the heavy nitrogen (primarily released at lower temperatures, mostly ⱕ500°C) released at slightly higher temperatures. All these acid residues with the exception of those from EET83309 and ALH78019 contain light nitrogen released from carbon combusting mostly between 600 and 800°C, accompanied by primordial noble gases (Figs. 4 and 5). This implies that the carrier of primordial noble gases as well as light nitrogen present in bulk ureilite samples survives the HF/HCl treatment, whereas the heavy nitrogen present in the bulk samples is partially removed, implying the presence of an acid-soluble heavy N carrier. XRD reveals that the residue is composed of mostly carbon (diamond, graphite, or both) and a minor amount of refractory oxides (thought to be poor in volatiles; Go¨ bel et al., 1978; Wacker, 1986). Except for HAV-A, A19-A, and E09-A, all acid residues have a single noble gas peak release between 600° and 800°C that is accompanied by light nitrogen. In the HF/HCl residue of the diamond-free ureilite ALH78019 (A19-A), in contrast to residues from other ureilites, most of the nitrogen and noble gases are combusted at temperatures less than 550°C, at which less than 1% of carbon combusts (Figs. 4e and 5e). Similar to the bulk sample, the acid-resistant residue of this ureilite has the lowest N content (82 ppm) among all the acid residues measured here. The residue contributes 48% of the total nitrogen found in the bulk sample (Table 4) indicating a significant part of nitrogen is carried by phases other than carbon, i.e., by silicates and metal. The nitrogen accompanied by primordial noble gas release has a minimum ␦15N value of –21‰, which is not as light as in the case of diamond-bearing ureilites. Most of the carbon that combusts at higher temperature is almost free of primordial noble gases but still contains significant amounts of nitrogen with a maximum ␦15N of 19‰. The minor amount of light nitrogen that was seen in the bulk sample at the highest temperatures (Fig. 2e) is not seen here and seems to be removed by the acid treatment (Rai et al., 2002). For the acid residue E09-A, nitrogen release peaks at 500°C, whereas the heaviest nitrogen is observed at 700°C with ␦15N ⫽ 153‰. The heavy nitrogen released from E09-A in the 700-900°C interval of combustion is also accompanied by noble gas release. Although the combustion temperature overlaps with that for graphite, the association with noble gases nevertheless suggests that it may not be graphite. Grady and Pillinger (1987) considered the different ␦13C signatures at 600 to 700°C (– 4.7‰) and 700 to 850°C (–7.0‰) for EET83309 as possible evidence for the presence of two types of graphite. Although it might be tempting to follow their interpretation and to attribute the two ␦15N signatures (87‰ at 600°C and the much higher value of 153‰ at higher temperatures) to two types of graphite, the corelease of noble gases suggests otherwise. A possibility might be cohenite, which is expected to combust at ⬎900°C and has been suggested for ALH82130 (Grady and Pillinger, 1986). A phase identification by XRD would be helpful in this context. Because the peak carbon combustion range in the acid residues is similar to the combustion characteristics reported for graphite and diamond (Grady et al., 1985) it is difficult, based on combustion alone, to ascertain whether gases are released from either diamond or graphite. Using both combustion and

2223

pyrolysis, however, this task can be achieved easily, because during pyrolysis diamond releases its gases at higher temperature compared with graphite (Table 5; Go¨ bel et al., 1978). Separate splits from the HF/HCl residue from LEW85328 have been analyzed by combustion and pyrolysis to check the release pattern of light nitrogen, with the release profile by pyrolysis shown in Figure 6. The light nitrogen is indeed released in the 1600° to 1850°C interval and is accompanied by argon with low 40Ar/36Ar. Another release peak of N (and Ar) at 1400°C during pyrolysis is most likely due to amorphous carbon, which can be seen in the HF/HCl residue as a release at ⱕ550°C on combustion (and which is also indicated by its absence in the HClO4 residue). Admixture of the ␦15N signature of the amorphous C, which is heavier than that of diamond can explain the ␦15N profile in the pyrolysis. The very high release temperature on pyrolysis together with the combustion release profile of the acid-resistant residue from LEW85328 (Fig. 4b) indicates that the carrier of the noble gases and light nitrogen is diamond and not graphite. This is confirmed by the observation that light nitrogen and primordial noble gases are unaffected by the perchloric acid treatment. 3.3.2. HClO4 residues Of the eight acid residues that were analyzed in this study, seven were treated with perchloric acid (HClO4) to obtain “oxidized residues.” This was intended to remove amorphous carbon as well as graphite. Coarse-grained graphite seems to survive this treatment, however. The results of nitrogen and argon analyses have also been compiled in Table A2 (Appendix), and summary of nitrogen data is given in Table 3. Nitrogen and argon isotopic compositions and abundances and percentage C release (measured as CO ⫹ CO2 pressure) are shown in Figures 7 and 8. The release profiles for all the oxidized residues (except ALH78019 and EET83309) are similar to those shown by the corresponding acid residues. The light nitrogen released at 600° to 800°C in HF-HCl-resistant residues survived the perchloric acid treatment along with primordial noble gases, whereas a major part of heavy nitrogen was lost by this treatment. Because of the removal of heavy nitrogen, light nitrogen is better resolved (except for Hav-O). For L28-O, the light nitrogen forms a plateau in ␦15N (⫽ –106 to –113‰) at 600 to 900°C and is associated with peak primordial noble gas release and peak carbon combustion. A small amount of heavy N (⬍5%) is also present, but its amount is small compared with bulk sample and HF/HCl residue. In the case of Hav-A and Hav-O, the amount of heavy nitrogen released below 550°C is nearly the same, but the ␦15N in the latter is closer to atmospheric nitrogen. The argon peak observed at ⱕ500°C in Hav-A is removed by the perchloric acid treatment. The heavy nitrogen peak is associated with a minor amount of carbon combustion. This residue had been treated with H2SO4 rather than HCl and prepared a long time ago (Go¨ bel et al., 1978), which may have caused trapping of atmospheric nitrogen thereby diluting the indigenous heavy nitrogen signature expected at low temperatures as well as the light nitrogen expected at higher combustion temperatures by continued release of atmospheric nitrogen. At 550°C, a relatively large amount of carbon combusted but was accompanied

2224

V. K. Rai, S. V. S. Murty, and U. Ott

Fig. 7. Stepwise release patterns of 36Ar and C along with corresponding 40Ar/36Ar ratios in the HClO4 treated residues of ureilites. Carbon release was monitored by measuring the CO ⫹ CO2 pressure using a convectron gauge.

Fig. 8. Stepwise pyrolysis release profiles for nitrogen and argon for HF/HCl residue of monomict ureilite LEW85328 (L28-A2).

Nitrogen components in ureilites

only by very low 36Ar, which indicates that some gas poor carbon survived the perchloric acid treatment (Fig. 5a). 3.3.3. ALH78019 In the oxidized residue most of the primordial noble gases were lost, similar to the behavior of phase Q from chondrites. Only less than 5% of 36Ar present in A19-A was released from A19-O with atmospheric 40Ar/36Ar. A large amount of carbon combusted at high temperature (ⱖ800°C), but with negligible release of noble gases. Although the nitrogen released from A19-O was almost double that released from A19-A, nitrogen in A19-O seems to be dominated by an atmospheric component (Rai et al., 2002). 3.3.4. EET83309 The release patterns of nitrogen and argon from E09-O are shown in Figures 7g and 8g. The sample yields a relatively small amount of nitrogen (65 ppm) with ␦15N of 10.5‰ and 342 ⫻ 10⫺8 ccSTP/g of 36Ar for the total. As in the case of ALH78019, more than 90% of the noble gases were removed by perchloric acid treatment. The release of nitrogen peaks at 500°C with the isotopic composition lying between 0 and 20‰ except for a slight increase in ␦15N at 800°C where less than 3% of nitrogen is released. The loss of noble gases by the perchloric acid treatment and absence of light nitrogen (with ␦15N ⱕ –100‰) indicates the absence of diamond, in agreement with a preliminary XRD analysis (Grady and Pillinger, 1987). Nevertheless, the presence of a small amount of light nitrogen in E09-B and E09-A could be due to the presence of a small amount of very fine-grained diamond. As in the case of ALH78019 (see discussion in Rai et al., 2002), it might have been lost in the perchloric acid treatment through colloidal solution (Amari et al., 1994). 4. DISCUSSION

4.1. Comparison to Literature Data For most of the ureilites studied in this work, acid-resistant residues have been analyzed for nitrogen for the first time. There are some cases in which literature data are available for comparison, but the match is not exact. Nevertheless, the isotopic profiles are similar. For a HF/HCl residue from EET83309, Grady and Pillinger (1988) reported an extreme ␦15N value of 540‰, whereas a value of up to only 153‰ is observed in our study. An earlier measurement of an acid residue from Nilpena yielded a total ␦15N value of ⫺74‰, with a lowest value of ⫺83‰ (Russell et al., 1993). In contrast, the acid-resistant residue from Nilpena analyzed in our study yielded a higher bulk ␦15N of – 42‰, but the lowest value of –110‰, which is even lower than that observed by Russell et al. (1993). The Nilpena residue analyzed by these authors and that used in our work are splits of the same sample analyzed for noble gases by Ott et al. (1984). The lowest value of ␦15N observed in any acid-resistant residue from ureilites is –118.1‰ (Novo Urei; Russell et al., 1993), which is close to the lowest value of ⫺114.1‰ measured for L28-A in our study. These differences in all likelihood are due to heterogeneity of the residues in terms of the proportions of different carbon

2225

phases in a given split. Similarly, three bulk samples of ALH77257 analyzed by Yamamoto et al. (1998) revealed similar release profiles for N and ␦15N but yielded N (ppm) contents of 1.21, 6.05, and 6.81 with respective minimum ␦15N (‰) values of –72.2, –101.2, and –99.5, whereas for the same meteorite Grady et al. (1985) reported 29.8 ppm N with minimum value of –13.6‰ for ␦15N. Clearly the distribution of different N carriers in ALH77257 is highly heterogeneous. 4.2. Number of N Components Required and Their Carriers Based on ␦15N, release temperature (either during pyrolysis or combustion), and association with noble gases and carbon, the following nitrogen components can be distinguished among the ureilites analyzed in this study. In addition to these trapped N components, imprints of cosmogenic N component can be seen in the bulk samples of those ureilites with high exposure ages at higher temperatures of extraction. A light nitrogen (␦15N ⱕ ⫺100‰) component released at very high temperature on pyrolysis and between 600 and 800°C on combustion of a C phase that is accompanied by primordial noble gases. 2A. A relatively light nitrogen (␦15N ⫽ ⫺21‰) component released at low temperatures on combustion (ⱕ500°C) of a very small amount of carbon with high concentration of primordial noble gases. This component has been observed only in ALH78019. 2B. A heavy nitrogen component (10 –95‰) released at very low temperatures on combustion (ⱕ500°C) that is carried by all the monomict ureilites. It is associated with negligible amounts of noble gases and little combustion of carbon. Noble gases associated with this carrier most likely have been lost during parent body processing, which might also be responsible for the variable ␦15N. We argue that the components 2A and 2B are related and should be considered as a single component with the original composition represented by 2A. Component 2B with variable ␦15N is derived from component 2A through partial loss of noble gases and N, resulting in the increase of ␦15N through Rayleigh fractionation (see section 4.7.). 3. A very heavy nitrogen (␦15N ⱖ 50‰, or possibly ⱖ 150‰) component accompanied by large amounts of noble gases but associated with little carbon that combusts only at low temperatures (ⱕ500°C), as observed in EET83309. This is a separate carbon carrier, which can not have been generated from component 2A. 4. Nitrogen carried by the majority of the carbon in the diamond-free ureilite ALH78019 with ␦15N ⱖ19‰, combusting at temperatures ⱖ650°C. 5. Nitrogen carried by acid-soluble phases with highly variable nitrogen composition, having nominal ␦15N in the range of –25 to 600‰ (inferred from mass balance). Given the heterogeneous distribution of carbon in ureilites, however, such nominal values have some inherent uncertainties and can serve illustrative purposes only in the current context. The concentration of acid-soluble nitrogen in ureilites (Table 4) is high compared with values in other differentiated meteorites such as HEDs and angrites (Miura and Sugiura, 1993; Murty, 1997b). The

1.

2226

V. K. Rai, S. V. S. Murty, and U. Ott

nominal range of ␦15N values inferred for this component among monomict ureilites (⫺25 to 155‰) is not as wide as among polymict ureilites (⫺35‰ to 600‰). This may be due to the presence of heavy N components in foreign clasts in case of the latter. Basically, the acid residues from monomict ureilites as well as polymict ureilites show two isotopically very distinct major nitrogen components, heavy nitrogen, and light nitrogen. In monomict ureilites, in general, heavy nitrogen (0 – 87‰) is not as heavy as in polymict ureilites (77–153‰). The light nitrogen, on the other hand, shows a relatively narrow range of ␦15N, between –100 and –125 ‰ (component 1; Figs. 4a and 4b). Peak release of light nitrogen is observed between 600 and 800°C on combustion and at ⱖ1400°C on pyrolysis. It is always accompanied by peak noble gas release and peak carbon combustion and can be identified as diamond. The combustion characteristics of the noble gas carrier in ALH78019 are consistent with amorphous carbon. Combining all observations, we infer that the trapped gases in ALH78019 are carried by a fine grained carbon rich phase similar to phase Q to which we refer as amorphous carbon (Rai et al., 2002). The amorphous C is less refractory compared with diamond and prone to lose its trapped gases during parent body processing, e.g., the shock event. Nitrogen, in contrast to the noble gases, can form chemical bonds with carbon, which could be the cause for survival of a significant fraction of nitrogen during such processes. The possibility needs to be explored that amorphous carbon of the type found in ALH78019 was initially present in all ureilites and that the nitrogen it carries (component 2A) may be related to some other components. To illustrate, let us assume that amorphous carbon of ALH78019 type contains about 1000 ppm (or more) of nitrogen initially with ␦15N of –20‰. More than 90% (or close to 99.5%) loss of nitrogen (assuming Rayleigh fractionation) is required to increase the ␦15N by 40‰ (or 100‰) into the range of observed heavy compositions. This does not seem implausible in the case of amorphous carbon in monomict ureilites, which contains a few to tens of ppm heavy nitrogen (component 2B). Because the 15N enrichment in polymict ureilites is much higher, much higher loss would be required in their case. Hence, fractionation loss does not seem to be a plausible cause for the origin of heavy N in polymict ureilites (component 3) unless the initial concentration of nitrogen in amorphous carbon was orders of magnitude higher. It seems more likely that the amorphous C in polymict ureilite EET83309 is a distinct component (compared with ALH78019), and that, with ␦15N ⱖ 50 ‰ and with large concentration of noble gases, it must have formed by a different process. Unlike the noble gases, which are depleted in graphite as indicated by the negligible release of argon along with peak carbon combustion in A19-A (Fig. 5e), graphite contains an appreciable amount of nitrogen, with ␦15N of 19‰. This is an important piece of information because it is difficult to measure the nitrogen composition of graphite in other ureilites in the presence of diamond that is not only highly enriched in nitrogen but the nitrogen of which is also very light. In the absence of any reliable measurement of nitrogen composition in graph-

ite from diamond-bearing ureilites, this is our best information on the composition of nitrogen in graphite from ureilites. The release patterns for EET83309 show a number of similarities to those of the diamond-free ureilite ALH78019: (i) low temperature release (in HF/HCl residue) of noble gases with only minor combustion of carbon, (ii) loss of noble gases by oxidizing acid treatment, (iii) presence of smooth coarsegrained crystals of graphite observed by SEM, and (iv) no indication for the presence of diamond (in a preliminary XRD study; Grady & Pillinger, 1987). Major differences between ALH78019 and EET83309 exist in nitrogen composition and noble gas release, which in the case of EET83309 is bimodal with the second peak coinciding with peak CO2 release. Nitrogen in E09-A is highly enriched in 15N compared with A19-A (Table 5). Although not much light nitrogen is associated with the noble gas peak release, a decrease in ␦15N has been observed. Assuming that this is due to release of a light nitrogen component, we derive its ␦15N the following way. We take the mean value (100‰) of the ␦15N signatures at 700 and 900°C fractions to represent the heavy nitrogen component to which a light nitrogen component is added to get the observed ␦15N of ⫺22.2‰. The ␦15N of this light nitrogen component turns out to be ⬃⫺120‰, similar to nitrogen in diamond. Assuming for diamond a nitrogen concentration of ⬃400 ppm, this translates into that about 56% of N released in 800°C would be due to diamond, a diamond concentration of 0.15% for the acid residue and about 60 ppm diamond in bulk EET83309. This is much less than the upper limit for the amount of diamond (about 0.36 mg diamond/g) in ALH78019 (Rai et al., 2002). What argues against this interpretation is the loss of most of the trapped gases by perchloric acid treatment along with absence of light nitrogen in the oxidized residue. Still, the possibility exists that the diamond in this residue is very fine grained and might have been lost in perchloric acid solution, similar to the loss of presolar diamond observed during alkali permanganate oxidation (KMnO4) as a colloid by Amari et al. (1994). Because the bulk sample of ALH78019 also showed small amounts of light nitrogen on pyrolysis at 1600°C onward (but this light nitrogen signature is not seen in acid residues), a similar mechanism can be construed to be responsible for loss of diamond (if present) for this sample as well. In any case, the present N isotopic data for EET83309 confirm that it, like ALH78019, is a diamond-free ureilite, as has been suggested by Grady and Pillinger (1987). 4.3. Nitrogen in Acid-Soluble Phases Table 4 compiles mass balance calculations for all the noble gases and nitrogen from the present work as well as literature data. Nominal mass balance for many acid residues accounts for more than 100% of noble gases contained by bulk samples. This is the case for our samples as well as for data from literature that exhibit similar behavior, most likely due to heterogeneity of the bulk ureilite samples with respect to the distribution of carbon phases. Similar calculations for nitrogen, on the other hand, reveal that, although it is highly enriched in acid insoluble phases, a significant part is carried by acidsoluble phases. In two cases (ALH81101 and ALH82130), the nitrogen accounted for by the residues is far less than 10% of the total. This may be due to the apparent high nitrogen content

Nitrogen components in ureilites

2227

Fig. 9. Percentage release of nitrogen and 36Ar, at each temperature step in the interval 550°C – 800°C (where mostly light nitrogen is released) have been plotted against their respective percentage C combusted, for individual carbon rich residues as well as two of the bulk samples (analyzed by combustion). The correlation with carbon is better for nitrogen than for argon.

of the bulk samples and the presence of terrestrial weathering products, because both are “finds” from Antarctica and release the majority of nitrogen from the bulk samples (A01-B and A30-B) in the initial temperature steps. Table 5 summarizes the characteristics of various carriers of nitrogen inferred from acid residues and bulk samples of ureilites. The ␦15N values for the acid-soluble phases have been calculated based on simple mass balance. The ␦15N of the graphite is the composition of nitrogen released along with peak C combustion in acid-resistant residue of ALH78019 (A19-A). 4.4. Correlation of Nitrogen and Argon with Carbon Phases From nitrogen and noble gas analyses of seven bulk samples of ureilites (three of them repeat analyses of ALH77257) and seven physically separated phases, Yamamoto et al. (1998) found, for the C-rich temperature fractions, no correlation between N/C and 36Ar/C ratios. Based on that, they concluded that the carrier of light nitrogen is not always the same as that

of noble gases. Our conclusions are different. We have shown that both nitrogen and noble gases released at 600 to 900°C (having light nitrogen) are well correlated with the amounts of carbon combusted (Fig. 9). Also, Go¨ bel et al. (1978) have shown that the noble gases are carried mostly by diamond and within the carrier, the concentrations vary by one to two orders of magnitudes, so 36Ar/C ratios vary by one or two orders of magnitude between different ureilites, which is what has been seen by Yamamoto et al. (1998). As for their analyses of carbon-rich temperature fractions of the bulk samples of ALH77257, they found that N/C is variable with 36Ar/C essentially remaining constant. This can be explained by variable proportions of carbon and complementary phases in their samples. With significant N also contained in noncarbon phases, this results in variable N/C ratios among their samples. Argon, on the other hand, resides mainly in carbon phases, and hence, for a given ureilite, 36Ar/C is constant. The other two ureilites (Y-790981 and Y-791538) in their study show similar N/C ratios as ALH77257, but 36Ar/C ratios differ by one order of magnitude confirming the Go¨ bel et al. (1978) observations. The separated phases of Yamamoto et al. (1998) were neither pure

2228

V. K. Rai, S. V. S. Murty, and U. Ott

silicates nor pure carbon, i.e., they had carbon and silicates in variable proportions thus showing intermediate ratios of N/C. A clue toward the carrier of light N in carbon-rich residues comes also from an understanding of the correlation of nitrogen and argon with the amount of carbon combusting in each step. In Figure 9, the percentage release of nitrogen and argon for each temperature step (in the temperature interval 550 – 800°C, where light nitrogen is released) have been plotted versus the percentage of carbon combusted at that temperature, for all acid residues of diamond-bearing ureilites together. Here it can be seen that nitrogen is better correlated with C compared with Ar. This is because nitrogen is present in all the carbon phases in the residues, whereas the noble gases are present primarily in one of them (diamond; Go¨ bel et al., 1978). So wherever diamond is the principal C phase, the correlations of both Ar and N with C are better, whereas in cases where graphite also is contributing, the Ar vs. C correlation is disturbed (more than N/C) because the graphite is noble gas free. 4.5. Nitrogen to Argon Ratio It has been clearly seen in all the release patterns from acid-resistant residues and oxidized residues that the carriers of heavy nitrogen and light nitrogen have very different nitrogen to argon ratios. Except for the two ureilites ALH78019 and EET83309, the heavy nitrogen (N released at ⱕ500°C) in both monomict and polymict ureilites is depleted in noble gases relative to nitrogen. Most of the trapped noble gases are released along with light nitrogen (N released above 550°C). Figures 10a and 10b are plots of 14N/36Ar ratios for heavy and light nitrogen vs. ␦15N for acid-resistant residues of monomict and polymict ureilites, respectively. Low-(ⱕ500°C) and hightemperature (ⱖ500°C) fractions are shown separately. It can be seen clearly that 14N/36Ar for the heavy nitrogen carrier is an order of magnitude higher than for light nitrogen carrier (hightemperature fraction) for both monomict and polymict ureilites. As discussed earlier, this is most likely due to loss of noble gases from a pristine amorphous carbon carrier (like the one in low shocked ALH78019) during parent body processes. Relatively smaller loss of nitrogen would have occurred compared with noble gases because of higher retentivity of nitrogen (as N is chemically bound). Diamond, being more refractory, would most probably have retained close to its original 14N/36Ar ratio. ALH78019 (monomict) and EET83309 (polymict) do not fit in this scenario. Because these ureilites do not contain the lowtemperature heavy and high-temperature light nitrogen as shown by the residues from other ureilites, 14N/36Ar of individual temperature steps for these two ureilites have been plotted separately in Figure 10c. Ranges for 14N/36Ar ratios in diamond from all the ureilites and in amorphous carbon from diamond-free ureilite are similar, an indication that the amorphous carbon carrier and diamond initially may have had identical 14N/36Ar ratios. 4.6. Trapping of Nitrogen into Diamond and Amorphous C The variations of 14N/36Ar among the various carbon phases should be helpful in understanding the mechanism of their incorporation. If both N and Ar are incorporated into ureilite carbon phases by ion implantation, the value of 14N/36Ar that

Fig. 10. a. Nitrogen to argon (14N/36Ar) ratios for low- and hightemperature-released components (see text for definition) from the acid residues (HF/HCl as well as HClO4) of monomict ureilites plotted vs. ␦15N. b. Same as Figure 10a but for polymict ureilites. c. 14N/36Ar ratios for individual temperature steps plotted against ␦15N for the diamond-free ureilites ALH78019 and EET83309.

would be expected (for singly ionized nitrogen) from a reservoir with solar composition and an electron temperature of 8000K (inferred from best match to observed noble gas elemental abundances; Go¨ bel et al., 1978) would be ⬃30, not much different from the solar abundance ratio. It can be seen clearly that the 14N/36Ar ratios measured in ureilite residues are two to three orders of magnitude higher than that expected from implantation as “planetary” gas. This implies that, most likely, the dominating mechanisms for incorporation of N and noble gases into the C phases were different. Although for noble gases ion implantation is favored (Go¨ bel et al., 1978), most of the nitrogen seems to be structurally incorporated into the carbon phases—a fact repeatedly noted, as in the earlier argument, resulting in higher 14N/36Ar ratios; ion implanted N would only make a minor contribution. 4.7. Origin of Diamond in Ureilites Ever since diamond was discovered in ureilites, its origin has been a puzzle. Many of the features of ureilites suggest that the diamond was produced in situ by shock conversion of graphite. Although most ureilites show evidence of shock, it is not clear whether the shock is really responsible for diamond formation (by conversion of graphite) or whether the diamond was al-

Nitrogen components in ureilites

2229

Fig. 11. N/C ratios and ␦15N values of the carbon carrier phases that have been identified for the ureilites have been plotted. Lines show trends expected, in the residual reservoirs, for the evolution of the N isotopic composition through Rayleigh fractionation due to loss of atomic N.

ready present when ureilites underwent the shock event. Laboratory simulation studies on diamond produced by chemical vapor deposition in an artificial noble gas atmosphere under low-pressure plasma conditions have been interpreted as supporting a nebular origin (Fukunaga et al., 1987; Matsuda et al., 1991; Fukunaga and Matsuda, 1997). Here, we try to rationalize the origin of diamond using nitrogen isotopes along with noble gas isotopes as tracers. It has been suggested that diamonds are produced either from gas-rich amorphous carbon (of the type present in ALH78019) or shock conversion of gas-rich pristine graphite (that lost its gases subsequently during parent body processes). We have tried to evaluate the relation between various carbon phases in ureilites based on their N/C ratios and ␦15N. In Figure 11, the N/C ratios and the ␦15N values for various carbon carrier phases that have been identified among monomict and polymict ureilites have been plotted. The expected isotopic shifts in nitrogen due to mass fractionation through loss of atomic N are also shown, from amorphous C (found in ALH78019) and the diamond (from the mean value of the diamond field). It can be clearly seen that the formation of diamond from gas-rich amorphous carbon in ALH78019 is impossible by mass fractionation, whereas it is possible to generate the ␦15N values observed in the amorphous C from all ureilites (except EET83309), starting from that of ALH78019. Graphite lies closer to the mass fractionation line starting from diamond (here it has been assumed that diamond signature is the same as that of the pristine graphite from which it has been shock converted), but it requires a very large gas loss. Loss of nitrogen should always be accompanied by relatively more argon

(noble gas) loss that leads to an isotopic shift in 38Ar/36Ar by 6 to 13%. This is not observed (0.1898 ⫾ 0.0001 for diamond [mean value from Go¨ bel et al., 1978] and 0.1893 ⫾ 0.0001 for graphite in A19-A [this work]), which indicates that nitrogen in diamond and graphite are not related by mass fractionation. Hence the formation of diamond from graphite does not seem to be possible. Furthermore, the large differences in nitrogen compositions between bulk ureilites and their acid residues combined with the similarity of nitrogen composition of diamond from monomict and polymict ureilites (that have very large difference in bulk nitrogen isotopic composition) argue against the in situ origin of diamond in ureilites. It seems more likely that all the three carbon phases have been produced from the gas phase in the nebula and later have been incorporated into ureilites. Further evidence that diamond formation is unrelated to parent body processes, and hence that the diamond is not produced in situ, comes from the ALH82130 ureilite, which is unusual in several respects (see %Fo and ⌬17O in Table 1). Among others, its mineral compositions are significantly different from other ureilites, and it is the most reduced specimen of ureilite identified thus far (Berkley et al., 1985). It is also of low shock grade and, although XRD of an HF/HCl residue did not reveal any diamond lines (Ott et al., 1986), the observation of light nitrogen with ␦15N of ⬃–100‰ provides evidence for its presence. It can be seen clearly that the diamond inferred to be in this ureilite has nitrogen with a composition similar to that of diamond from other ureilites, although its ⌬17O and %Fo of olivine cores are very different. ⌬17O can be used as an indicator for silicate source homogenization and %Fo an indi-

2230

V. K. Rai, S. V. S. Murty, and U. Ott

cator for parent body equilibration, so the large differences between this highly reduced and other ureilites indicate that ALH82130 originated in very isolated region on the ureilite parent body (or on a different parent body). Nevertheless, the composition of nitrogen in diamond is very similar to other diamond-bearing ureilites. Finally, the mere presence of diamond in ALH82130 (of low shock grade) and in the almost unshocked ureilite DaG 868 (Takeda et al., 2001) provides another argument against the role of shock in the formation of diamond. 4.8. Origin of Heavy and Light Nitrogen Grady and Pillinger (1988) were the first to identify the heavy nitrogen in polymict ureilites. They suggested that it was of external origin and most likely injected into the ureilites parent body by an impactor. This may need reevaluation, given our results on monomict ureilites as well as some recent reports in literature (Rai et al., 2000; Yamamoto et al., 1998): heavy nitrogen is present in significant amounts in both monomict and polymict ureilites, except that in polymict ureilites it seems much heavier. The behavior toward combustion and towards acid treatment suggests amorphous carbon as the most probable carrier of heavy nitrogen and diamond as the carrier of light nitrogen. There are at least three ways to understand the relation between the isotopic signatures of these two components. One possibility is that both amorphous carbon and diamond trapped their gases from the solar nebula at different locations having different nitrogen isotopic compositions and that the difference represents local nebular heterogeneity. Another possibility for the presence of both heavy and light nitrogen is fractionation during incorporation, brought about by a change in the physicochemical conditions, which led to the formation of the three different forms of carbon. Diamonds are formed in a relatively narrow range of physicochemical conditions (Werner and Locher, 1998), so they show a narrow range of ␦15N, whereas amorphous carbon can be formed in a wider range (Matsuda et al., 1991), resulting in a broader range of ␦15N. A third possibility is that all carrier phases initially trapped nitrogen with the same composition, with differential loss of gases from some of the carriers leading to the observed variations in nitrogen composition. Our preferred interpretation is that the primary cause for the differences is change in physicochemical conditions in the nebula, with later partial loss due to parent body processes leading to finer changes superimposed on the primary differences, in particular, between the heavy nitrogen phases of monomict and polymict ureilites. Because the noble gases in the carbon phases are of nebular origin, the majority of nitrogen can also be considered to be from nebular source and as such might help in obtaining the composition of nebular nitrogen. Although the noble gas isotopic compositions are uniform in different C-phases of ureilites, such is not the case with nitrogen, implying that some additional physiochemical processes (presently not understood) are operating during the incorporation of N into these C-phases. These physiochemical processes might be causing varying extents of isotopic fractionation, to the nebular nitrogen with uniform composition, leading to the observed differences in the ␦15N values among the C-phases of ureilites. Laboratory simulation experiments are needed to understand the sign and

magnitude of the isotopic fractionation, under physiochemical processes prevalent during the formation of ureilite C-phases, and to derive the isotopic composition of nebular nitrogen. 4.9. Carbon and Evolution of Ureilite Parent Body Broadly, models of ureilite petrogenesis can be divided into two categories: primitive and igneous. Most of the primitive models can successfully account for the observed oxygen isotope heterogeneity, the enrichment of noble gases in carbon, and the enrichment of trace siderophile elements. In the nebular sedimentation model (Takeda and Yanai, 1978; Takeda et al., 1980, 1989) olivine and clinopyroxene preferentially sedimented toward the equatorial plane acquiring preferred orientation and layering. Carbonaceous material condensed and was incorporated into this thin mafic layer later. One of the major problems of this model is that a primitive nonchondritic parent body is required, and thus far no evidence has been found for such a body to have ever existed in the early solar system. On the other hand, the major challenge to all the igneous models is to explain how the primitive oxygen isotope signature, the noble gases, and the trace siderophile elements could have survived extensive igneous processes. Some of the igneous models assume that the ureilite parent body formed as a result of a planetary differentiation process from carbon rich magma, either magmatic cumulate or partial melt residue (Goodrich et al., 1987; Berkley and Jones, 1982). Another assumes collision of the differentiated ureilite parent body (still largely molten because of primordial heating) with a comet (or asteroid) rich in carbon, noble gases, and trace siderophiles (Warren and Kallemeyn, 1989). In the explosive volcanism model (Taylor et al., 1993; Warren and Kallemeyn, 1992), ureilites are assumed to be products of internal differentiation of a chondritic parent body, but partial melt residues rather than cumulates. From our noble gas and nitrogen data for bulk as well as various chemically separated phases of ureilites we have concluded that nitrogen isotopic compositions have not been homogenized between diamond, graphite, and silicates (or acidsoluble phases in general). Furthermore, acid-soluble phases from different ureilites not only show large variations in nitrogen isotopic compositions but also exhibit relatively large N concentrations (compared with silicates from other achondrites). This indicates that the major silicate reservoir of ureilites also may contain variable nitrogen isotopic compositions, as is the case for oxygen isotopes. The implication is that the carbon as well as the acid-soluble phases were never a part of the same magma. In view of these considerations, two of the proposed models for the origin of ureilites seem most suitable. These are 1) the partially disruptive model and b) the nebular sedimentation model. The partially disruptive model, although it can account for the oxygen isotopic anomaly, has difficulties in explaining the variations in nitrogen isotopic compositions in the acid-soluble phases. Therefore, a model similar to the nebular sedimentation model (Takeda and Yanai, 1978; Takeda et al., 1980, 1989) seems most appropriate. Support for this conclusion comes from variations in C isotopic composition both between different ureilites as well as within individual ureilites (Grady et al., 1985). Based on nitrogen and noble gas data, we suggest that graphite, diamond, and amorphous carbon all condensed in the solar nebula under reducing conditions.

Nitrogen components in ureilites

Olivine and clinopyroxene were also condensed directly from the gas phase, which resulted in their oxygen isotopic anomaly. The silicates may have condensed later than carbon in the same location, or they may have condensed concurrently at different locations in the nebula and later accreted together to form the ureilite parent body. 5. CONCLUSIONS

Ten ureilites have been studied for nitrogen and noble gases. Of these, HF/HCl- or (in the case of Havero¨ ) HF/H2SO4resistant residues have been prepared for eight ureilites. From these seven residues were prepared that were also resistant to HClO4. Analysis was by pyrolysis, combustion, or a combination of both. We infer that at least five distinct nitrogen components are needed to explain the nitrogen isotopic systematics of ureilites. Among the major components, a light nitrogen component with ␦15N ⱕ–100‰ is carried by the diamond present in both monomict and polymict ureilites. It is accompanied by primordial noble gases. Based on the degree of secondary alteration, presence of two types of amorphous carbon carriers is suggested. One of these is found in diamond-bearing ureilites and has lost a significant fraction of trapped noble gases during parent body processing. Its nitrogen, which is not associated with noble gases, is relatively heavy with variable ␦15N (ⱖ20‰). The other is little affected by parent body processes and is present only in the two diamond-free ureilites (ALH78019 and EET83309) and in Havero¨ . It contains large amounts of noble gases but the nitrogen isotopic compositions are very different (␦15N ⫽ ⫺21‰ in ALH78019 vs. much heavier in case of EET83309). Apparently, these two pristine amorphous C phases contain different nitrogen components, which are unlikely to be related by mass fractionation. Based on absence of light nitrogen and the low release temperature of nitrogen and noble gases, the polymict ureilite EET83309 also seems to be diamond-free. Of more than 90 ureilites known so far, this is the second diamond-free ureilite and the first diamond-free polymict ureilite. Most of the nitrogen released from this ureilite is relatively heavy. The presence of light nitrogen (with a narrow range of ␦15N) in diamond from both monomict and polymict ureilites, having very large variations in their bulk nitrogen composition argues against an in situ origin of diamond. Also the large differences in ␦15N of diamond, graphite, and amorphous carbon rules out the possibility that diamond was formed from graphite or amorphous carbon by in situ shock conversion. It has been proposed that the heavy nitrogen in polymict ureilites is of foreign origin, but the presence of heavy nitrogen in some of the monomict ureilites argues against this. It seems more likely that both heavy and light nitrogen were produced by nonequilibrium fractionation processes during trapping of these gases into their carriers in solar nebula because of differences in physiochemical conditions at their formation locations. Additional minor modification of nitrogen isotopic compositions may have resulted from secondary alteration on the ureilite parent body. Acknowledgments—Thorough and critical reviews by K. J. Mathew, I. Franchi, and an anonymous reviewer and editorial remarks by Rainer Wieler are gratefully acknowledged.

2231 REFERENCES

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Wilkening L. L. and Marti K. (1976) Rare gases and fossil tracks in the Kenna ureilite. Geochim. Cosmochim. Acta 40, 1265–1473. Wright I. P. and Pillinger C. T. (1989) Carbon isotopic analysis of small samples by use of stepped-heating extraction and static mass spectrometry. In new frontiers in stable isotope research: Laser probes, ion probes and small sample analysis (eds. W. C. Shanks et al.) USGS Bull. 1890, 9 –34. Yamamoto T., Hashizume K., Matsuda J-I., and Kase T. (1998) Multiple nitrogen isotopic components coexisting in ureilites. Meteorit. Planet. Sci. 33, 857– 870. Table A1. Nitrogen and argon in bulk samples of monomict, diamond-free, and polymict ureilites.

Temp.

N (ppm)

␦15N (‰)

36 Ar (10⫺8 ccSTP/g)

40

Ar/36Ar

PCO2 ⫹ CO mtorr

LEW85328 (bulk), L28-B, pyrolysis, 77.4 mg 300c 1.5 75.0 2.9 1.63 ⫾0.4 0.02 400c 1.4 70.5 1.9 0.286 0.9 0.004 500c 1.3 79.0 2.4 2.01 0.6 0.04 800 1.0 96.0 3.1 24.2 0.4 0.2 1000 0.4 ⫺8.1 6.2 31.7 0.4 0.5 1200 3.5 ⫺97.1 75.2 0.546 0.7 0.001 1400 3.1 ⫺106.2 108.1 0.133 0.4 0.001 1401 1.5 ⫺109.8 22.4 0.010 0.8 0.001 1450 1.2 ⫺117.6 10.9 0.007 0.6 0.001 1500 — — 8.1 0.329 0.002 1600 0.3 ⫺116.4 2.3 0.327 0.5 0.008 1850 0.2 ⫺94.8 0.1 94.4 2.1 0.1 Total 15.4 ⫺40.2 243.6 1.45 0.6 0.01 LEW85328 (bulk), L28-B1, combustion and pyrolysis, 26.33 mg 300c 1.8 12.3 0.31 19.2 2 ⫾0.9 0.1 400c 4.2 18.7 1.91 17.8 0 0.2 0.1 450c 0.5 15.2 0.43 9.59 0 0.5 0.07 500c 3.0 23.2 12.3 2.06 0 0.2 0.03 600c 3.4 27.8 24.8 3.43 21 0.1 0.01 700c 2.5 ⫺31.6 60.4 0.965 398 0.2 0.001 800c 3.3 ⫺98.8 107.4 0.116 750 0.3 0.001 900c 0.8 ⫺61.8 24.8 1.06 95 0.5 0.01 1000c 0.1 ⫺1.6 0.65 11.0 0 1.3 0.1 1200 0.1 14.1 2.4 22.9 2.3 0.3 1500 0.2 ⫺93.7 5.0 1.66 1.3 0.01 1850 — — 1.2 1.61 0.10 Total 20.1 ⫺10.0 241.5 1.34 0.3 0.02

Nitrogen components in ureilites (Table A1 Continued)

Temp.

N (ppm)

2233

(Table A1 Continued)

␦15N (‰)

36 Ar (10⫺8 ccSTP/g)

40

Ar/36Ar

PCO2 ⫹ CO mtorr

Kenna (bulk), Ken-B, pyrolysis, 49.49 mg 400c 1.2 18.8 8.2 0.308 ⫾1.4 0.001 800 2.0 24.5 8.9 0.343 1.0 0.001 1000 2.3 ⫺75.8 3.8 0.092 0.4 0.006 1200 4.7 ⫺79.2 15.9 0.006 0.5 0.001 1300 4.2 ⫺96.7 19.8 0.036 1.1 0.001 1350 0.8 ⫺123.4 5.3 0.278 2.6 0.046 1400 0.4 ⫺103.0 5.4 0.141 0.8 0.002 1500 1.2 ⫺125.6 7.3 0.462 1.0 0.003 1600 0.4 ⫺99.8 0.4 23.7 1.5 0.1 1850 0.3 ⫺75.6 0.1 28.3 3.6 0.4 Total 17.5 ⫺70.4 75.0 0.297 1.0 0.005 Lahrauli (bulk), Lah-B, pyrolysis and combustion, 29.89 mg 300c 8.7 8.7 0.9 8.68 7 ⫾0.2 0.08 400c 11.1 9.0 1.8 8.47 13 0.2 0.11 500c 11.6 13.0 5.0 16.2 10 0.1 0.1 600c 9.3 3.6 9.7 2.49 66 0.5 0.01 700c 7.8 ⫺32.2 31.2 0.540 225 0.1 0.001 800c 5.3 ⫺23.8 7.3 9.76 144 0.1 0.01 900c 1.4 ⫺5.6 0.4 23.7 2 0.3 0.1 1200 1.4 ⫺2.8 0.8 48.5 0.1 0.2 1500 1.0 ⫺80.9 3.2 1.65 0.3 0.02 1850 — — 0.9 17.7 0.1 Total 52.2 ⫺2.3 77.3 4.70 0.2 0.02 Havero¨ (bulk), Hav-B, pyrolysis, 76.53 mg 400c 8.5 16.5 0.2 9.56 ⫾0.4 0.25 1000 4. 54.3 0.8 12.2 0.4 0.1 1200 1.0 19.6 2.2 0.112 0.6 0.006 1400 3.9 ⫺38.4 20.6 0.480 0.2 0.002 1600 3.9 ⫺77.8 57.2 0.333 0.3 0.001 1800 0.8 ⫺22.9 0.3 188 0.3 1 Total 22.1 ⫺4.0 81.4 1.20 0.3 0.001 ALH81101 (bulk), A01-B, pyrolysis, 42.34 mg 400c 23.4 4.4 0.2 54.2 ⫾0.4 0.1 500c 7.4 6.4 0.5 16.1 0.4 0.1

Temp.

N (ppm)

800

11.8

␦15N (‰)

36 Ar (10⫺8 ccSTP/g)

40

Ar/36Ar

4.0 0.2 59.5 0.4 0.1 1000 2.6 ⫺15.6 0.2 15.5 0.4 0.2 1200 3.0 ⫺59.6 7.6 1.33 0.4 0.01 1400 2.4 ⫺85.9 21.1 0.031 0.5 0.005 1600 2.8 ⫺84.7 9.8 3.22 0.4 0.01 1850 — — — Total 53.2 ⫺8.6 39.7 1.96 0.4 0.01 ALH82130 (bulk), A30-B, pyrolysis, 50.24 mg 400c 30.4 0.8 0.3 35.7 ⫾0.4 0.1 500c 12.0 6.7 0.7 15.6 0.4 0.1 800 4.0 ⫺3.7 0.3 18.7 1.0 0.1 1000 1.4 ⫺45.4 0.6 2.77 0.4 0.03 1200 2.0 ⫺93.4 5.9 0.355 0.7 0.002 1400 0.9 ⫺76.3 10.8 0.421 0.8 0.001 1600 2.9 ⫺100.3 2.1 1.79 0.5 0.06 1800 0.5 ⫺93.3 0.1 165 0.4 1 1850 0.7 ⫺28.5 — — 0.8 Total 54.9 ⫺10.7 20.6 2.51 0.5 0.01 ALH78019 (bulk), A19-B, pyrolysis, 42.20 mg 400c 0.9 ⫺16.0 4.1 1.37 ⫾ 0.5 0.02 1000 2.8 ⫺9.3 11.7 1.34 1.8 0.01 1200 0.4 ⫺19.5 12.9 0.097 0.4 0.002 1400 1.6 ⫺3.5 46.1 0.012 0.4 0.002 1600 0.4 ⫺47.5 34.7 0.089 2.9 0.004 1800 0.1 ⫺45.7 0.6 23.4 3.2 0.2 Total 6.3 ⫺13.5 110.1 0.360 1.3 0.005 Nilpena (bulk), Nil-B, combustion and pyrolysis, 49.15 mg 300c 1.4 18.4 0.1 213 ⫾0.2 1 400c 4.9 26.7 0.1 47.5 0.1 0.1 500c 5.0 31.2 8.2 15.1 0.2 0.1 600c 3.5 0.6 13.5 2.53 0.2 0.01 700c 4.7 ⫺89.0 18.7 1.03 2.0 0.01 800c 3.8 ⫺79.5 15.0 1.26 0.3 0.01 900c 3.7 ⫺93.5 8.9 4.82 0.3 0.01 1000c 3.1 ⫺92.4 8.9 3.72 0.2 0.01

PCO2 ⫹ CO mtorr

2 10 30 537 1050 856 605 450

2234

V. K. Rai, S. V. S. Murty, and U. Ott

(Table A1 Continued)

Temp.

N (ppm)

Table A2. Nitrogen and argon in acid residues of monomict, diamond-free, and polymict ureilites.

␦15N (‰)

36 Ar (10⫺8 ccSTP/g)

40

Ar/36Ar

PCO2 ⫹ CO mtorr Temp.

1050c

1.1

1200

0.7

1500

1.0

1850

0.1

Total

32.9

EET87720 (bulk), 300c 0.1 400c

5.7

500c

11.6

800

7.9

1000

3.9

1200

1.7

1400

1.7

1500

0.7

1600

0.6

1850

0.2

Total

34.1

EET83309 (bulk), 300c 0.4 400c

2.7

500c

2.2

800

1.1

1000

1.3

1200

1.1

1400

1.5

1600

0.7

1850

—

Total

10.8

⫺104.3 6.1 3.09 0.3 0.01 ⫺15.2 0.9 21.3 1.2 0.1 ⫺69.1 1.4 0.35 0.8 0.02 ⫺69.4 0.4 7.16 1.0 0.15 ⫺37.6 82.0 3.93 0.5 0.01 E20-B, pyrolysis, 71.41 mg 107.3 0.4 22.0 ⫾0.6 0.5 118.8 0.4 17.3 0.4 0.1 155.5 2.0 11.4 0.4 0.1 172.7 1.2 16.1 0.4 0.1 194.5 3.3 0.790 0.4 0.005 31.9 7.1 0.292 0.5 0.001 ⫺55.5 15.1 0.013 0.4 0.001 ⫺84.8 5.2 0.077 0.6 0.001 ⫺99.9 0.6 7.71 1.1 0.03 ⫺91.3 0.1 31.7 0.9 0.3 130.4 35.0 1.76 0.4 0.01 E09-B, pyrolysis, 45.90 mg 69.0 0.3 18.08 ⫾2.9 0.04 69.9 5.1 0.446 0.5 0.003 66.1 22.5 0.376 0.4 0.001 49.3 7.1 1.22 0.8 0.01 107.3 15.5 0.519 0.4 0.002 49.4 33.6 0.079 0.7 0.001 ⫺9.3 40.6 0.204 0.5 0.001 ⫺6.3 12.9 0.446 0.5 0.002 — 0.1 15.1 0.2 54.1 137.7 0.372 0.6 0.001

N (ppm)

␦15N (‰)

36 Ar (10⫺8 ccSTP/g)

40

Ar/36Ar

PCO2 ⫹ CO (mtorr)

184

c ⫽ combustion. Uncertainties in the concentration of N and Ar are ⫾10%.

Havero¨ (HF/H2SO4 residue), Hav-A, combustion, 3.28 mg 200 9.8 7.7 0.8 58.3 ⫾0.6 0.6 300 38.4 4.4 23.6 44.4 0.2 0.1 400 49.9 13.7 74.2 49.6 0.7 0.1 500 74.8 2.6 195.4 32.9 0.7 0.1 600 25.1 ⫺46.2 55.9 12.9 1.2 0.1 650 32.2 ⫺97.8 313.5 0.984 0.4 0.004 700 16.0 ⫺104.2 264.2 0.242 1.9 0.001 800 5.2 ⫺59.0 90.4 0.363 0.9 0.001 1000 4.7 ⫺46.4 408.3 0.228* 1.2 0.001 1050 1.1 4.2 5.8 8.29 5.7 0.15 Total 257.2 ⫺20.9 1432 8.64 0.6 0.02 Havero¨ (HClO4 residue), combustion, Hav-O, 0.56 mg 300 42.4 2.6 2.2 140 ⫾2.4 1 400 221.2 1.5 28.1 38.2 0.3 0.1 450 54.7 ⫺14.1 82.8 9.85 0.4 0.04 550 31.6 ⫺42.5 162.9 0.200 1.2 0.006 650 13.7 ⫺32.2 2789 0.194 4.4 0.001 800 10.8 ⫺67.9 1912 0.393 2.3 0.009 1000 16.7 ⫺46.0 1668 1.47 2.5 0.01 Total 391.1 ⫺9.2 6646 0.898 0.9 0.006 LEW85328 (HF/HCl residue), L28-A1, combustion, 1.21 mg 200 1.4 — 0.91 2.22 0.10 300 21.1 12.4 29.5 1.11 2.1 0.01 400 27.5 25.5 243.9 0.306 3.8 0.001 500 23.3 11.6 998.6 0.192 4.0 0.001 600 95.7 ⫺97.9 4030 0.049 0.4 0.001 700 197.4 ⫺106.7 8994 0.017 0.3 0.001 800 51.7 ⫺114.1 4383 0.029 0.3 0.001 900 2.3 ⫺35.2 7.7 19.2 12.0 0.8 1000 4.0 ⫺40.7 0.5 129 1.8 1 Total 424.4 ⫺82.7 18689 0.049 1.0 0.001 LEW85328 (HF/HCl residue), L28-A2, pyrolysis, 1.00 mg 300 — — 4.4 0.637 0.209 600 3.0 8.2 25.4 9.25

0 0 0 44 510 924 553 418 35 0 2484

2 12 27 184 160 148 69 602

0 4 10 134 845 924 488 8 2 2415

Nitrogen components in ureilites (Table A2 Continued)

(Table A2 Continued)

Temp. 800 1000 1200 1400 1500 1600 1700 1800 1800r 1850

N (ppm)

␦15N (‰)

⫾1.9 19.4 0.3 2.2 25.9 0.9 0.3 27.0 4.9 160.3 12.0 0.3 112.9 ⫺34.4 0.4 74.0 ⫺81.2 0.1 108.9 ⫺100.5 0.4 75.9 ⫺100.5 0.6 Added to 1800°C fraction 12.1 ⫺101.1 2.0 12.4

1851 ⫺48.8 0.4 LEW85328 (HClO4 residue), 200 2.6 — Total

300

561.8

11.8

2235

36 Ar (10⫺8 ccSTP/g)

40

Ar/36Ar

0.04 1.54 0.02 15.8 10.5 0.1 572.5 0.014 0.001 1527 0.016 0.001 4456 0.033 0.005 2267 0.068 0.001 2837 0.250 0.002 800.3 0.064 0.004 168.2 2.01 0.02 37.8 4.78 0.08 4.1 135 1 12739 0.204 0.038 L28-O, combustion, 2.07 mg 0.1 141 4 3.3 5.41 0.08 26.0 0.914 0.004 50.4 0.771 0.003 1586 0.092 0.001 4157 0.023 0.001 5145 0.048 0.001 3656 0.134 0.001 0.3 —

PCO2 ⫹ CO (mtorr)

Temp. 200

23.0

0

11.9 6 ⫾0.6 400 1.3 1.3 44 2.9 500 7.1 ⫺4.1 156 1.2 600 61.4 ⫺105.6 890 1.3 700 154.2 ⫺106.9 960 0.9 800 168.7 ⫺107.0 954 0.5 900 75.0 ⫺112.7 790 0.3 1000 7.4 ⫺63.9 0 4.4 Total 489.5 ⫺102.1 14624 0.072 3800 0.8 0.001 ALH81101 (HF/HCl residue), A01-A, combustion, 1.06 mg 200 3.8 27.1 0.2 294 0 ⫾1.1 1 300 3.1 21.0 1.2 53.1 0 3.1 0.2 400 23.9 14.9 5.8 16.4 0 0.3 0.1 500 45.2 13.5 20.4 3.96 36 0.1 0.02 600 36.3 ⫺57.3 492.5 0.016 685 0.1 0.001 700 21.2 ⫺94.5 708.6 0.004 627 0.9 0.001 800 0.5 ⫺57.7 3.5 29.5 2 1.0 0.1 1000 6.3 ⫺4.9 0.8 37.5 0 4.6 1.8 Total 140.3 ⫺21.4 1233 0.364 1350 0.6 0.003 ALH81101 (HClO4 residue), A01-O, combustion, 1.06 mg

N (ppm) 5.6

␦15N (‰)

36 Ar (10⫺8 ccSTP/g)

40

Ar/36Ar

PCO2 ⫹ CO (mtorr)

16.9 0.2 197 ⫾1.0 1 300 21.4 23.6 0.3 89.3 0.3 0.6 400 65.4 15.4 0.3 53.4* 0.6 0.7 500 70.1 12.4 5.8 0.502 0.2 0.073 600 63.4 ⫺88.1 280.9 0.249 0.5 0.002 700 107.3 ⫺107.7 1475 0.015 0.2 0.002 800 61.7 ⫺63.2 228.2 0.419 1.3 0.002 1000 1.0 ⫺18.0 1.2 3.45 16.2 1.29 Total 340.4 ⫺44.3 1992 0.134 0.4 0.002 ALH82130 (HF/HCl residue), A30-O, combustion, 1.32 mg 800p 1.7 87.3 10.7 9.17 ⫾2.4 0.03 200 — — 2.4 182 1 300 0.1 — 3.1 11.5 0.3 400 9.9 19.2 14.3 4.09 4.8 0.06 450 11.4 19.5 28.3 4.90 0.7 0.02 500 2.3 26.0 17.3 4.04 3.1 0.05 600 27.2 ⫺63.6 186.6 22.8 0.6 0.1 700 47.6 ⫺53.3 168.0 141 0.7 1 800 6.5 ⫺51.2 206.9 2.39 1.4 0.01 900 0.2 — 2.5 40.4 0.2 1000 0.3 — 0.8 25.6 1.0 1800p 1.1 ⫺23.9 1.6 304 2.7 1 Total 108.3 ⫺37.7 642.6 46.48 1.2 0.02 ALH82130 (HClO4 residue), A30-O, combustion, 1.82 mg 800p 11.6 8.1 1.5 52.6 ⫾0.6 0.1 200 — 0.1 285 — 1 300 0.2 — 1.3 227 1.0 400 8.0 18.5 0.3 51.1 4.4 2.1 450 3.3 14.4 0.9 28.5 2.4 0.3 500 7.5 ⫺5.3 15.1 0.331 1.0 0.042 600 23.1 ⫺80.7 123.2 0.487 0.5 0.005 700 42.0 ⫺103.3 150.6 0.416 0.4 0.008 800 3.0 ⫺13.9 59.4 2.66 0.9 0.02 900 — 0.3 193 1

0 0 0 85 503 663 86 0 1337.0

12 27 27 745 290

90 924 1010 168

2236

V. K. Rai, S. V. S. Murty, and U. Ott (Table A2 Continued)

(Table A2 Continued)

Temp. 1000

N (ppm) —

␦15N (‰)

36 Ar (10⫺8 ccSTP/g)

1.1

40

Ar/36Ar

PCO2 ⫹ CO (mtorr)

247 1 1800p 9.0 2.0 0.7 279 2.2 1 Total 108.1 ⫺56.3 354.6 3.53 1.1 0.02 ALH78019 (HF/HCl residue), A19-A, combustion, 1.35 mg 300 13.3 1.0 261.0 0.254 1 ⫾1.4 0.001 400 12.5 ⫺21. 987.3 0.547 1 4.7 0.001 450 19.7 ⫺21.0 197.7 0.006 1 0.6 0.002 550 6.9 ⫺11.1 98.8 0.114 14 0.8 0.004 650 11.8 ⫺9.9 27.9 0.049 172 1.3 0.032 800 10.8 18.9 140.8 0.844 795 2.6 0.018 1000 7.3 10.1 68.1 2.79 1020⫹ 0.4 0.09 117‡ Total 82.3 ⫺7.1 1782 0.521 2121 1.7 0.006 ALH78019 (HClO4 residue), A19-O, combustion, 1.00 mg 300 12.2 ⫺22.7 1.5 268.1 0 ⫾0.4 0.5 400 19.8 ⫺8.2 2.7 249.4 0 0.8 0.8 450 8.7 ⫺12.3 4.1 285.4 0 1.2 0.8 550 18.7 ⫺10.9 33.2 277.9 27 0.4 0.7 800 72.2 ⫺2.3 18.4 295.1 206 0.3 0.3 1050 25.3 3.2 — — 1240 3.6 Total 157.0 ⫺5.3 59.9 282.2 1473 1.0 0.6 EET83309 (HF/HCl residue), E09-A, combustion, 1.63 mg 200 5.6 ⫺15.6 — — 0 ⫾0.7 300 5.9 20.3 5.7 0.624* 0 0.9 0.001 400 22.7 40.5 662.0 0.094 0 0.6 0.001 500 79.5 57.3 1543 0.137 120 0.4 0.001 600 7.7 87.4 878.2 0.235 715 2.1 0.001 700 2.1 152.7 1320 0.144 815 3.6 0.001 800 1.1 ⫺22.3 628.5 0.067 360 1.4 0.001 900 1.0 46.5 178.6 0.025 80 8.3 0.005 1000 0.9 24.4 12.1 3.38 4 1.3 0.13 Total 126.4 51.8 5229 0.151 2094 0.7 0.001 EET83309 (HClO4 residue), E09-O, combustion, 1.61 mg 200 5.4 — 2.8 42.1 0 0.3 300 8.7 11.7 1.2 27.7 0 ⫾0.4 0.1 400 13.6 9.4 7.3 10.4 0 0.3 0.1

Temp.

N (ppm)

500

31.8

␦15N (‰) 9.5 0.3 17.0 0.7 ⫺0.7 1.1 40.6 5.3 —

36 Ar (10⫺8 ccSTP/g)

51.6

40

Ar/36Ar

1.58 0.01 600 4.4 112.9 1.51 0.01 700 3.9 148.0 0.561 0.003 800 1.9 16.1 4.71 0.02 900 — 2.5 3.62 0.31 1000 — — — — Total 64.6 10.5 342.3 1.89 0.5 0.01 Nilpena (HF/HCl residue), Nil-A, combustion, 2.57 mg 200 1.3 18.8 — — ⫾3.8 300 23.1 77.2 5.0 2.75 0.4 0.01 400 62.5 52.6 43.0 0.970 0.4 0.002 500 18.1 24.4 195.4 0.347 0.5 0.002 600 107.9 ⫺98.1 1999 0.046 0.4 0.001 700 59.9 ⫺109.9 1493 0.016 0.2 0.001 800 1.0 ⫺5.2 20.7 1.81 4.6 0.01 1000 0.1 — — Total 273.9 ⫺42.5 3756 0.077 0.4 0.001 EET87720 (HF/HCl residue), E20-A, combustion, 1.76 mg 200 0.1 — 0.2 64.5 0.1 300 3.9 28.9 1.6 22.7 ⫾2.1 0.1 400 58.8 75.6 40.9 2.24 0.3 0.01 500 145.0 107.9 228.0 1.12 0.8 0.01 600 97.0 29.0 518.3 0.338 0.4 0.001 700 113.7 ⫺47.8 911.7 0.110 1.4 0.001 800 101.9 ⫺60.1 1857 0.047 0.5 0.001 900 8.2 ⫺17.2 903.3 0.066 1.9 0.003 1000 4.9 23.9 10.3 28.7* 0.8 0.04 Total 533.5 21.4 4471 0.183 0.8 0.002 EET87720 (HClO4 residue), E20-O, combustion 1.89 mg 200 0.2 — 0.1 300 5.5 10.1 0.2 86.6 ⫾1.9 1.2 400 28.3 19.2 3.3 12.6 2.6 0.1 500 20.3 8.0 26.0 0.878 0.4 0.005 600 37.2 ⫺55.0 583.7 0.406 0.3 0.001 700 61.0 ⫺94.0 2488 0.071 0.6 0.001 800 22.1 ⫺99.3 1054 0.065 0.2 0.007

PCO2 ⫹ CO (mtorr) 44 530 667 862 12 0 2115 0 2 23 270 1200 1150 13 0 2658 0 1 17 172 775 850 885 237 1 2938 0 1 20 104 830 1090 740

Nitrogen components in ureilites (Table A2 Continued)

Temp.

N (ppm)

900

4.4

1000

2.5

Total

181.6

␦15N (‰) ⫺43.5 1.0 ⫺34.4 1.9 ⫺52.3 0.8

36 Ar (10⫺8 ccSTP/g)

40

Ar/36Ar

0.8 — 4156

* as measured, (40Ar blank was higher than step). ‡ CO ⫹ CO2 pressure at 1050°C step.4. r: re-extraction; p: pyrolysis.

—

3

—

0

0.132 0.003 40

PCO2 ⫹ CO (mtorr)

2788

Ar measured in this

2237