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Abstract

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

Abstract– A bulk sample (split from Almahata Sitta #36) and an acid resistant residue (from #44) have been analyzed for noble gases and nitrogen by step-wise combustion/pyrolysis. In the bulk sample, He and Ne are a mixture of cosmogenic and trapped components. Cosmic- ray exposure ages of 13.8 and 16.0 Ma are calculated based on 3He, and 21Ne, respectively. Except for a small amount of cosmogenic 3He, He, and Ne in the acid-resistant residue are not significantly above blank level. Ar, Kr, and Xe in both the bulk and residue are dominated by a trapped component, but the elemental ratios are different. While the ratios of 36Ar/132Xe and 84Kr/132Xe are about 400 and 1, respectively, in all the combustion steps of the residue, the bulk sample has about an order of magnitude more 132Xe in the corresponding combustion steps. It seems, an acid soluble phase is the host of this Xe-rich carrier and is different from a similar phase observed in the ureilite Allan Hills 82130. Nitrogen in the bulk sample and acid residue are 21.1 ppm (−36.8‰), and 249.5 ppm (−74.3‰), respectively. Peak release of C (monitored as CO + CO2), N, Ar, Kr, and Xe occurred at the 700 °C combustion step of the residue, confirming diamond as the principal carrier for these gases. In the residue, the isotopic ratio 38Ar/36Ar shows a monotonic increase with release temperature.


Introduction

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

Ureilites are the second largest group of achondrites in our collection (Mittlefehldt et al. 1998), although the number of observed falls is only 7. Almahata Sitta (hereafter AS) is the most recently fallen ureilite, the least weathered in our collection, and is classified as an anomalous polymict ureilite, anomalous on account of some collected fragments being rich in pores (up to 20%) and carbonaceous matter (Jenniskens et al. 2009). It is the first meteorite to be associated with a known small asteroid of size approximately 4 m, called 2008 TC3, predicted and observed to hit Earth on October 7, 2008 in the Nubian Desert of Sudan (Yeomans 2008; Borovi✓ka and Charvát 2009; Jenniskens et al. 2009). AS has three major lithologies that are not homogeneously distributed at gram sample level, with several clasts found scattered among these lithologies (Zolensky et al. 2010).

Ureilites are enigmatic meteorites showing both primitive and igneous characteristics. Heterogeneous oxygen isotopic composition, falling along the CCAM line in the three isotope plot (Clayton and Mayeda 1988), occurrence of large amounts of trapped noble gases, mostly hosted in elemental C (Göbel et al. 1978; Rai et al. 2003a), and coexistence of multiple nitrogen components (Yamamoto et al. 1998; Rai et al. 2003b) are some of the primitive features, while the mineralogy, texture, and chemistry suggest them to be highly fractionated rocks (Goodrich 1992; Mittlefehldt et al. 1998). Although the carrier of trapped noble gases is shown to be elemental C, mostly diamond (Göbel et al. 1978; Rai et al. 2003a) or amorphous C, in case of Allan Hills (ALH) 78019 (Wacker 1986; Rai et al. 2002), there are some suggestions that graphite is a likely candidate as well (Okazaki et al. 2003). Earlier work has identified the typical elemental and isotopic ratios of N, N/C, and N/36Ar in the host phases of monomict and polymict ureilites (see table 5 in Rai et al. 2003b).

Different models have been proposed for the formation of ureilites that describe them as multistage igneous cumulates (Berkley et al. 1976; Goodrich et al. 1987; Berkley 1989), residual partial melts (Boynton et al. 1976; Wasson et al. 1976; Spitz and Boynton 1991; Scott et al. 1993), collision products of primitive planetesimals (Takeda 1987; Takeda et al. 1989; Warren and Kallemeyn 1989), and a result of smelting of an olivine rich parent body (Warren and Kallemeyn 1992; Walker and Grove 1993; Singletary and Grove 2003). Early smelting or partial melting events suggested by Hf-W systematics may prevent the homogenization of the primitive features like oxygen isotopic composition and the presence of primordial noble gases in the ureilite parent body (Lee et al. 2009).

The fresh nature of AS, the corroborating information about the size of the asteroid prior to impact, and the unusual properties of this meteorite make it an interesting case to study the volatiles nitrogen and noble gases.

Sample Preparation

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

As part of a consortium study, we received two fragments from samples #44 and #36. Sample #36 is from a 58 g meteorite found at Long. = 32.51850° and Lat. = +20.71195°, with a density of 2.67 g cc−1, while sample #44 consisted of small fragments (possibly broken from a larger fallen meteorite during impact) lying in the sand at Long. = 32.49690° and Lat. = +20.70178°, with total weight of 2.3 g. Our bulk sample is from #36 and hence is a fresh interior chip.

We dissolved the piece from #44 (178.4 mg) to obtain acid-resistant residue, with the hope that any adhering terrestrial material will dissolve in acids and does not contribute to the noble gases. The HF/HCl acid residue was prepared by the procedure described earlier (Rai et al. 2003b). Briefly, the finely powdered sample was treated alternately with 10 N HF/6 N HCl and 6 N HCl. This procedure was repeated six times. The residue was then washed with water under ultrasonication, several times, until the pH became neutral. After subsequent washes with alcohol and acetone, the residue was air dried. The yield at this stage was approximately 7 mg (about 4% of starting mass, and in the range observed for most ureilites; Rai et al. 2003a). Residues from ureilites, produced by the same procedure by earlier workers, have shown them to be mostly C phases (Göbel et al. 1978; Ott et al. 1984).

For the residue, we only investigated the morphological features by scanning electron microscope (SEM) and took energy dispersive X-ray spectra to identify qualitatively the major elements. In most grains, only a C line was seen. Figure 1 gives the SEM pictures of the residue. The residue is mostly made of crystallites of graphite (dark gray in color) of approximately 10 μm, with coatings of amorphous C. In addition, whitish crystals of diamond, few microns to submicron size are also in abundance. The amorphous C phase, seen as coating on graphite crystals, seems to be a minor phase.

image

Figure 1.  Scanning electron microscope pictures of the HF/HCl resistant residue of Almahata Sitta. Most of the residue is a cluster of crystalline structures. Energy dispersive X-ray spectra of most crystals show only C peak. Graphite (G) with layered structure (dark gray) and whitish crystals of diamond (D) of few microns to submicron sizes are seen in all four pictures. Amorphous carbon, seen as a fine coating on the dark gray graphites, seems to be low in abundance.

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The bulk sample (as received) and the residue were packed into gold foil (to enable combustion up to 1030 °C) and loaded into the noble gas extraction system. The samples were degassed overnight under vacuum at 150 °C, with an infra red lamp, to drive off the adsorbed gases. After obtaining satisfactory system blanks, the sample analysis was started.

Experimental

The bulk samples were combusted under 2 Torr O2 pressure at the following temperatures (in °C, 300, 400, 600, 800, and 1000) before pyrolyzing at 1200, 1400, and 1800 °C in a Mo crucible. The acid residue was only combusted at 300, 400, 450, 500, 600, 700, 800, and 1030 °C. For combustion, O2 was generated from Cu/CuO finger, until the required pressure was reached in the sample chamber, a double-walled quartz finger, with outer layer of secondary vacuum. The sample was kept at the desired temperature for 45 min, after isolating the Cu/CuO finger. At the end of the combustion, the remaining O2 was removed by exposing to Cu/CuO finger at 400 °C. At this stage, about 10% of extracted gas had been isolated for later processing for N2 analysis and the rest was cleaned on Ti/Zr and Ti/Pd getters to remove active gases. The heavy noble gases (Ar, Kr, and Xe) were held back on a charcoal finger kept at liquid nitrogen temperature, and the He, Ne fraction is admitted into the mass spectrometer (VG1200; MicroMass, UK) for analysis. Subsequently, the Ar, Kr, and Xe fraction was admitted into the mass spectrometer. After a quick scan of the peak heights of 36Ar, 84Kr, and 132Xe, for abundances, Ar, Xe, and Kr were run for isotopic analysis. The procedures have been described in earlier publications (Rai et al. 2003a, 2003b).

Blanks were run under identical conditions of sample analyses. For N and heavy noble gases, the blank contribution is <5% for all temperature steps. For 4He and Ne, the blank contribution is <15% for the bulk sample, but for the residue sample, blank contribution is up to 50% at some temperatures (≥600 °C) making the isotopic ratios susceptible to blank correction, although the signals at 4He and 22Ne allowed estimation of their abundances. Hence, we only report 4He and 22Ne amounts for the residue sample. 3He on the other hand has a negligible blank, making it possible to detect even small amounts with ease.

Air standards were analyzed to ascertain the sensitivity and mass discrimination corrections. The data reported have been corrected for blanks, mass discrimination, and interferences from CO2++ (22Ne), 40Ar++ (20Ne), and CO (N2) as detailed earlier (Murty 1997; Rai et al. 2003a, 2003b). In our earlier work on the measurement of Ar isotopic composition in ureilites, we identified a pressure dependence on the mass discrimination on the 38Ar/36Ar ratio and worked out the correction factor to take this into account (Rai et al. 2003a). The Ar isotopic data presented here have been corrected for the pressure-dependent mass discrimination, as detailed in Rai et al. (2003a).

Results

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

The data of all noble gases for the totals of the bulk sample and acid residue are given in Table 1. N data, cosmogenic He, Ne, production rates, and exposure ages are listed in Table 2. Table 3 gives the individual temperature data, for both bulk and acid residue.

Table 1.   Noble gas abundances and isotopic ratios for the totals in the bulk sample (AS#36) and HF/HCl residue (prepared from AS#44) of Almahata Sitta. Abundances are in ccSTP g−1 units. Errors in isotopic ratios are at 95% confidence limits. Errors in concentrations are ±10% (for He, Ne, Ar) and ±15% (for Kr and Xe).
Sample (wt. mg)4He (10−6)22Ne (10−6)36Ar (10−6)3He/4He (10−4)20Ne/22Ne21Ne/22Ne38Ar/36Ar40Ar/36Ar84Kr (10−10)132Xe (10−10)
  1. Note: AS = Almahata Sitta.

Bulk (30.62)13.00.1230.435178.6 ±15.12.986 0.0920.7360 0.01160.1954 0.00036.810 0.043 32.34 102.3
Residue (1.23)6821.23453.30.1908 0.00020.2058 0.001117,10612,211
Table 2.   Nitrogen, trapped and cosmogenic noble gas amounts in AS samples. Production rates P3 and P21 are in units of 10−10 ccSTP g−1 Ma−1 units (for details of calculation, see text). Errors in N are ±10% and errors in δ15N correspond to 95% CL. The trapped gas amounts are normalized to 132Xe. Errors in cosmic-ray exposure ages (T3 and T21) are ±15%.
SamplesN (ppm)δ15N (‰)3Hec (10−8)21Nec (10−8)P3P21T3 (Ma)T21 (Ma)Ratios (iX/132Xe)
4He20Ne36Ar84Kr132Xe
  1. Note: AS = Almahata Sitta.

Bulk21.1−36.80 ± 0.7823.29.08167.956.713.816.0126827.142.50.316≡1.00
Residue249.5−74.30 ± 0.6112.6  55811.53711.401≡1.00
Table 3.   Data for temperature fractions of bulk and HF/HCl residue of Almahata Sitta samples. Data for the totals are listed in Table 1.
Temp. (°C)4He (10−6)22Ne (10−8)36Ar (10−6)3He/4He (10−4)N (ppm)δ15N (‰)20Ne/22Ne21Ne/22Ne38Ar/36Ar40Ar/36Ar14N/36Ar (103)84Kr/36Ar (10−3)132Xe/36Ar (10−3)C releasea (%)
  1. Note: P* = pyrolysis, while at other temperatures, it is combustion.

  2. aC release has been monitored as the evolved (CO + CO2).

  3. bFor the residue sample, only 3He concentrations are listed under the column (3He/4He), in 10−8 ccSTP g−1 units.

Bulk sample #36 (30.62 mg)
 3001.090.4470.005124.1 ± 10.52.7724.4  0.89.511 0.1780.0728 0.00090.1951 0.000239.1 0.188618.036.4∼0
 4000.780.3930.005272.4  23.11.8231.8 0.59.998 0.1690.0952 0.00130.2008 0.000940.8  0.559813.141.8∼0
 6001.900.5540.024181.6  15.41.8027.7 0.98.964 0.1500.1660 0.00270.1919 0.000925.9  0.1118.39.951.8∼0
 8004.561.0430.096107.1   9.13.27−65.3 0.56.731 0.2670.4319 0.01790.1893 0.00057.78  0.0154.29.244.827.8
 10002.011.260.156173.3  14.76.15−94.3 0.84.040 0.2070.6019 0.02300.1893 0.00014.38  0.0563.25.715.542.8
 1200P*0.561.680.058917.9  77.73.89−23.1 0.81.771 0.0500.8359 0.01120.2008 0.00094.45  0.03106.45.310.07.4
 1400P*1.423.220.078175.9  14.90.821−69.4 2.41.162 0.0450.9667 0.01370.2074 0.00012.41  0.0116.88.014.812.2
 1800P*6.513.670.01136.5   3.10.573−20.4 0.51.094 0.0310.8895 0.00910.2273 0.00024.12  0.1980.710.48.99.8
HF/HCl residue from #44 (1.23 mg)
 30081.016.972.80 ∼0b16.618.74 0.24  0.1890 0.00021.185  0.0089.53.052.23∼0
 40067.912.265.450.95b23.9512.0 0.6  0.1891 0.00020.3540  0.00317.03.322.46∼0
 45046.012.376.300.69b18.338.82 0.87  0.1896 0.00010.1906  0.00394.653.462.76∼0
 50024.97.3323.41.06b14.954.62 0.46  0.1905 0.00020.6197  0.00841.023.242.972.3
 600165.924.99105.33.00b32.23−96.0 1.2  0.1902 0.00050.1815  0.00110.492.962.2621.0
 700138.822.47164.51.85b77.42−115.3 0.1  0.1910 0.00010.1412  0.00020.752.192.7939.0
 80080.816.4378.01.93b49.14−106.8 0.8  0.1910 0.00010.1969  0.00121.04.312.7815.6
 103076.410.6267.53.16b16.92−114.4 1.1  0.1916 0.00010.2165  0.00030.45.142.9821.9

He and Ne

For the bulk sample, He, Ne isotopic ratios clearly indicate that they are a mixture of trapped and cosmogenic components. Cosmogenic 3He has been estimated by assuming radiogenic 4He to be negligible, because the U content in ureilites is approximately 1 ppb (Higuchi et al. 1976), and taking (3He/4He)t = 1.4 × 10−4 and (3He/4He)c = 0.2. We calculated 3Hec (in units of 10−8 ccSTP g−1) of 23.2 and 12.6 for the bulk and acid residue, respectively.

In Ne three isotope plot the temperature steps of the bulk sample (Fig. 2) fall along the mixing line between cosmogenic and a trapped component (Ne-U). There is no solar Ne in AS as observed in some polymict ureilites (Ott et al. 1993). We calculated the cosmogenic ratio (22Ne/21Ne)c = 1.063 ± 0.017, by taking the trapped composition as Ne-U (20Ne/22Ne = 10.4 ± 0.3 and 21Ne/22Ne = 0.030 ± 0.005; Ott et al. 1985) and estimated the 21Nec = 9.08 × 10−8 ccSTP g−1 (listed in Table 2).

image

Figure 2.  Ne three isotope plot for the bulk sample of Almahata Sitta. Cosmogenic (Göbel et al. 1978), solar wind (SW) (Heber et al. 2009), Ne-U (Ott et al. 1985), and air (Ozima and Podosek 2002) values are also indicated.

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For the calculation of production rates, we used the average chemical composition of ureilites (arithmetic mean of six ureilites, as listed in Vdovykin 1976) (in wt%, Mg = 21.81; Al = 0.25; Si = 18.6; S = 0.33; Ca = 0.71; Fe = 14.62; Cr = 0.508; Ti = 0.07; Mn = 0.29; Ni = 0.131). We derived the production rates based on the empirical equation of Eugster and Michel (1995) for diogenites and for shielding depth indicated by the ratio (22Ne/21Ne)c = 1.063. The production rates (in units of 10−8 ccSTP g−1 Ma−1) obtained by us (P3 = 1.68; P21 = 0.567) and the exposure ages derived (T3 = 13.8 Ma; T21 = 16.0 Ma) are in the range of values reported by Welten et al. (2010) and Ott et al. (2010), within the uncertainties of ±15%. Exposure ages of ureilites span a range of values, with a low value of 0.1 Ma for ALH 78019 to a high value of 46.8 Ma for Elephant Moraine (EET) 83309, with no appreciable age clustering (Aylmer et al. 1990; Rai et al. 2003a). Exposure age of 16 Ma for AS is thus normal for an ureilite.

3He is mostly cosmogenic in AS. Release patterns of 3He and CO + CO2 are shown in Fig. 3 for both the bulk sample and the acid residue. 3He has a bimodal release in both cases. The main target element for 3He is pure carbon for the residue and the peaks at 600 and 1030 °C combustion steps correspond to diamond- and coarse-grained graphite. Assuming quantitative retention of 3He by both these phases, one can infer about equal proportion of diamond and graphite in the acid residue. The C release in the temperature intervals 500–700 and 800–1030 °C, where diamond and graphite usually combust (Rai et al. 2003a) further support this inference. Confocal Raman imaging spectroscopy has shown that the major C phase in AS is highly crystalline graphite (Ross et al. Forthcoming). Amorphous carbon combusts below 500 °C, wherein negligible pressure of CO + CO2 has been observed, suggesting that amorphous C is a minor phase in the residue. For the bulk sample, on the other hand, target elements for 3He are not limited to C. The low temperature (LT) peak of 3He could be mostly contributed by C phases while the high temperature (HT) release could be due to a mixture of C (protected in metal/silicate phases) and other phases like metal/silicate.

image

Figure 3.  Release pattern of 3He is shown along with C release (monitored as CO + CO2 pressure) for the bulk and residue samples of Almahata Sitta.

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Ar, Kr, and Xe

The data for the totals of Ar, Kr, and Xe are given in Table 1. The isotopic composition of Kr and Xe is similar in all temperature steps for both bulk and residue samples and grossly matches that of Kenna (Wilkening and Marti 1976) and will not be discussed in this study. The 38Ar/36Ar ratio for the total of the residue matches with the average ureilite value of 0.190 ± 0.001 derived from the values of acid residues of seven ureilites from Göbel et al. (1978). The higher value of 38Ar/36Ar of 0.1954 for the bulk sample is due to the cosmogenic Ar. Due to the large amount of trapped Ar (compared to cosmogenic Ar) even in the bulk sample, deriving the cosmogenic 38Ar will have a lot of uncertainty, so we do not use cosmogenic 38Ar for estimating the cosmic-ray exposure age of AS. The 40Ar/36Ar ratios of the bulk and residue are 6.8 and 0.2, while the lowest values measured are 2.4 (in 1400 °C pyrolysis) and 0.14 (700 °C combustion) in the bulk and residue, respectively. The higher values in the bulk are due to the in situ-produced radiogenic 40Ar from 40K decay. Compared to the lowest 40Ar/36Ar value of (2.9 ± 1.7) × 10−4 in the acid residue of Dayalpur (Göbel et al. 1978), the value for AS is high, even though the trapped Ar amount is very high. This may indicate either the presence of air contamination, K content in acid residue, or the presence of a K carrying phase that tagged along with C phases. But the elemental ratios of noble gases clearly rule out air contamination as the possible cause.

Nitrogen

Nitrogen data for the totals are given in Table 2 and for the individual temperature steps are compiled in Table 3. N release pattern, along with the C release (monitored as CO + CO2) for the bulk and the residue, is shown in Fig. 4. The bulk sample has 21.1 ppm N with δ15N of −36.8‰, with a peak release at 1000 °C, which also shows the lightest N composition (−94.3‰). Isotopic composition of N up to 600 °C has a signature of approximately +30‰, then reaches a minimum of −94.3‰ at 1000 °C and in subsequent steps shows an excursion between −20‰ and −69.4‰. The HF/HCl residue has an N content of 249.5 ppm with a composition of −74.3‰. There is a minor peak at 400 °C with +12‰, and a major release at 700 °C (−115‰) and about 26% N is released in the subsequent combustion steps at 800 and 1030 °C.

image

Figure 4.  Release patterns of C (monitored as CO + CO2 pressure), N, and δ15N (upper panels) and Ar, Kr, and Xe (lower panels) for the bulk and the residue samples of Almahata Sitta.

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Discussion

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

Cosmic-Ray Exposure Age

The agreement, within experimental uncertainties, between the exposure ages based on 3He and 21Ne suggests no 3He loss in #36. Also, our value of T21 = 16.0 ± 2.4 Ma for AS#36 is in agreement with the value of 14.1 Ma, as well as the average exposure age of 15 ± 2 Ma, from the four fragments #4, #36, #44, and #47 (Welten et al. 2010). The Bern plot of Welten et al. (2010) also does not indicate any 3He loss, although partial loss of 3He is indicated for #1 (Ott et al. 2010). Of the five fragments of AS (1, 4, 36, 44, and 47) for which exposure ages are determined, fragments #1 and #44 are from shallower depth, while #47 is more deeply shielded (based on their (22Ne/21Ne)c ratios). Considering that the size of AS has been estimated to be approximately 4 m in diameter, no complex exposure is indicated. It is surprising that an approximately 4 m object with a porosity of up to 55% survived for 16 Ma since breakup from its parent asteroid until fall on Earth.

Trapped Gases

One of the features of ureilites is the presence of large concentrations of trapped noble gases, and a principal N component with an isotopic signature of approximately −100‰, hosted in diamonds (Göbel et al. 1978; Rai et al. 2003a, 2003b). Additional N components have also been identified in the carriers amorphous carbon and graphite (Rai et al. 2003b) while it has been demonstrated that graphite is devoid of noble gases (Wacker 1986; Rai et al. 2002), although amorphous C may contain noble gases in some ureilites (Wacker 1986; Rai et al. 2002, 2003a). Noble gas abundances and release patterns in the ureilite AS show some feature not observed earlier and hold clues to the noble gas carriers and gas trapping process(es).

Release pattern of C phases has been monitored during combustion as pressure of CO + CO2 by the Convectron pressure gauge. During the subsequent pyrolysis of the bulk sample, there is also release of CO + CO2, due to oxidation of C that is entrapped within silicates, by the solid state reaction with the oxygen from silicates. In Fig. 5, the release patterns of C, N, Ar, Kr, and Xe from the acid residue for AS have been compared with those of typical monomict, polymict, and diamond-free ureilites and from the most reduced and low-shocked ALH 82130 having unusual noble gas elemental ratios, 36Ar/132Xe ∼ 19 and 13 for bulk sample and HF/HCl residue, as against the average values of ∼100 and ∼200, respectively (Rai et al. 2003a). A sharp release of N, Ar, Kr, and Xe, at the same temperature as peak C release also occurs, is seen for the typical monomict (Lewis Cliff [LEW] 85328) and polymict (EET 87720) ureilites. ALH 78019 has peak release (∼90%) of noble gases and approximately 50% N at 400 °C (wherein a negligible amount of C is released), while most C release occurs at >600 °C, with a negligible amount of noble gases, but approximately 40% N. Studies of ALH 78019 have given a clear indication that its major C phase (crystalline graphite) contains no detectable noble gases and the minor C phase (amorphous carbon) is the principal noble gas carrier (Rai et al. 2002). The C release pattern of AS residue shows about 50% release in the 500–700 °C interval and the remaining 50% combusting at 800–1030 °C steps and carrying approximately 40% of Ar, Kr, Xe, and approximately 25% N in the later two steps. AS release pattern has features that partly resemble those from ALH 78019, EET 87720, and other monomict ureilites.

image

Figure 5.  Release patterns of C (monitored as CO + CO2 pressure), N, Ar, Kr, and Xe for the residue sample of Almahata Sitta are compared with those from Allan Hills (ALH) 78019 (diamond-free ureilite), Lewis Cliff (LEW) 85328 (typical monomict ureilite), Elephant Moraine (EET) 87720 (typical polymict ureilite), and ALH 82130 (low-shocked ureilite showing Xe enrichment), data for which is taken from Rai et al. (2003a).

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Elemental Ratios

The elemental ratios of trapped noble gases in ureilites span a considerable range of values for 132Xe/36Ar and 84Kr/36Ar, but in a plot of 132Xe/36Ar versus 84Kr/36Ar they mostly plot along the mass fractionation line (MFL), with very few exceptions, such as in the cases of ALH 82130 and EET 83309 (Ott et al. 1993; Rai et al. 2003a), indicative of a relative enrichment of 132Xe in these two ureilites. In Fig. 6, the ratios of 36Ar/132Xe and 84Kr/132Xe for different temperature fractions of AS bulk and residue have been compared. For the AS residue, the 36Ar/132Xe ratios fall within the average ureilite range and the 84Kr/132Xe ratios also match the average ureilites range within a factor of two. For the AS bulk sample, on the other hand, both the ratios 36Ar/132Xe and 84Kr/132Xe are lower by factors of about 10 and 5, respectively, relative to the average, indicating enrichment of heavier noble gases. The elemental ratios for the totals of AS bulk and AS residue are plotted in Fig. 7, wherein the MFL, along which most of the data of bulk and acid residues of ureilites fall (see fig. 12, Rai et al. 2003a), is also shown. While the point AS bulk falls above the MFL, the point AS residue falls along the line. The deviation of AS bulk from MFL can be explained by enrichment of 132Xe. Similar Xe enrichment has been observed in the monomict ureilite ALH 82130 (bulk and acid residue) and polymict ureilite EET 83309. Data for both these ureilites are also shown in Fig. 7. The different features in the bulk and residue samples suggest the loss of a phase during acid dissolution from the bulk sample, where heavier noble gases are enriched. A similar feature has been observed in the bulk samples of ALH 82130 (monomict) and EET 83309 (polymict) (Ott et al. 1993; Rai et al. 2003a), but there are differences in the acid resistance of the heavy noble gases-enriched phase(s) as well as in enrichments in these three ureilites. ALH 82130 has only Xe enrichment in the bulk which survived the HF/HCl treatment and to some extent, even after treatment with HClO4 (Rai et al. 2003a). EET 83309 showed enrichments in both 84Kr and 132Xe, which survived the HF/HCl treatment, but on further treatment with HNO3, the Xe enrichment vanished and the 84Kr enrichment still survived.

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Figure 6.  Elemental ratios 36Ar/132Xe and 84Kr/132Xe at different extraction temperatures for the bulk and residue of Almahata Sitta are compared.

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image

Figure 7.  The elemental ratios 132Xe/36Ar and 84Kr/36Ar are plotted against each other. These ratios vary by more than two orders of magnitude among ureilites, but normally fall along the mass fractionation trend (shown as the line mass fractionation line [MFL], taken from fig. 12 of Rai et al. 2003a). The data of Almahata Sitta (AS) residue fall close to MFL, while the data for AS bulk fall much above the MFL line, indicating excess 132Xe in AS bulk. Similar Xe excess has been earlier observed in the case of ureilite Allan Hills 82130 (data shown are from Ott et al. 1986 [for A30-B2] and Rai et al. 2003a [for A30-B1, A30-Res]).

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Another important difference in these three cases is in the variation of the elemental ratios with temperature. There is a general increase of the ratios 36Ar/132Xe and 36Ar/84Kr with extraction temperature, during combustion release from C-rich residues for ALH 82130 and EET 83309 (Rai et al. 2003a), whereas the ratios are remarkably uniform at all temperatures, in the case of AS residue (see Fig. 6). This clearly suggests the presence of at least two different carrier phases for noble gases in AS: one is an HF/HCl resistant C phase having the elemental ratios similar to the average values found among other ureilites, while the other phase is either acid soluble or lost as a colloid during the process of preparing the residue, and having relatively higher (by an order of magnitude) Xe. It is possible that the Xe enrichment is a result of partial loss of Ar and Kr, from a labile phase in a heating event. This could have happened in the breakup event leading to the ejection of AS meteoroid from the parent asteroid.

Nitrogen Components and Carriers

The bulk sample shows at least three isotopic components (see Fig. 4): a component with δ15N of 31.8‰ released in the LT (300–600 °C) interval, a lighter N component having <−94‰, with peak release at 1000 °C, and an admixture of this light component with a component heavier than −20‰ in the temperature fractions 1200–1800 °C. The slightly lower δ15N values for the 300 and 600 °C fractions (compared to the 400 °C fraction) is due to a small amount of adsorbed air and a small release of the more tightly bound lighter N, respectively. Such heavy N has been observed in other ureilites as well (Rai et al. 2003b) and has been identified with amorphous C, having a lower combustion temperature (Rai et al. 2002). The peak release at the 1000 °C combustion, with the lightest composition is due to the diamond, which has a δ15N of −115‰, as observed in both monomict and polymict ureilites (Rai et al. 2003b). In the subsequent pyrolysis steps, the diamonds that are protected inside silicates and metal and hence not accessible to the external oxygen, and another metal/silicate phase, carrying N with δ15N ≥−20‰ are coreleased, resulting in the excursions in the observed N isotopic structure. This metal/silicate phase with δ15N ≥−20‰ could be the clast material observed in AS (Zolensky et al. 2010) and other polymict ureilites (Mittlefehldt et al. 1998) and attributed as the possible carrier of heavy N in polymict ureilites (Grady and Pillinger 1988).

The N release from the HF/HCl residue is bimodal with a minor peak at 400 °C (with δ15N ∼ 12‰) and a major peak at 700 °C (with δ15N ∼ −115‰). Slightly lower δ15N values in the 300, 450, and 500 °C fractions, as compared to the +12‰ value in the 400 °C fraction are due to a small amount of adsorbed air in the 300 °C fraction and a small release of light N in the 450 and 500 °C fractions, respectively. Although both bulk and residue samples show light N with −115‰ as a major N component, consistent with the expected combustion/pyrolysis steps for diamond to be the carrier, the LT steps in bulk and residue show heavy δ15N that differ by approximately 20‰. This cannot be attributed to air contamination in the LT steps of residue, as the corresponding 40Ar/36Ar ratios are ≤1. The N/C ratios of the LT fractions (≤3 × 10−3) rule out organic contamination during sample preparation. The N/C of the 300–600 °C fractions of the bulk sample is 0.0456 (assuming 4% C in the analyzed sample). If the LT N release in the residue is also represented in the bulk, an additional noncarbonaceous, acid soluble N phase with δ15N > 30‰ is needed in the bulk sample. The elemental ratio N/36Ar in the LT of residue and bulk are 7 × 103 and 588 × 103, respectively, clearly indicating the possibility of a noncarbonaceous N carrier, considering the fact that most trapped noble gases are hosted by C phases. The numerous clasts observed in all three major lithologies of AS (Zolensky et al. 2010) could be the host phase of this heavy N carrier.

Argon Isotopic Variations

Argon in the residue is purely of trapped origin for the isotopes 36Ar and 38Ar, while 40Ar could have a small radiogenic contribution, depending on the K abundance. The isotopic ratio 38Ar/36Ar is expected to be representative of the trapped component only, except for fractionation effects due to possible loss processes or acquisition mechanism. The ratio for the total of the residue 0.1908 ± 0.0002 matches with the average value for ureilite residues 0.190 ± 0.001 (average value of seven ureilite residues, taken from Göbel et al. 1978). However, the ratios for the temperature steps show a monotonic increase from 0.1890 (in 300 °C fraction) up to 0.1916 (in 1030 °C fraction). Figure 8 shows the trend of 38Ar/36Ar ratio against release temperature. There is almost a linear increase of the isotopic ratio with temperature. Although the total value falls within the average ureilite range, the values at HTs clearly are above this range. It is possible that the observed trend is an artifact of the correction employed for the pressure dependence on the 38Ar/36Ar ratio observed in our earlier work (Rai et al. 2003a), but this can be ruled out by the following observation. In the same plot, the 36Ar amount (the most abundant isotope in the ureilite Ar) in each temperature fraction is also shown. The peak amount of 36Ar and the peak value of 38Ar/36Ar do not show a parallel. If the observed variation in the 38Ar/36Ar ratio is a consequence of the pressure of Ar in the mass spectrometer, a parallel behavior is expected, safely ruling out the pressure effect as the plausible cause of this variation. Also, our earlier observation is a lowering of the 38Ar/36Ar ratio as the pressure of Ar increases and not as observed in the release pattern.

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Figure 8.  The variation of the isotopic ratio 38Ar/36Ar with extraction temperature is shown for the residue sample of Almahata Sitta (AS). The amount of 36Ar released in each temperature fraction is also shown. Average value of 38Ar/36Ar for ureilite residues (Göbel et al. 1978) and the value for the total of AS residue are also shown.

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This isotopic variation can be due to two possible reasons related to either release process or due to incorporation process. The observed isotopic change could be due to diffusive fractionation from a uniform reservoir, with progressive enrichment in the heavy isotope at higher temperature fraction that is released later. However, a very low diffusion coefficient for diamond (Göbel et al. 1978; Ozima 1989), the principal noble gas carrier, at the temperatures of interest rules out this possibility. An increase of the elemental ratios 36Ar/132Xe and 36Ar/84Kr as the temperature of combustion increases has been interpreted as due to noble gas incorporation into diamonds by ion implantation (Rai et al. 2003a). This process, resulting in a change of the elemental ratios by more than an order of magnitude is expected to result in a measurable change in isotopic ratio of at least the lighter element Ar. The isotopic changes in the 38Ar/36Ar ratios observed in AS residue will be in accordance with the expectation of heavy Ar isotope enrichment. But the magnitude of change (1.37%) in the Ar isotopic ratio is expected to result in an elemental ratio change of more than two orders of magnitude in 36Ar/132Xe, while the observed elemental ratios (335–448) are surprisingly very uniform (within 30%) in all temperature fractions. The above two possibilities can hence be ruled out.

The observed variations could be due to a two component mixing, where two reservoirs of trapped Argon are present with normal (average ureilite composition) and with higher 38Ar/36Ar ratio, the latter being sited in the phase with HT release. Figure 9 is a plot of 38Ar/36Ar versus the corresponding δ15N for the temperature steps of residue sample. The data shown in Fig. 9 fall into two clusters. One of the clusters corresponds to LT fractions with normal 38Ar/36Ar and heavy δ15N and the second one with higher 38Ar/36Ar and light δ15N. As already discussed we can use the N compositions to identify the LT and HT phases as amorphous C and diamond, respectively. The question then is why Ar composition is enriched in the HT (diamond) component, while the δ15N agrees with the composition found for diamonds in other ureilites.

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Figure 9. 38Ar/36Ar versus δ15N for the individual temperature steps of Almahata Sitta residue. The low temperature (LT) and high temperature (HT) data cluster into two fields, indicating the presence of two different components.

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Noble Gas Carriers

Earlier work has shown that the principal noble gas carriers in ureilites are amorphous C and diamond (Göbel et al. 1978; Wacker 1986; Rai et al. 2003a), while graphite is devoid of noble gases (Rai et al. 2002). An additional noble gas carrier, probably kamacite has been suspected (Göbel et al. 1978). There have been hints of graphite being a noble gas carrier in ALH 78019 (Okazaki et al. 2003). The largest concentration of noble gases has so far been observed in the Dayalpur residue B-2-1, at 78.3 × 10−5 ccSTP 36Ar g−1 (Göbel et al. 1978). The AS residue with a 36Ar amount of 45.3 × 10−5 ccSTP g−1 ranks second, among the data available on ureilites. Compared to the bulk sample of AS (represented here by fragment #36), the residue is enriched by factors of 1043, 529, and 119, respectively in trapped 36Ar, 84Kr, and 132Xe. This nonuniform enrichment is due to the presence of a phase in the bulk sample (of AS#44) with enrichments in 84Kr and 132Xe, compared to the residue. We did not measure the bulk sample AS#44. Welten et al. (2010) have reported 36Ar amount (in ccSTP g−1 units) in both AS#36 (13.1 × 10−8) and AS#44 (1390 × 10−8), and they differ by about a factor of 100. Our value of 36Ar in AS#36 is high by a factor of 3.3, compared to Welten et al. (2010) value. Further, the reported concentrations of Ar, Kr, and Xe in AS#1 and AS#47 also show a factor of five variations (Ott et al. 2010). These variations are due to the heterogeneous distribution of the carrier phases of trapped noble gases in the few milligrams range of sample sizes being analyzed. Ar, Kr, and Xe are dominated by the trapped component and mainly carried by the diamond phase, as indicated by the co-release of C, Ar, Kr, and Xe at the 700 °C combustion step of the residue and the accompanying δ15N (see Fig. 4). But about 40% C release along with the accompanied noble gases has occurred in the 800 and 1030 °C combustion steps for the residue (see Fig. 4). While most of the C release in these later two steps is likely contributed by the noble gas-poor coarse-grained graphite (see SEM pictures of residue in Fig. 1) (Rai et al. 2002), the noble gases must have been contributed by the coarse (∼μm)-sized diamonds which combust at higher temperature. The oxygen made available during combustion is enough to completely oxidize the total C in the sample. Hence, the combustion reaction is not O2 limited. Rather, it is reaction rate limited, due to the fact that coarser diamond takes a longer time to get oxidized due to the smaller surface area. This results in the release of gases from coarser diamonds in higher temperature combustions. The accompanying δ15N, with typical −115‰ signature of diamonds further affirms the diamond origin for the noble gases released in the 800 and 1030 °C fractions. SEM pictures of the residue clearly show large graphite crystals (dark gray crystals showing layering) and more than one size range of much smaller diamond crystals (whitish colored crystals) and both these phases have been coated with amorphous C. The coarse-grained graphite might be combusting at >700 °C, and hence may overlap with the region where the coarse diamonds (around a few microns in size) are combusting and releasing their trapped noble gases and nitrogen, explaining the continued release of N and Ar, Kr, Xe at 800 and 1030 °C combustion steps. Diamond aggregates ranging up to several micrometers in dimension, contained within graphite grains have been identified by confocal Raman imaging spectroscopy (Zolensky et al. 2010). Although it is not possible to estimate the relative proportions of graphite and the diamond aggregates (of several microns size) contained within the graphite, visual indications from SEM pictures suggest graphite to be the predominant C phase combusting in the 800–1030 °C interval. As graphite is gas poor, to account for the approximately 40% release of Ar, Kr, Xe by the coarse-sized diamonds necessarily implies a larger concentration of noble gases in the coarse diamonds (as compared to the submicron diamonds), predominantly releasing their gases in the 600–700 °C combustion steps.

Noble Gas Trapping Mechanism

Ureilite noble gases are highly fractionated in favor of heavy elements with respect to the solar component. Despite orders of magnitude variations in the absolute amounts in several ureilites, their isotopic composition is similar (Göbel et al. 1978; Rai et al. 2003a) and closely matches with “Q component” (Busemann et al. 2000). Amorphous carbon and diamond have been shown to be the carrier phases of noble gases in ureilites. The possible trapping mechanisms into these C phases to produce the observed elemental pattern have been thoroughly discussed in the literature (Rai et al. 2003a and references therein) and will not be repeated here. Rayleigh distillation (Ozima et al. 1998), solubility or physical adsorption (Wacker 1989), and ion implantation from nebular plasma (Göbel et al. 1978; Matsuda et al. 1991; Matsuda and Yoshida 2001; Rai et al. 2003a) are the processes considered. Rai et al. (2003a) have favored the ion implantation mechanism from nebular plasma based on the observed change in the elemental ratios 132Xe/36Ar and 84Kr/36Ar with depth in the diamond grains and its match with the ion implantation simulation (Ziegler et al. 1985). Artificial ion implantation experiments into synthetic nanodiamonds also favored this mechanism for gas acquisition by presolar diamonds (Koscheev et al. 2001). The uniform elemental ratios at all combustion steps for the AS residue and the variation of the isotopic ratio 38Ar/36Ar require a different mechanism or a subsequent alteration to explain these observations. The elemental and isotopic ratios of heavy noble gases in AS may hold clues in providing information on the noble gas trapping in C phases of ureilites, but more work is needed to decipher this information.

Conclusions

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

Cosmogenic and trapped noble gases and nitrogen have been investigated in a bulk sample and an acid-resistant C-rich residue of the anomalous ureilite AS. He and Ne are a mixture of trapped and cosmogenic components. The cosmic-ray exposure age based on 21Ne (16 Ma) is in the range of values observed for other ureilites.

The uniform elemental ratios at all combustion temperatures for the residue and the increase in the ratio of 38Ar/36Ar are contrary to the expectations of noble gas acquisition by diamonds through ion implantation mechanism from hot plasma (Rai et al. 2003a). Argon isotopic variation can be best explained by a two component mixing, as also indicated by the accompanying N isotope composition. But at present there is no satisfactory mechanism to explain how Ar becomes enriched in heavy isotope in the component released at HT.

Earlier studies have shown the presence of solar Ne in some polymict ureilites (Ott et al. 1993) while trapped Ne in AS is Ne-U. Some polymict ureilites have also been observed to host a heavy N component, with δ15N up to +600‰ (Grady and Pillinger 1988) and also containing an amorphous C phase having δ15N ≥50‰ and carrying noble gases. Such an amorphous C phase is also missing in AS. The polymict ureilite EET 83309 has a C phase (probably a gas-bearing graphite or cohenite) carrying noble gases and N with δ15N ≥153‰, and combusting at <700 °C (Rai et al. 2003b). AS does not have this type of C phase either.

Two features found in AS#36 and #44, the uniform elemental ratios in the residue at all release temperatures and the increase in the isotopic ratio 38Ar/36Ar with increasing extraction temperature have not been found in any other ureilite. This makes AS a truly anomalous polymict ureilite. Some of these anomalous features could be primary (nebular/parent body related) and some could be due to atmospheric ablation and consequent loss/gain of noble gases. The impact of asteroid 2008 TC3 was accompanied by three intense detonations at 45–35 km altitude, with meteoric plasma reaching temperatures of approximately 3650 K (Borovi✓ka and Charvát 2009). Considering the fact that most of the recovered AS pieces are small (<379 g), and in particular, the residue has been prepared from small fragments (#44), it is possible that some of the results may be partially affected by gas loss/exchange during the fragmentation and fireball flares. Further investigations will be needed to ascertain the primary/ablation causes and their implications to the noble gas incorporation into the C phases of ureilites.

Acknowledgments

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

Acknowledgments— Suruchi Goel has helped in the preparation of the acid residue. Dipak Panda and Suruchi Goel are thanked for the SEM work. Students and staff of the University of Khartoum collected the meteorites. We thank R. Okazaki and Rainer Wieler for critical reviews and the AE, A. J. T. Jull for useful comments. Discussions with U. Ott have been helpful in improving the presentation. S. V. S. M. thanks the Department of Space, Government of India for the financial support. P. J. is supported by a NASA Planetary Astronomy grant.

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  2. Abstract
  3. Introduction
  4. Sample Preparation
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. Acknowledgments
  10. References
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