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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Experimental and Raman Spectra
  6. Results and Discussion
  7. Summary
  8. References

Abstract– We performed micro-Raman spectroscopic analyses of the carbon vein in five ureilites: Allan Hills (ALH) A77257, Northwest Africa (NWA) 3140, Shişr 007, Yamato 790981 (Y-790981), and Yamato 791538 (Y-791538). The graphite peaks showed that the graphite structure in ureilite is well developed, especially compared with the carbonaceous material in carbonaceous chondrite. The domain sizes of the graphite were 45–110 Å. We observed shifts in the diamond peak positions to higher wave numbers with a large full width at half maximum (FWHM), especially for NWA 3140. Although the FWHM of a diamond peak is not a crucial diagnostic test for a chemical vapor deposition (CVD) origin of diamond, the shift of the diamond peaks to higher wave numbers could be a strong indicator that supports the CVD origin as these shifts have only been observed in CVD diamonds. We discuss the origin of diamond from various aspects, and confirm that the CVD model is the most plausible. We conclude that all carbon material (graphite, amorphous carbon, diamond, etc.) condensed on the early condensates in the primitive solar nebula.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Experimental and Raman Spectra
  6. Results and Discussion
  7. Summary
  8. References

Ureilites are a unique group of olivine-pyroxene achondrite that contain a carbonaceous matrix. Their silicates are coarse-grained (millimeter-sized) and considered to be either cumulates from ultramafic magmas or partial melt residue (Goodrich 1992; Mittlefehldt 2004). The boundaries of these silicate minerals are filled with carbonaceous veins (about 2–6 wt.%) that are graphite, diamond, metallic iron, and troilite (Grady et al. 1985). Ureilites have a high concentration of carbon compared with carbonaceous chondrites, and the presence of large (micrometer-sized) diamond (Miyamoto et al. 1993) is one of their characteristic features (Vdovykin 1970). An interesting and puzzling feature is that the silicate minerals clearly show igneous texture while the oxygen isotopes indicate a primitive nebula process. The oxygen isotopic data lie on a line having a slope close to 1 in a three-isotope plot, which differ from the data observed for magmatically differentiated achondrites (Clayton and Mayeda 1988). The carbonaceous vein also contains large amounts of planetary-type noble gas in which heavy noble gasses are enriched compared with the solar pattern (Weber et al. 1971, 1976). In carbonaceous chondrites, the planetary noble gas is carried by phase Q (Lewis et al. 1975), and noble gas in ureilites is very similar to Q gas (Göbel et al. 1978). Phase Q may be a very tiny phase in meteorites, or the release from Phase Q may result from the rearrangement of the carbon phase into which gasses were implanted (Ott et al. 1981; Ott 2002; Matsuda et al. 2010b).

Many stimulating discussions have occurred regarding the origin of diamond in ureilites. Three origins have been proposed: “hydrostatic pressure” (Urey 1956; Ringwood 1960; Carter and Kennedy 1964), “impact shock” (Lipschutz 1964; Anders and Lipschutz 1966), and “chemical vapor deposition (CVD)” (Fukunaga et al. 1987; Matsuda et al. 1991, 1995). In the “hydrostatic pressure” model, diamond was produced under high pressure and temperature inside the ureilite parent body. In the “impact shock” model, diamond formed under high pressure upon impact with the earth or during breakup of the parent body. The “CVD” model considers that diamond formed under a plasma condition in the primitive solar nebula. The first hypothesis is now disregarded as the diamond has a preferred orientation and there is the presence of lonsdaleite, a high-pressure type diamond with a hexagonal structure. Because the hydrostatic pressure is not uniaxial it cannot produce a preferred orientation; also, it is difficult to achieve a sufficiently high pressure to produce lonsdaleite inside the parent body. Thus, there remain two main hypotheses for the origin of diamonds in ureilites, “impact shock” or “CVD.”

Elemental and isotopic studies of noble gasses (Matsuda et al. 1991, 1995) and nitrogen (Rai et al. 2002, 2003a) clearly support the CVD origin. The trapping efficiency and patterns of noble gasses (Matsuda et al. 1991, 1995) and the large difference between the nitrogen isotopic ratios (Rai et al. 2002, 2003) for diamond and for other carbon phases in ureilite are not compatible with the in situ conversion of graphite or amorphous carbon into diamond. However, mineralogical observations of compressed graphite in the vicinity of diamond seem to support an impact shock origin (Nakamuta and Aoki 2000; Hezel et al. 2008).

Raman spectroscopy is a useful and nondestructive tool for examining the nature of carbon material, and it has been applied to carbon material in ureilite (Miyamoto et al. 1988, 1993; Kagi et al. 1991, 1994). Recently, several studies were conducted that focused in particular on diamond in new ureilites (Hezel et al. 2008; Le Guillou et al. 2010; Ross et al. 2011). However, a Raman spectroscopic study of graphite should also be very important. Therefore, we conducted a Raman spectroscopic study to obtain information on the formation process of diamond and the nature of graphite in ureilites.

Samples

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Experimental and Raman Spectra
  6. Results and Discussion
  7. Summary
  8. References

We examined five ureilites in this study: Allan Hills A77257 (ALHA77257), Shişr 007, Northwest Africa 3140 (NWA 3140), Yamato 790981 (Y-790981), and Yamato 791538 (Y-791538). ALHA77257, Y-790981, and Y-791538 are Antarctic meteorites. Shişr 007 was found in Dhofar, Oman, in 2001 (Russell et al. 2002), and NWA 3140 was found in Morocco in 2004 (Russell et al. 2005). Mineralogical and crystallographic studies of pyroxene in ALHA77257, Y-790981, and Y-791538 have been widely performed (Takeda 1987, 1989; Tribaudino 2006; Mikouchi et al. 2010). Y-791538 contains orthopyroxene as its main component (Takeda 1989), which is very different from typical ureilite. The rim of silicate materials is reduced in all samples and there are metallic substances in the matrix. Shişr 007 had not been examined previously using Raman spectroscopy, but Raman data for diamond and graphite in the other four ureilites are given in the literature (Miyamoto et al. 1988, 1993; Kagi et al. 1991, 1994; Karczemska et al. 2009; Ross et al. 2011). The nitrogen and noble gas isotopic compositions and carbon abundance of ALHA77257 and Y-791538 are provided by Yamamoto et al. (1998).

Goodrich (1992) classified ureilites into four grades based on shock level: very low (shock pressures <0.2 Mb), low, medium, and high (shock pressures >1 Mb). They classified Y-791538, ALHA77257, and Y-790981 as low?, low, and medium, respectively. Cloutis et al. (2010) classified Y-791538 and ALHA77257 as low-medium and low, respectively. Russell et al. (2002) reported that Shişr 007 experienced moderate shock. The shock level of NWA 3140 is unknown.

In the present study, first we prepared four types of samples (bulk, acid residue, etched residue, and thin section) of Y-791538, Shişr 007, and NWA 3140 for the Raman study. The “bulk” sample was simply clashed by a hammer. The “acid residue” sample was treated with a cycle of 3 M HCl and 1 M HCl-10 M HF to remove its silicate fraction, in the same manner of Lewis et al. (1975). Starting weights were 0.4–1 g, and repeated the cycle until the solution did not appear yellow anymore. The obtained residues were 3.5% (Y-791538), 2.9% (Shişr 007), and 2.0% (NWA 3140), which are roughly compatible to the carbon content of ureilites. The “etched residue” sample was treated by H2O2 to remove damaged graphite by weak oxidation. The “thin section” sample was polished, without diamond paste to avoid contaminating the diamond (Kagi et al. 1991). We obtained Raman data for these four types of samples, and finally concluded that the thin section is the best sample in the Raman study for the study of diamond and graphite. Ureilites contain large crystals of silicate materials, and the size of diamond in ureilites is probably a few micrometer or less (Vdovykin 1970; Miyamoto et al. 1993). Thus, it is difficult to obtain the spectra of diamonds from the bulk sample. For the acid residue and the etched residue, whole carbonaceous veins blur into each other, and large peaks of graphite bands cover the small diamond band. Thus, the chemical residues make it difficult to resolve the diamond peaks. Furthermore, we are concerned that the chemical treatment may affect the carbon structure as in the case of carbonaceous chondrite (Matsuda et al. 2010b). As a result, we prepared thin sections for all five ureilites to collect the Raman data.

Experimental and Raman Spectra

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Experimental and Raman Spectra
  6. Results and Discussion
  7. Summary
  8. References

Raman spectra were obtained with a Kaiser HoloLab 5000 micro-Raman spectrometer (manufactured by Kaiser Optical Systems, Inc.) equipped with a 532 nm YAG laser, holographic transmission grating, and a charge coupled device (CCD) detector. We used a laser power of 0.5–1.5 mW at the sample surface and obtained the Raman spectrum using 10 accumulations of 20 s each. The excitation laser spot size was approximately 2 μm in diameter. We obtained a typical Raman spectrum of diamond (the diamond band appeared at about 1332 cm−1) and graphitic carbon (the G band appeared at about 1580 cm−1 and the D band appeared about 1350 cm−1) from all samples. The spectra were accompanied by chevron shape fluorescence backgrounds. We subtracted the fluorescence with a multipoint baseline and adjusted the Lorentzian profile curves in the 1100–1800 cm−1 range. The peak position, intensities, and full width at half maximum (FWHM) of the three bands were obtained from these curve fits to define the spectral parameters characterizing diamond and graphite. It has been reported that in the case of carbonaceous chondrites the high laser power changed the sample into a more amorphous state (Matsuda et al. 2009; Morishita et al. 2011). However, an energy of 0.5–1.5 mW does not have a large effect on the Raman parameters. As we will see later, the carbon in ureilite appears as well-ordered graphite and diamond, and the heating due to laser irradiation is generally small for such low energy.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Experimental and Raman Spectra
  6. Results and Discussion
  7. Summary
  8. References

We obtained diamond bands and G and D bands of graphite from five ureilites. The diamond band is obtained either solely or together with graphite peaks. Figure 1 shows the typical shape of the Raman spectrum for each ureilite. NWA 3140 and Y-791538 have the lowest intensity diamond bands. Other than the diamond and graphite bands, we occasionally obtained a small peak around 1315 cm−1. It is most likely that this peak is due to lonsdaleite that is hexagonal diamond and one of the polymorphs of diamond and graphite. It has been reported that the FWHM of the lonsdaleite peak is about 5 times wider than that of the diamond band; also, the intensity of the peak is about 500 times less than that of the diamond band and is located around 1324 cm−1 (Smith and Godard 2009). Nevertheless, peaks similar to ours have been detected also by Gogotsi et al. (1998) and Hu et al. (2009), who determined that the peak comes from lonsdaleite. Therefore, we believe that the peak we obtained around 1315 cm−1 indeed originates from lonsdaleite.

image

Figure 1.  Typical Raman spectra (after baseline correction) for thin sections of five ureilites: Alan Hills A77257 (ALHA77257), Shişr 007, Northwest Africa 3140 (NWA 3140), Yamato 790981 (Y-790981), and Yamato 791538 (Y-791538). The x-axis shows wave numbers and the y-axis shows spectrum intensity. All five spectra have a diamond band around 1332 cm−1, a graphite G band around 1580 cm−1, and a graphite D band around 1350 cm−1.

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Graphite Band

The data for the G and D bands of graphite are shown in Fig. 2 and summarized in Table 1. Many G band positions are around 1580 cm−1 (Table 1), although some scatter toward higher positions is observed (Fig. 2a). This is very different from the data observed for carbonaceous meteorites (Bonal et al. 2006; Busemann et al. 2007; Matsuda et al. 2010), for which the peak positions of the G bands at the higher wave numbers range from 1580 to 1605 cm−1 and are inversely correlated with FWHMG (FWHM of G band) (Busemann et al. 2007). A beautiful inverse correlation was obtained even in a single meteorite, the C3V Allende chondrite (Matsuda et al. 2010b). In addition, FWHMG of carbonaceous chondrites ranges from 50 to 100 cm−1 and is much larger than that of ureilite, for which FWHMG ranges from 20 to 50 cm−1 (Table 1). The higher shift in the G band in carbonaceous chondrites compares to the normal G band position of graphite (1580 cm−1), and an inverse correlation with FWHMG shows that the carbon material in carbonaceous chondrite is poorly graphitized carbon to amorphous carbon (Matsuda et al. 2010b). According to Ferrari and Robertson (2000), the G band position of graphite at about 1580 cm−1 shifts higher to 1600 cm−1 when it evolves to nanocrystalline graphite, and then shifts even lower to 1520 cm−1 when it changes to amorphous carbon. The G band position at around 1580 cm−1 and the small values of FWHMG for ureilites suggest that the graphite in ureilites is well ordered. The downward shift trend for the G band position from 1580 cm−1 with increasing FWHMG of carbonaceous materials in carbonaceous chondrites indicates that they are in a more amorphous state (Matsuda et al. 2010b).

image

Figure 2.  Raman spectra data of G and D band of graphite. (a) FWHMG (full width at half maximum for G band) versus peak position of the G band, (b) FWHMD (full width at half maximum for D band) versus peak position of the D band, and (c) ID/IG (peak intensity ratio of D band to G band) versus peak position of the G band.

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Table 1. Average values of Raman data for G and D bands of graphite in ureilites.
SamplePeak center (G band) (cm−1)FWHMG (cm−1)Peak center (D band) (cm−1)FWHMD (cm−1) ID/IG
  1. *Number of analyses.

ALHA772571585 ± 525 ± 91358 ± 1478 ± 451.1 ± 1.2
 (27*) (27*)  
NWA 31401584 ± 424 ± 91355 ± 849 ± 320.50 ± 0.14
 (71*) (67*)  
Shişr 0071583 ± 338 ± 141356 ± 1060 ± 380.66 ± 0.27
 (47*) (28*)  
Y-7909811587 ± 630 ± 141362 ± 1477 ± 540.80 ± 0.81
 (54*) (38*)  
Y-7915381581 ± 123 ± 61355 ± 758 ± 220.44 ± 0.26
 (41*) (17*)  

The diagram of FWHMD (FWHM of D band) against the peak position of the D band (Fig. 2b) shows a very rough positive correlation. The D band positions of ureilites are high compared with those in carbonaceous chondrites (Bonal et al. 2006; Busemann et al. 2007; Matsuda et al. 2010), but the FWHMD in ureilites are small (20–130 cm−1 from Table 1) compared with those in carbonaceous chondrites (60–350 cm−1). Again, this indicates that the graphitic structure is more developed in ureilites.

The intensity ratio of the D and G bands (ID/IG) is plotted against G band position in Fig. 2c. It is known that the D and G band intensity ratio is a good indicator of the degree of graphitization (Tuistra and Koenig 1970). Note, however, that the behavior of the D band is complex. The D band intensity is very localized at the graphite surface, especially where the crystalline structure is not perfect even though the G band intensity is uniform over the entire graphite (Pimenta et al. 2007). In line with this, Sadezky et al. (2005) states that the D band intensity may not be an indicator for graphitization, especially in soot, while Zickler et al. (2006) showed that the relationship between the crystallite size and the ID/IG ratio breaks down for crystallite sizes below 2 nm. However, we consider the relationship to still be useful as the graphite in ureilites is well developed judging from Raman spectroscopy.

The crystallite size La (graphite domain dimension, Å) is determined from the following relationship with the ID/IG ratio (Tuistra and Koenig 1970; Vidano et al. 1981).

La = CL(λL) × (ID/IG)−1 (1)

where λL is the wavelength of the excitation laser and CL(λL) is a factor depending on the excitation laser wavelength, given as follows.

CL(λL) = C0 + λL × C1 (2)

Dresselhaus et al. (2000) gave the values C0 = −126 Å and C1 = 0.033 for the VIS region 400 nm < λL < 700 nm. The obtained ID/IG ratios for ureilites are 0.44–1.1 (Table 1), which then corresponds to sizes between 45 to 110 Å (45 ± 49, 99 ± 28, 75 ± 31, 62 ± 63, and 113 ± 67 Å, respectively, for five ureilites in Table 1). The obtained values are surely larger than the lower limit of the crystallite size (20 Å) for the relationship indicated by Zickler et al. (2006). Kagi et al. (1991) reported 70–180 Å for the domain size of graphite, and our values are roughly identical considering the large variations (about 100%) in the ID/IG ratios (Table 1).

Diamond Band

The diamond peak positions and their FWHM are shown in Fig. 3 and are summarized in Table 2. Most peak positions are located at 1332 cm−1 (normal diamond), the exception being NWA 3140, for which some peak positions are shifted to higher wave numbers (Fig. 3). The data positions for Y-791538 and Y-790981 are very similar to those obtained by Miyamoto et al. (1993). Shişr 007, Y-791538, and Y-790981 seem to have a narrow range of peak positions and small FWHM (Fig. 3; Table 2). Interestingly, NWA 3140 has peak positions at higher wave numbers with large FWHM. For NWA 3140, Ross et al. (2011) reported an average peak position at 1332.2 cm−1 with an FWHM of 8.0 cm−1. These values are quite different from our data (1336.1 cm−1 for the peak position and an FWHM of 33 cm−1) and show that NWA 3140 is normal. Karczemska et al. (2009) also conducted a Raman study of NWA 3140, and reported the presence of diamond having a high wave number around 1337 cm−1 and large FWHM in some places, although normal peak positions were obtained in other portions. Thus, it appears that NW 3140 is very inhomogeneous regarding diamond. A shift toward a higher wave number seems to be related to the presence of internal stresses in diamond (Knight and White 1989). We will discuss this below in relation to the origin of diamond.

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Figure 3.  Raman spectra data of diamond. FWHM (full width at half maximum of diamond band) is plotted against the peak position.

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Table 2. Average values of Raman data of diamond bands in ureilites.
SamplePeak center (cm−1)FWHM (cm−1)
  1. *Number of analyses.

ALHA772571332.8 ± 1.315.5 ± 8.2
 (47*) 
NWA 31401336.1 ± 8.633 ± 24
 (17*) 
Shişr 0071330.3 ± 0.89.7 ± 4.0
 (54*) 
Y-7909811332.0 ± 1.011.9 ± 3.2
 (63*) 
Y-7915381330.6 ± 0.79.6 ± 1.9
 (40*) 

On the Origin of Diamond in Ureilites

Miyamoto et al. (1993) measured the Raman spectra of diamonds synthesized under shock-induced high pressure and by CVD, as well as the Raman spectra of two ureilites, Y-790981 and Y-791538. In Fig. 4, we compared their measured peak positions and FWHM of synthesized diamonds with our data of diamonds in ureilites. As stated above, our data for the two Yamato ureilites agree well with those obtained by Miyamoto et al. (1993). One datum of ALHA77257 obtained by Miyamoto et al. (1988) also agrees with our data. Our obtained ureilite data are located at the right-bottom corner of the shock-produced diamonds, but they fit much more nicely in the area for CVD diamonds (Fig. 4). This suggests that the origin of diamonds in ureilites is CVD. Similar agreement between FWHM of ureillite and CVD diamonds is also found for other ureilites (Hezel et al. 2008; Le Guillou et al. 2010; Ross et al. 2011). However, Heymann (1989) reported that the FWHM of a diamond in the Canyon Diablo iron meteorite is only 7 + 2 cm−1. Furthermore, El Goresy et al. (2001) reported that the diamond in the Ries crater has a FWHM of only 4.5 cm−1. From this, the above authors concluded that the FWHM is not a diagnostic tool for determining the origin of diamond. The problem could be related to the duration time of the meteorite impact shock being 103–106 times longer than that in the laboratory experiments, and makes the Raman band of meteorite diamond narrower compared with the shock-produced diamond in the laboratory (Hezel et al. 2008).

image

Figure 4.  Comparison of our Raman data for diamonds in ureilites (average values of individual ureilites) with those of shock-produced and CVD diamonds (Miyamoto et al. 1993).

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While the FWHM of the Raman diamond peak may not be a crucial tool for determining the origin of diamond, the peak position could be such a tool. It is apparent in Fig. 3 that for NWA 3140 it is shifted toward higher wave numbers for large FWHM. In general, it is easy to understand the downshift of the peak position with larger FWHM. This is due to the effect of small grain size (e.g., Yoshikawa et al. 1993), the presence of lonsdaleite (e.g., Miyamoto et al. 1993), and the heating effect of laser power (e.g., El Goresy et al. 2001). Surely, the shock-produced diamonds are located in the area of lower wave numbers and larger FWHM (the left-upper part in Fig. 4).

Meanwhile, our ureilite diamond data show the trend that the peak position shifts to a higher wave number with larger FWHM (Fig. 4). This trend is also observed even in a single ureilite such as ALHA77257 and NWA 3140 (Fig. 3). The shift toward higher wave number indicates the presence of internal stress in diamond caused by a mismatch between diamond and substrate and is often observed in CVD diamonds (Knight and White 1989; Miyamoto et al. 1993). Knight and White (1989) reported that the effect is most noticeable in diamond films deposited on hard substrates such as alumina or carbides. Matsuda et al. (1991) proposed that formation of diamond in ureilites occurred on high-temperature condensates in the nebula. This model can well explain the chemical feature of carbon vein material, e.g., the enrichment in refractory siderophiles such as Re, Ir, W, etc. (high-temperature condensates) and carbon and noble gasses (Janssens et al. 1987; Matsuda et al. 1991). Thus, the shift of the peak center to a higher wave number is easily explained with this CVD model, but would be difficult using the impact shock model.

The main reason to insist on the shock-origin hypothesis has been that diamond in ureilites is always found in conjunction with graphite (Hezel et al. 2008; Ross et al. 2011). However, this is not strong evidence for the shock-origin hypothesis. It is not easy to produce pure CVD diamond under a plasma condition. We produced the CVD diamonds from a gaseous mixture of H2 and CH4 (Fukunaga et al. 1987; Matsuda et al. 1991). Graphite is easily produced together with the diamond when the proportions of H2 and CH4 are slightly changed. Especially, high CH4 content seems to favor the growth of graphite (Fukunaga et al. 1987). Thus, the presence of diamond in conjunction with graphite is not a unique indicator for the shock-origin hypothesis. Even in the primitive solar nebula, diamond and graphite could have been easily produced together with a slight change in nebula condition.

Nakamuta and Aoki (2000) reported that the basal spacing for part of the graphite coexisting with diamond is slightly smaller compared with normal spacing, which has also been used as evidence for shock origin (e.g., Hezel et al. 2008). However, if the shock occurred for a mixed material of graphite and diamond, the graphite in the vicinity of diamond should be compressed because of the hardness of diamond. Thus, this again does not strongly support the shock-origin hypothesis. Nakamuta and Aoki (2000) wrote in their abstract that the intensity of diamond to graphite is correlated with the shock level of the meteorites. However, the shock level of ALHA77257, which had highest diamond-to-graphite ratio, is less than that of Y-791538, and the correlation is not perfect. As written in Matsuda et al. (1991), the diamond content is about the same (approximately 50% of the total carbon) in highly shocked ureilite (Goalpara) and in moderately shocked ureilite (Novo Urei). Takeda et al. (2001) reported the presence of diamond even in DaG 868 that is supposed one of the most weakly shocked ureilite and that the amount of diamond is comparable to that of ALHA77257. Takeda et al. (2001) proposed a catalytic transformation of graphite to diamond at relatively low pressure for the origin of diamond in DaG 868, but the diamond yield is surely proportional to the shock pressure in the laboratory experiments (Matsuda et al. 1995) and it is curious that the diamond-to-graphite ratio has no correlation with the shock level. To support the shock origin for ureilite diamonds it is often cited that there is no diamond in the ureilite ALHA78019 (very low-shocked), but the detection of diamond is not so simple (Matsuda et al. 1991). Ott et al. (1984) reported the presence of diamond in lightly shocked ureilite Nilpena although Jaques and Fitzgerald (1982) reported that there was no diamond in it.

The most severe constraints for the origin of diamond in ureilites are the noble gasses and nitrogen. Shock traps the noble gasses in diamond within a closed system, but it is difficult to explain the fractionation and the trapping efficiency of noble gasses using the shock model (Matsuda et al. 1995). Most telling, the different nitrogen isotopic compositions of diamond and graphite (and amorphous carbon) are very difficult to explain by the shock model (Rai et al. 2002, 2003a). Thus, Le Guillou et al. (2010) have admitted that there are CVD diamonds in a condensation process. Meanwhile, they consider that amorphous carbon could be the product of diamond post-shock annealing, but again, it is difficult to explain the difference in nitrogen isotopes between amorphous carbon and diamond. Amorphous carbon is also produced by CVD, and the elemental abundance patterns of noble gasses are similar to those in CVD diamond (Fukunaga and Matsuda 1997). Thus, we suggest that graphite, amorphous carbon, and diamond are directly formed on the early condensates in the primitive solar nebula. This concept well explains the nitrogen, and noble gas feature of the carbon vein in ureilites. It is likely that the chemical conditions and ionization mechanism necessary to form CVD diamond and other carbon are present in the primitive solar nebula (Matsuda et al. 1988, 1991). The CVD diamonds (and other carbon phases, too) are produced by the decomposition of hydrocarbons of gas phases under the thermodynamically metastable condition. It is interesting that the decomposition requires the presence of a large amount of hydrogen. The major chemical component of the primitive solar nebula is hydrogen, which is very suitable to produce CVD carbon phases. The formation of diamond, amorphous carbon, and graphite depends on the sp3/sp2 bond ratio of the raw material carbon phase. The carbon having sp3 hybrid orbital favors the diamond formation but that having sp2 bond favors the graphite formation. In the primitive solar nebula, CO having sp2 bond is dominant at high-temperature conditions, but it gradually transforms to CH4 having sp3 bond as the temperature decreases. The reaction of CO to CH4 depends also on the total pressure of the primitive solar nebula. The CO begins to transform to CH4 at about 1000–1300 K when the total pressure is 10−3 to 10−2 atm (Matsuda et al. 1991). Laboratory-synthesized CVD diamonds are produced at these temperatures. If the total pressure of the nebula is lower, the transformation starts at lower temperatures. The ionization mechanism in the primitive solar nebula is possible such as a hot plasma conditions (Arrhenius and Alfvén 1971), lightning, solar wind, cosmic rays, etc. (Matsuda et al. 1991). After this deposition of carbonaceous materials, a shock event with silicate minerals occurred in some later stage on a parent body and the compressed feature of graphite in the vicinity of diamond formed. It is also likely that the boundaries of silicate minerals were filled with the CVD carbonaceous material at this shock event.

Summary

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Experimental and Raman Spectra
  6. Results and Discussion
  7. Summary
  8. References

We studied carbonaceous material included in five ureilites (ALHA77257, NWA 3140, Shişr 007, Y-790981, and Y-791538) using micro-Raman spectroscopy.

  • 1
     We prepared four kinds of samples (bulk, acid residue, etched residue, and thin section). The thin section provided the best sample for studying diamond and graphite in ureilites.
  • 2
     The graphite in ureilites is well ordered compared with the carbon material in carbonaceous chondrites. The domain dimensions of graphite were estimated to be 45–110 Å.
  • 3
     The shifts of the diamond peak position to higher wave number seem to support the CVD origin of ureilite diamonds.
  • 4
     Although the mineralogical evidence of graphite and diamond appears to be explained by both the shock and the CVD models, it is almost impossible to explain the elemental and isotopic features of noble gasses and nitrogen for graphite, amorphous carbon, and diamond in ureilites by the shock model.
  • 5
     We consider that graphite, amorphous carbon, and diamond were chemically deposited on the early condensates in the primitive solar nebula. Then a shock event occurred on ureilite and several shock-induced features (compressed graphite in the vicinity of diamond, etc.) were produced at that time. This model well explains all the features of the carbon veins in ureilites.

Acknowledgments—  This research was supported by a Grant-in-Aid for Scientific Research 18104010 (J.M.). We would like to thank Drs. U. Ott and S.V. S. Murty for their review comments on the manuscript. The manuscript has been greatly improved by their valuable comments.

Editorial Handling—  Dr. Ian Lyon

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Experimental and Raman Spectra
  6. Results and Discussion
  7. Summary
  8. References