Heavily fractionated noble gases in an acid residue from the Klein Glacier 98300 EH3 chondrite

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Geochimica et Cosmochimica Acta 74 (2010) 5134–5149 www.elsevier.com/locate/gca

Heavily fractionated noble gases in an acid residue from the Klein Glacier 98300 EH3 chondrite Daisuke Nakashima a,b,*, Ulrich Ott a, Ahmed El Goresy c, Tomoki Nakamura d,1 a Max-Planck-Institut fu¨r Chemie, J.-J.-Becher-Weg 27, D-55128 Mainz, Germany Geochemical Research Center, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan c Bayerisches Geoinstitut, Bayreuth Universita¨t, D-95447 Bayreuth, Germany d Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan b

Received 7 January 2010; accepted in revised form 3 June 2010; available online 11 June 2010

Abstract Noble gases were measured both in bulk samples (stepped pyrolysis and total extraction) and in a HF/HCl residue (stepped pyrolysis and combustion) from the Klein Glacier (KLE) 98300 EH3 chondrite. Like the bulk meteorite and as seen in previous studies of bulk type 3 E chondrites (“sub-Q”), the acid residue contains elementally fractionated primordial noble gases. As we show here, isotopically these are like those in phase-Q of primitive meteorites, but elementally they are heavily fractionated relative to these. The observed noble gases are different from “normal” Q noble gases also with respect to release patterns, which are similar to those of Ar-rich noble gases in anhydrous carbonaceous chondrites and unequilibrated ordinary chondrites (with also similar isotopic compositions). While we cannot completely rule out a role for parent body processes such as thermal and shock metamorphism (including a later thermal event) in creating the fractionated elemental compositions, parent body processes in general seem not be able to account for the distinct release patterns from those of normal Q noble gases. The fractionated gases may have originated from ion implantation from a nebular plasma as has been suggested for other types of primordial noble gases, including Q, Ar-rich, and ureilite noble gases. With solar starting composition, the corresponding effective electron temperature is about 5000 K. This is lower than inferred for other primordial noble gases (10,000–6000 K). Thus, if ion implantation from a solar composition reservoir was a common process for the acquisition of primordial gas, electron temperatures in the early solar system must have varied spatially or temporally between 10,000 and 5000 K. Neon and xenon isotopic ratios of the residue suggest the presence of presolar silicon carbide and diamond in abundances lower than in the Qingzhen EH3 and Indarch EH4 chondrites. Parent body processes including thermal and shock metamorphism and a late thermal event also cannot be responsible for the low abundances of presolar grains. KLE 98300 may have started out with smaller amounts of presolar grains than Qingzhen and Indarch. Ó 2010 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

*

Corresponding author. Present address: Department of Geoscience, University of Wisconsin-Madison, 1215W. Dayton St., Madison, WI 53706, USA. Tel.: +1 608 261 1523; fax: +1 608 262 0693. E-mail address: [email protected] (D. Nakashima). 1 Present address: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan. 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.06.006

Primordial noble gases contained in primitive chondrites were incorporated into the chondrites during their formation, and the compositions were established either in the early solar system or in the Sun’s parent molecular cloud (Huss and Alexander, 1987). Q noble gases, a representative of primordial noble gas components, were found to be retained in hydrofluoric and hydrochloric acid-resistant residues (HF/HCl residues) from the Allende CV3 chondrite (Lewis et al., 1975). Further treatment of the residues with

Fractionated primordial noble gases in KLE 98300

an oxidant (HNO3) leads to loss of the Q noble gases, and Lewis et al. (1975) named the carrier of Q noble gases phase-Q. Thereafter it was established that phase-Q consists of carbonaceous matter (e.g., Ott et al., 1981). Also, as revealed by numerous noble gas studies of HF/HCl residues, all chondrite groups contain Q noble gases (Srinivasan et al., 1977, 1978; Alaerts et al., 1979a,b; Moniot, 1980; Matsuda et al., 1980; Schelhaas et al., 1990; Wieler et al., 1991, 1992; Huss et al., 1996; Busemann et al., 2000). Accordingly, phase-Q must have been widely distributed in the early solar system, and refining the signatures of Q noble gases and understanding their origin are important in understanding solar system evolution. Q-type noble gases have highly fractionated elemental compositions relative to solar abundances favoring the heavy noble gases (4He/20Ne/36Ar/84Kr/132Xe = 374/3.2/76/ 0.81/1 on average), and isotopic compositions are slightly fractionated from those of solar wind (cf., Busemann et al., 2000; Ott, 2002). Ozima et al. (1998) suggested that the isotopic compositions could be explained by massdependent fractionation of noble gases having solar composition. The elemental compositions, however, cannot readily be explained by the same mass-dependent fractionation and require other fractionation factors, e.g., such involving ionization potential, atomic size, and so forth (Ozima et al., 1998). Despite many efforts, the origin of Q noble gases is still unknown. HF/HCl-resistant residues of primitive meteorites contain also carbonaceous matter that carries isotopically “anomalous” components. The anomalies can only be explained by stellar nucleosynthesis prior to solar system formation, and the carrier phases graphite, diamond, and silicon carbide (SiC) have been identified as presolar grains (e.g., Anders and Zinner, 1993; Ott, 1993). They are important for understanding not only stellar nucleosynthesis but also the thermal histories of meteorite parent bodies, because presolar grain abundances tend to decrease with increasing petrologic type (Huss and Lewis, 1995). Similarly, Q noble gas elemental ratios are also subject to change during parent body processes such as thermal metamorphism and aqueous alteration (Busemann et al., 2000). The 36Ar/84Kr/132Xe ratios in Q are negatively correlated with petrologic type, i.e., more primitive chondrites tend to show higher 36Ar/84Kr/132Xe ratios. Patzer and Schultz (2002) found through bulk noble gas measurements of 57 enstatite chondrites that enstatite type 3 (E3) chondrites contain noble gases that are “Xe rich” relative to Q noble gases (36Ar/132Xe as low as 37). The Xerich noble gases (also named “sub-Q” noble gases by Patzer and Schultz, 2002) were suggested to be a product of elemental fractionation of Q noble gases. Since the more primitive E3 chondrites tended to show the lower elemental ratios, it was suggested that sub-Q noble gases were produced from Q noble gases not by parent body processes but that their composition was established prior to accretion to the parent body (Patzer and Schultz, 2002). Note, however, that so far the very existence of a distinct sub-Q noble gas component has been controversial. Since Q noble gases show significant variation in elemental compositions (Busemann et al., 2000), sub-Q noble gases may just be one end of the Q noble gas

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elemental variation (Busemann, 2002; Ott, 2002). The low elemental ratios of sub-Q noble gases may also be explained by contamination with atmospheric Xe (Crabb and Anders, 1982; Nakashima and Nakamura, 2006; Okazaki et al., in press). In addition, there is no information on carrier phases and detailed isotopic compositions. If sub-Q noble gases are fractionated Q noble gases, they may also be present in HF/HCl-resistant residues, as the Q noble gases. In fact, noble gases in an acid residue from the Qingzhen EH3 chondrite were found to be isotopically and elementally Q-like (Huss et al., 1996). However, also Qingzhen bulk shows Q-like rather than sub-Q noble gas compositions (Patzer and Schultz, 2002), and thus the presence of Q- rather than sub-Q noble gases in its acid residue is not surprising. To make progress in this context, it is important to select, for studies of acid-resistant residues, E3 chondrites that in bulk analyses already show sub-Q noble gas signatures. In our study, noble gases were measured in a HF/HCl residue from the Klein Glacier (KLE) 98300 EH3 chondrite, for which we also report bulk noble gas data showing the presence of sub-Q noble gases (see also the abstract by Nakashima et al., 2006). Gases in the residue were measured by stepped pyrolysis and by stepped combustion. The former facilitates deconvolution of noble gas components with different origins and trapping sites, whereas the latter enables us to release noble gases preferentially from carbonaceous oxidizable phases. The results show that, in fact, noble gases in the acid residue are isotopically Q-like but elementally fractionated relative to Q noble gases. KLE 98300 is known to be one of the least modified enstatite chondrites by thermal metamorphism, but the shock state of the meteorite is unclear. In order to improve the situation we have been searching for shock features by observing polished thin sections of KLE 98300. Based on the indication for the presence of elementally, but not isotopically, fractionated Q-type noble gases, we discuss a possible scenario for their origin. In doing so we also considered the possible effects of parent body processes. These include thermal and shock metamorphism, as well as a late thermal event which has been suggested to have occurred on the EH3 chondrite parent body based on results of mineralogical studies (e.g., Ehlers and El Goresy, 1988) and was also supported by a young Rb–Sr age (2 Ga; Torigoye and Shima, 1993). 2. EXPERIMENTAL PROCEDURES Three polished thin sections of KLE 98300 were prepared and studied microscopically in transmitted and reflected light so as to identify the shock features. A bulk sample of the KLE 98300 (1.035 g) was put into a Teflon beaker, and concentrated HCl was added. The sealed beaker was heated at 80 °C for 24 h. This treatment was repeated until the sample was finely disintegrated. Subsequently, the sample was treated alternately with concentrated HCl and HF (1:1) and concentrated HCl at 80 °C for 24 h until solutions turned colorless. CS2 was used to remove sulfur, and the residue was washed with concentrated HCl for three times, followed by washes with water. The recovered residue was 9.48 mg (0.91% yield). Hereafter

145 ± 7 83.5 ± 5.1 106 ± 4 0.205 ± 0.007 0.201 ± 0.004 0.202 ± 0.004 4.97 8.72 13.7 a

The errors in gas concentrations less than 20%, and include all errors of experiments, standards, etc. nd = not determined.

nd 0.733 ± 0.182 nd 2.38 ± 4.83 nd 0.174 522 100 622 500 1200 Total HF/HCl residue combustion 0.22 mg (KR-2)

0.00513 ± 0.00011 nd

366 ± 1 84.5 ± 0.2 15.0 ± 0.2 17.2 ± 0.8 279 ± 1 148 ± 1 0.215 ± 0.001 0.244 ± 0.001 0.212 ± 0.002 0.188 ± 0.002 0.188 ± 0.001 0.220 ± 0.001 1.13 2.62 1.35 0.350 1.09 6.54 0.826 ± 0.017 0.830 ± 0.017 0.0767 ± 0.0084 0.289 ± 0.018 0.0455 ± 0.0155 0.760 ± 0.015 1.03 ± 0.03 1.06 ± 0.04 2.83 ± 0.32 5.85 ± 0.25 10.3 ± 0.9 1.52 ± 0.04 0.314 0.161 0.0542 0.108 0.131 0.768 698 102 8.20 5.90 0.211 814 500 900 1200 1500 1700 Total HF/HCl residue pyrolysis 9.26 mg (KR-1)

0.00525 ± 0.00006 0.00464 ± 0.00010 0.00156 ± 0.00053 0.00533 ± 0.00105 nd 0.00513 ± 0.00006

1513 ± 89 0.248 ± 0.004 1.79 0.786 ± 0.005 1.02 ± 0.02 1.51 0.00696 ± 0.00020

1122 ± 59 3957 ± 210 1049 ± 55 326 ± 18 1116 ± 26 0.190 ± 0.002 0.271 ± 0.015 0.599 ± 0.005 0.218 ± 0.002 0.211 ± 0.005 2.26 0.299 0.0659 0.629 3.26 0.461 ± 0.002 0.794 ± 0.003 0.804 ± 0.003 0.788 ± 0.003 0.753 ± 0.008 4.96 ± 0.03 0.820 ± 0.006 0.831 ± 0.006 0.857 ± 0.009 1.37 ± 0.02 0.943 0.588 0.362 0.101 1.99 0.00698 ± 0.00018 0.00609 ± 0.00019 0.00703 ± 0.00011 0.0155 ± 0.0002 0.0357 ± 0.0026 468 366 44.3 0.157 878 500 900 1200 1700 Total

40

Ar/36Ar 38

Ara (108 cm3/g) 36

Ne/22Ne 21

Ne/22Ne 20

Nea (108 cm3/g) 20

He/4He 3

Hea (108 cm3/g) 4

868

We observed euhedral enstatite crystals (5–80 lm; Fig. 1) protruding FeNi metal and troilite throughout the polished

Bulk 27.79 mg

3.1. Shock features

Bulk 66.22 mg

3. RESULTS

Fraction (°C)

we call the residue “KR” (KLE 98300 Residue). A small fraction of KR was studied with a field emission scanning electron microscope (FESEM; Zeiss LEO1530) equipped with an energy-dispersive X-ray spectrometer (EDS) at MaxPlanck-Institute for Chemistry in Mainz, in order to check for the completeness of dissolution. Qualitative analysis with EDS showed besides carbon the presence of S, and small amounts of Na, Mg, Al, Si, and Fe. In some cases, accompanying a high carbon peak, small Mg and Al peaks were observed, suggestive of spinel (acid-resistant mineral; Lewis et al., 1975). KR was split into two subsamples used for two distinct noble gas analyses: 9.26 mg (KR-1) for stepped pyrolysis (500, 900, 1200, 1500, and 1700 °C) and 0.22 mg (KR-2) for stepped combustion (500 and 1200 °C). KR-1 was wrapped in Pt-foil, while KR-2 was wrapped in Ni-foil. In the case of noble gas extraction by pyrolysis, the heating duration was 30–40 min including the time to reach the nominal temperature. In combustion, the sample was heated at the nominal temperature for 20 min under 5–6 torr O2 generated by heating CuO at 800 °C. In both cases, the extracted gases were purified by Ti getters and then separated into four fractions (He–Ne, Ar, Kr, and Xe) using charcoal traps. Concentrations and isotopic ratios were measured with noble gas mass spectrometers at Max-Planck-Institute for Chemistry in Mainz: MAP215-50 for stepped pyrolysis and MAP215 for stepped combustion. Sensitivities and discrimination correction factors for the mass spectrometers were calibrated by measuring known amounts of standard gases. Blanks for noble gases at 1700 °C (stepped pyrolysis) were 4 He = 1.1  1011, 20Ne = 3.2  1012, 36Ar = 1.0  1011, 84 Kr = 7.4  1014, 132Xe = 2.3  1014 cm3, whereas those at 1200 °C (stepped combustion) were 4He = 1.2  109, 20 Ne = 5.7  1012, 36Ar = 4.7  1012, 84Kr = 9.7  1014, 132 Xe = 2.3  1014 cm3. Blank corrections were applied to all noble gas data, and isobaric interferences were handled by the blank corrections. The interference from benzene at mass 78 may not have been properly corrected, however, and the derived 78Kr/84Kr ratios are higher than that of KrQ (0.00603; Busemann et al., 2000) or Kr-Air (0.00609; Basford et al., 1973). One-r errors are given for concentrations and isotopic ratios in Tables 1–3, and include statistical uncertainties for measured blanks, samples, and standard gases. The derived noble gases were deconvoluted into noble gas components with different origins, as described in Appendix. We also analyzed two bulk samples of KLE 98300 (including nitrogen): one by total extraction at 1700 °C (27.79 mg) and one by stepped pyrolysis at 500, 900, 1200, and 1700 °C (66.22 mg). Blanks for noble gases at 1700 °C were 4He = 2.3  1011, 20Ne = 3.2  1012, 36Ar = 3.8  1012, 84Kr = 1.1  1013, 132Xe = 2.5  1014 cm3. Preliminary results from these measurements have been reported in Nakashima et al. (2006).

Ar/36Ar

D. Nakashima et al. / Geochimica et Cosmochimica Acta 74 (2010) 5134–5149

Table 1 Concentrations and isotopic ratios of He, Ne, and Ar in bulk samples and in a HF/HCl residue of KLE 98300.

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Fractionated primordial noble gases in KLE 98300

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Table 2 Concentrations and isotopic ratios of Kr in bulk samples and in a HF/HCl residue of KLE 98300.

Bulk 66.22 mg

Fraction 84Kra (1012 cm3/g) (°C)

78

80

82

83

500 900 1200 1700 Total

487 137 87 207 919

0.00675 ± 0.00014 0.00674 ± 0.00054 0.00650 ± 0.00043 0.00693 ± 0.00180 0.00676 ± 0.00044

0.0409 ± 0.0004 0.0478 ± 0.0007 0.0442 ± 0.0008 0.0432 ± 0.0014 0.0428 ± 0.0008

0.200 ± 0.002 0.206 ± 0.002 0.211 ± 0.004 0.209 ± 0.002 0.204 ± 0.004

0.200 ± 0.002 0.203 ± 0.002 0.202 ± 0.002 0.204 ± 0.002 0.202 ± 0.004

977

0.00666 ± 0.00022 0.0445 ± 0.0007 0.207 ± 0.002 0.203 ± 0.001 0.304 ± 0.002

Bulk 27.79 (mg) HF/HCl residue Pyrolysis 9.26 mg (KR-1)

Kr/84Kr

Kr/84Kr

0.202 ± 0.004 0.206 ± 0.003 0.203 ± 0.003 0.204 ± 0.007 0.201 ± 0.010 0.204 ± 0.003

Kr/84Kr

0.201 ± 0.002 0.205 ± 0.005 0.204 ± 0.006 0.201 ± 0.007 0.202 ± 0.011 0.203 ± 0.005

86

Kr/84Kr

0.304 ± 0.002 0.305 ± 0.005 0.303 ± 0.005 0.294 ± 0.002 0.302 ± 0.005

500 900 1200 1500 1700 Total

415 566 529 190 100 1801

0.00985 ± 0.00127 0.00803 ± 0.00091 0.00881 ± 0.00095 0.01088 ± 0.00218 0.00763 ± 0.00419 0.00896 ± 0.00066

HF/HCl residue Combustion 500 0.22 mg (KR-2) 1200 Total

1589 2197 3786

0.01533 ± 0.00539 0.0378 ± 0.0057 0.209 ± 0.009 0.201 ± 0.016 0.308 ± 0.020 0.00753 ± 0.00319 0.0391 ± 0.0038 0.202 ± 0.008 0.205 ± 0.010 0.305 ± 0.013 0.01081 ± 0.00293 0.0386 ± 0.0034 0.201 ± 0.008 0.203 ± 0.010 0.307 ± 0.014

a

0.0408 ± 0.0013 0.0395 ± 0.0021 0.0404 ± 0.0013 0.0413 ± 0.0025 0.0436 ± 0.0044 0.0405 ± 0.0016

Kr/84Kr

0.309 ± 0.006 0.309 ± 0.005 0.310 ± 0.003 0.308 ± 0.009 0.309 ± 0.012 0.309 ± 0.011

The errors in gas concentrations less than 20%, and include all errors of experiments, standards, etc.

thin sections as found by Keil (2007), which may suggest that KLE 98300 is an impact-melt breccia. However, the KLE 98300 thin sections do not show eutectic textures of metal + troilite with metal spherules in troilite (and vice versa), as they are observed in shock-induced melt veins in chondrites (e.g., Chen et al., 2002). In addition, feathery graphite (Fig. 1), which occurs associated with FeNi metal and troilite, does not depict any deformation kinking nor is there any shock-produced diamond (El Goresy et al., 2001). Thus KLE 98300 does not show the shock features observed in heavily shocked materials, and for this reason we conclude that KLE 98300 is not an impact-melt breccia. Weaker shock features are visible, however. Half of the pyroxene grains that we observed in the thin sections show undulose extinction, the other half show weak mosaicism. Also observed were planar fractures. Olivine grains, which were rarely found in the thin sections, exhibit only undulose extinction. Troilite does not show shock-induced finelyspaced twin lamellae, which suggests that any shock pressure experienced by the meteorite was less than 2 GPa. In summary, KLE 98300 exhibits only weak shock features, which is similar to the case of Qingzhen where also shock-induced twin lamellae were not observed (Shima et al., 1983; Lin and El Goresy, 2002). 3.2. Bulk noble gases Results of noble gas analyses are shown in Tables 1–3. Cosmic-ray exposure ages and radiogenic noble gas retention ages have been discussed in the previous report on bulk noble gases (Nakashima et al., 2006). Here we focus on the trapped noble gases with emphasis on the isotopic ratios of Ne and Xe and the elemental ratios of the heavy noble gases. Fig. 2 shows a Ne three-isotope diagram. Isotopic ratios of Ne released at 500 °C from the bulk sample fall on the tie line between Ne-Air and cosmogenic Ne, indicative of atmospheric contamination. For the higher temperature

fractions (as well as for the total extraction), Ne isotopic ratios are distributed around cosmogenic Ne. Thus, neon in the bulk samples is dominated by cosmogenic Ne. Fig. 3 shows a Xe three-isotope diagram. With increasing heating temperature, Xe isotopic ratios shift from Xe-Air towards Xe-Q. While in the 1200 °C fraction Xe isotopic ratios are consistent with those of Xe-Q within the errors, they shift to the lower right at 1700 °C, suggesting a contribution of fission Xe (244Pu or 238U) or HL-Xe (Ott, 2002). The 130Xe/126Xe ratio at 1700 °C (39.4 ± 1.0; Table 3) is closer to that of Xe-Q (39.9; Busemann et al., 2000) than that of Xe-HL (27.3; Huss and Lewis, 1994a), indicating that only the heavy isotopes are enhanced. Given fission decays of 244Pu and 238U provide only 129,131–136Xe, a fission Xe contribution appears most plausible. This interpretation of the 1700 °C release is supported by the results for the total extraction analysis, where the Xe isotopic ratios deviate slightly from the tie line between Xe-Air and XeQ to the right. This probably indicates a small contribution from fission Xe rather than HL-Xe (or air, for that matter), because the 130Xe/126Xe ratio (39.7 ± 1.5; Table 3) is close to that of Xe-Q. In any case, Xe in the bulk samples is dominated by Q-Xe. Fig. 4 shows an elemental ratio diagram for trapped 36 Ar, 84Kr, and 132Xe. The elemental ratios at 500 °C are plot in an area shifted from the Q or sub-Q range towards Air, suggestive of atmospheric contamination. In the 900 °C fraction, they are plotted below the mixing area, suggestive of preferential adsorption of terrestrial Kr and especially Xe during terrestrial weathering (elementally fractionated air; Crabb and Anders, 1981; Scherer and Schultz, 2000; Busemann and Eugster, 2002). In the 1200 and 1700 °C fractions, they are distributed below the subsolar and even below the Q range. This indicates the presence of sub-Q noble gases as suggested by Patzer and Schultz (2002). Ratios are especially low in the 1200 °C fraction (36ArTrapped/84Kr/132Xe = 4.2/0.8/1), even lower than subQ composition derived by Patzer and Schultz (2002). Given

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Bulk 66.22 mg

Fraction 132Xea (°C) (1012 cm3/g)

124

500 900 1200 1700 Total

51.1 31.1 107 146 336

0.00396 ± 0.00022 0.00453 ± 0.00034 0.00456 ± 0.00017 0.00462 ± 0.00009 0.00449 ± 0.00010

421

0.00431 ± 0.00012 0.00411 ± 0.00016 0.0810 ± 0.0010 5.20 ± 0.03

0.161 ± 0.001 0.813 ± 0.007 0.382 ± 0.003 0.321 ± 0.003 1753 0.161 ± 0.002 0.159 ± 0.001 0.160 ± 0.001 0.161 ± 0.001 0.163 ± 0.010 0.160 ± 0.001

Bulk 27.79 mg

Xe/

132

Xe

126

Xe/132Xe

128

129

130

131

134

136

0.00340 ± 0.00026 0.00368 ± 0.00033 0.00425 ± 0.00015 0.00406 ± 0.00009 0.00399 ± 0.00009

0.0739 ± 0.0008 0.0769 ± 0.0020 0.0827 ± 0.0010 0.0819 ± 0.0006 0.0805 ± 0.0010

1.11 ± 0.01 6.47 ± 0.05 6.43 ± 0.05 4.78 ± 0.02 4.91 ± 0.06

0.156 ± 0.003 0.156 ± 0.002 0.163 ± 0.003 0.160 ± 0.001 0.160 ± 0.002

0.794 ± 0.005 0.779 ± 0.014 0.824 ± 0.007 0.817 ± 0.005 0.812 ± 0.010

0.390 ± 0.005 0.383 ± 0.007 0.381 ± 0.004 0.384 ± 0.002 0.384 ± 0.005

0.328 ± 0.002 3.80 0.324 ± 0.003 169 0.317 ± 0.002 579 0.322 ± 0.001 546 0.321 ± 0.004 1298

865 1362 1983 1629 157 5995

0.00456 ± 0.00025 0.00435 ± 0.00014 0.00430 ± 0.00015 0.00470 ± 0.00015 0.00447 ± 0.00020 0.00446 ± 0.00009

HF/HCl residue 500 Combustion 1200 0.22 mg (KR-2) Total

4401 4274 8675

0.00436 ± 0.00036 0.00363 ± 0.00035 0.0808 ± 0.0051 1.10 ± 0.01 0.00441 ± 0.00047 0.00399 ± 0.00030 0.0792 ± 0.0042 1.16 ± 0.02 0.00438 ± 0.00030 0.00380 ± 0.00023 0.0800 ± 0.0034 1.13 ± 0.02

a

0.0794 ± 0.0009 0.0799 ± 0.0013 0.0783 ± 0.0008 0.0829 ± 0.0008 0.0835 ± 0.0039 0.0802 ± 0.0007

Xe/132Xe

HF/HCl residue 500 Pyrolysis 900 9.26 mg (KR-1) 1200 1500 1700 Total

b

0.00391 ± 0.00023 0.00405 ± 0.00020 0.00380 ± 0.00010 0.00407 ± 0.00006 0.00412 ± 0.00053 0.00395 ± 0.00007

Xe/132Xe

1.12 ± 0.01 1.11 ± 0.01 1.12 ± 0.00 1.18 ± 0.00 1.23 ± 0.02 1.14 ± 0.01

Xe/132Xe

Xe/132Xe

0.817 ± 0.008 0.812 ± 0.005 0.807 ± 0.003 0.815 ± 0.006 0.819 ± 0.008 0.812 ± 0.005

Xe/132Xe

0.380 ± 0.004 0.379 ± 0.002 0.382 ± 0.003 0.381 ± 0.003 0.376 ± 0.005 0.380 ± 0.003

Xe/132Xe

0.316 ± 0.003 0.318 ± 0.002 0.324 ± 0.002 0.321 ± 0.002 0.317 ± 0.004 0.320 ± 0.002

0.160 ± 0.007 0.814 ± 0.007 0.379 ± 0.006 0.316 ± 0.007 0.161 ± 0.007 0.817 ± 0.017 0.380 ± 0.006 0.321 ± 0.006 0.160 ± 0.005 0.815 ± 0.011 0.379 ± 0.006 0.319 ± 0.005

The errors in gas concentrations less than 5%, and include all errors of experiments, standards, etc. Radiogenic 129Xe concentrations are estimated by the equation: 129XeRad = 132Xe  [(129Xe/132Xe)  (129Xe/132Xe)Q]), where (129Xe/132Xe)Q is 1.042 (Busemann et al., 2000).

129

XeRadb (1012cm3/g)

66.8 94.2 158 221 29.7 570 247 495 743

D. Nakashima et al. / Geochimica et Cosmochimica Acta 74 (2010) 5134–5149

Table 3 Concentrations and isotopic ratios of Xe in the bulk samples and a HF/HCl residue of KLE 98300.

Fractionated primordial noble gases in KLE 98300

Fig. 1. Reflected light photomicrograph of KLE 98300. Euhedral enstatite crystals (dark gray; En) associate with FeNi metal (white; Me) and troilite (light gray; Tr). Enstatite crystals occurring inside FeNi metal and troilite protrude perpendicularly to the polished surface. Feathery graphite (dark gray; center; Gr) occurs with FeNi metal.

that Xe isotopic ratios at 1200 °C are consistent with those of Xe-Q (Fig. 3), atmospheric Xe contribution in this temperature fraction is negligible. In fact, if there is a contribution from atmospheric Ar and Kr to the noble gases released at 1200 °C, the elemental ratios of the pure primordial noble gas released in this step must be even lower. Thus, atmospheric noble gas contribution cannot be responsible for the low elemental ratios observed at 1200 °C. Thus, the combination of low Ar/Kr/Xe and XeQ like isotopic ratios in the 1200 °C fraction supports the conclusion that sub-Q noble gases are fractionated Q noble

Fig. 2. Ne three-isotope diagram. Ne data of KR-2 are not shown, because the amounts of Ne isotopes in KR-2 are at blank level. Numerals near the data points refer to the extraction temperatures in 100 °C. Ne isotopic ratios of Q (20Ne/22Ne = 10.67, 21Ne/22Ne = 0.0294)Q are from Ott (2002), those of Air (20Ne/22Ne = 9.80, 21 Ne/22Ne = 0.0290)Air from Eberhardt et al. (1965), those of A2 (20Ne/22Ne = 8.50, 21Ne/22Ne = 0.036)A2 from Huss and Lewis (1994a), and those of E(H) (20Ne/22Ne = 0.0827, 21Ne/22Ne = 0.00059)E(H) from Lewis et al. (1994).

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Fig. 3. Xe three-isotope diagram of the bulk samples of KLE 98300. The numerals near the data points refer to the extraction temperatures in 100 °C. The dashed line is a tie line between Xe-Air and Xe-Q. Xe isotopic ratios of Q (130Xe/132Xe = 0.1619, 136 Xe/132Xe = 0.3164)Q are from Busemann et al. (2000), those of Air (130Xe/132Xe = 0.1514, 136Xe/132Xe = 0.3294)Air from Basford et al. (1973), those of 244Pu–Xe (130Xe/132Xe = 0, 136Xe/132Xe = 1.13)Pu and 238U–Xe (130Xe/132Xe = 0, 136Xe/132Xe = 1.68)U from Ozima and Podosek (2002).

gases (Patzer and Schultz, 2002). The observation also suggests that noble gas analysis of a HF/HCl residue from KLE 98300 may be a useful approach in trying to learn more about the sub-Q noble gases. Assuming that noble gases with low elemental ratios released at 1200 °C are the indigenous primordial component,

Fig. 4. Elemental ratio diagram of 36Ar, 84Kr, and 132Xe in the bulk samples of KLE 98300. Numerals near the data points refer to the extraction temperatures in 100 °C. The 36Ar (but not 84Kr and 132Xe) abundances have been corrected for the cosmogenic contribution. The elemental ratios of terrestrial air (36Ar/84Kr/132Xe = 1340/27.7/ 1)Air are from Ozima and Podosek (2002), while those of subsolar are from Crabb and Anders (1981) and Patzer and Schultz (2002). Q noble gas elemental ratios of acid residues from carbonaceous and ordinary chondrites (gray area) are shown for comparison as well as typical Q ratios (cf., Busemann et al., 2000). Elemental ratios of 36 ArTrapped, 84Kr, and 132Xe in E3 chondrites (sub-Q range surrounded by a black line; Patzer and Schultz, 2002) are also shown for comparison. The shaded area represents a mixing area defined by Q range, sub-Q range, and Air.

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the two bulk samples are strongly affected by (un-) fractionated atmospheric noble gas contamination during terrestrial weathering, because two data points (Bulk step total and Bulk total extraction; Fig. 4) are plotted around (and below) the mixing area. 3.3. Isotopic compositions of KR noble gases Ne isotopic ratios for the HF/HCl residue KR-1 (stepwise pyrolysis) are shown in Fig. 2 (see also Table 1). In the first two steps, Ne is dominated by the cosmogenic component. Its presence is attributed to recoil implantation of spallation-produced Ne from surrounding minerals into acid-resistant phases and/or in-situ Ne production in residual non-carbonaceous minerals, possibly spinel (detected by FESEM), chromite, and daubre´elite (Crabb and Anders, 1982). The 1200 °C data point plots significantly below the Ne-A2-cosmogenic mixing line, indicative of a contribution from Ne-E (almost pure 22Ne). There are two varieties of Ne-E: Ne-E(L) hosted by presolar graphite and Ne-E(H) in presolar SiC (cf., Ott, 2002). The former is released at around 800 °C (Amari et al., 1990), while the latter is released above 1200 °C (Lewis et al., 1994). Therefore, the Ne-E component observed in the 1200 °C fraction is most likely Ne-E(H). Neon from the 1500 °C extraction plots on the tie line between cosmogenic Ne and Ne-A2. Considering that Ne-A2, which is carried by presolar diamond (Huss and Lewis, 1994a), is released ahead of Ne-E(H) during stepped pyrolysis of etched residues of, e.g., Qingzhen and Orgueil (Huss and Lewis, 1995), we consider it more likely that – instead of being a mixture of Ne-A2 and cosmogenic Ne–Ne released at 1500 °C is a mixture of Ne-E(H), Ne-Q, and cosmogenic Ne. In the highest temperature fraction, the Ne isotopic composition is close to that of Ne-Q, but also to Ne-Air within errors. Since Ar in this temperature fraction is isotopically atmospheric (Table 1), Ne may well also be atmospheric (blank). Overall, neon includes at least Ne-cosmogenic and Ne-E(H), possibly also Ne-A2 and Ne-Q. Isotopic ratios 3He/4He of KR-1 (Table 1) are clearly higher than those of trapped He (3He/4He 6 2.6  104; cf., Ott, 2002), indicative of a contribution of cosmogenicHe (3He/4He  0.2), similar to the case of Ne. Also, the 38 Ar/36Ar ratios of KR-1 are higher than in trapped Ar (0.188), which can be attributed to the presence of cosmogenic Ar (1.54) (Table 1). Since KR-1 releases noble gases both from residual minerals and from carbonaceous matter, the higher ratios may be explained by a contribution of cosmogenic Ar in the residual minerals. The 40Ar/36Ar ratios are higher than those in most other HF/HCl residues (mostly <3; e.g., Busemann et al., 2000), suggesting a contribution of radiogenic 40Ar and / or air. A radiogenic noble gas contribution is also seen in xenon (Table 3), where 129 Xe/132Xe ratios are high relative to (129Xe/132Xe)Q (1.042; Busemann et al., 2000). The observed radiogenic 129 Xe excesses (=132Xe  [129Xe/132Xe – 1.042]) may be explained by adsorption of radiogenic 129Xe onto the surface of the residual material during the chemical treatment if the excess becomes prominent in a low temperature fraction in the acid residue in contrast to a higher temperature from

the bulk sample, an effect observed by, e.g., Srinivasan et al. (1978) (see also Matsuda et al., 1999). However, the temperature fractions where the 129Xe excesses in KR-1 become prominent (1200–1500 °C; Table 3) are almost the same as in the case of the bulk sample (1200–1700 °C). This indicates that the 129Xe excesses in KR-1 are not due to adsorption of radiogenic 129Xe onto residual phases but are indigenously derived from residual phases. KR-2 had a small sample size, its noble gases were released from mostly oxidizable phases only, and the blank was relatively high. Probably for these reasons, 3He in the 1200 °C fraction and Ne in the 500 °C fraction were not detected. Despite noble gas release only from mostly carbonaceous matter in the case of KR-2, the 38Ar/36Ar ratios are high, which can be explained by incorporation of cosmogenic Ar into the carbonaceous matter by recoil. KR-2 contains not only more (radiogenic) 40Ar (1.5  105 cm3/g compared to 9.7  106 cm3/g for KR-1), but also shows a higher excess of radiogenic 129Xe (7.4  1010 cm3/g) than KR-1 (5.7  1010 cm3/g). 40Ar and radiogenic 129Xe therefore may reside in a common unidentified carrier that is combustible. Fig. 5 shows a Xe three-isotope diagram of 130Xe/132Xe vs. 136Xe/132Xe. Isotopic ratios are distributed around the composition of Xe-Q, although 130Xe/132Xe ratios are slightly lower than that of Xe-Q, indicative of a contribution of atmospheric Xe (16–17% to total 132Xe; Appendix). The 136Xe/132Xe ratios in the 1200 °C (KR-1 and KR-2) and 1500 °C (KR-1) fractions are slightly higher than that of Xe-Q, which can be attributed to a contribution from Xe-HL carried by presolar diamond (Huss and Lewis, 1994a), and its contribution to total 132Xe can be estimated as approximately 0.4–0.7% (Table 4; Appendix). Since KR1 shows evidence for the presence of Ne-E(H) (see Ne discussion above), the presence of Xe–S (G component in SiC; cf., Ott, 2002) can also be expected. However, its contribution to total 132Xe as estimated from Ne is less than 0.0015% (Appendix), so it is undetectable. In essence, Xe isotopic ratios show that Xe in KR is isotopically similar to Xe-Q. The same is true for Kr, although some atmospheric Kr contribution is seen (no figure; Table 2). 3.4. Elemental ratios of KR noble gases Uncorrected 36Ar/84Kr/132Xe ratios are 11/0.30/1 for total KR-1 and 16/0.44/1 for total KR-2. In order to derive the ratios in the elementally fractionated Q-type component (hereafter “EFQ” for descriptive purpose), we have applied corrections for contributions from cosmogenic, HL, and G (Ar–S, Kr–S, and Xe–S) components as well as atmospheric noble gases (Appendix). While in most analyses of HF/HCl residues Ar is dominated by the Q component (e.g., Alaerts et al., 1979a,b), this cannot be ascertained from the isotopic ratios in our case, unlike for Kr and Xe. Here we have estimated possible contributions of Ar from SiC and diamond from the Ne-E(H) and Xe-HL abundances, respectively. Atmospheric 36Ar is corrected based on the assumption that the measured 40Ar is atmospheric in origin (Appendix), but actual 40Ar includes both atmospheric and radiogenic 40Ar. Therefore our estimated atmospheric 36Ar

17 – – – – – 500 1200 Total KR-2

Concentrations of trapped noble gases are estimated based on the isotopic ratios (see Appendix). Trapped He and Ne concentrations of KR-2 are not determined, because of lack of discernible He and Ne. 4He in KR-1 is corrected only for cosmogenic and G components. The heavy noble gas abundances in the respective temperature fractions have been corrected for cosmogenic 36Ar (but not 84Kr and 132Xe) and atmospheric noble gases, while total abundances of 36Ar, 84Kr, and 132Xe have been corrected for cosmogenic, HL, and G components as well as atmospheric noble gases. The corrected 36Ar concentrations have ranges due to the correction for atmospheric 36Ar with an upper limit. b SiC abundance in the bulk meteorite is estimated from the 22Ne-E(H) concentration in KR-1, 22Ne-E(H) concentration in SiC (1.65  104 cm3/g; Lewis et al., 1994), and the residue fraction (0.91%) (see Section 4.3). c Diamond abundance in the bulk meteorite is estimated from the Xe-HL concentrations in KR, Xe-HL concentration in diamond (2  107 cm/g; Huss and Lewis, 1994b), and the residue fraction (0.91%) (see Section 4.3).

– 0.0376 0.0376 3.48 3.70 7.18 1.10 0.119 1.19 25.3–49.1 65.3–86.4 85.7–134

0.119–0.154 <0.0624 1.03–2.87 7.91 500 900 1200 1500 1700 Total KR-1

a

1.8 – – 0.0256 0.0143 – 0.0399 0.803 0.991 1.56 1.49 0.157 5.00 0.359 0.435 0.529 0.114 0.0934 1.53 0.007–0.009

ArEFQ (109 cm3/g)

36

SiCb (ppm) NeE(H) (109 cm3/g)

22

NeA2 (109 cm3/g)

22

NeEFQ (109 cm3/g)

20

Hecorrected (106 cm3/g)

4

Table 4 Concentrations of trapped noble gases in KR and abundances of presolar grains in the bulk meteoritea.

concentrations are upper limits, and the corrected 36Ar concentrations have possible ranges (Table 4). Corrections are not minor (Appendix but the corrected 36Ar/132Xe and 84 Kr/132Xe ratios are 5.9–12 and 0.31 for KR-1 and 12–19 and 0.17 for KR-2, not significantly different from the uncorrected ratios. It is possible in principle that the low elemental ratios result from adsorption of dissolved primordial noble gases onto KR during the chemical treatment, which leads to elementally fractionated noble gases favoring heavy elements (cf., Marty et al., 1983). As discussed in the previous subsection, however, there appears to be no adsorption of radiogenic 129Xe onto KR during the chemical treatment. The same should be true for primordial noble gases. Therefore, it is unlikely that the observed low elemental ratios are artifacts. Fig. 6 shows an elemental ratio diagram of corrected 36Ar, 84Kr, and 132Xe (see also Table 4). Both data bars of KR-1 and KR-2 are clearly below the Q range and even below the sub-Q range (as measured in bulk E3s). The same is true for the individual temperature fractions, except for the 1700 °C fraction, which appears to be affected by presence of atmospheric Ar (blank; see discussion in previous subsection). Thus, the heavy noble gases in KR are elementally strongly fractionated relative to Q noble gases. The ratio of corrected 4He to 132XeEFQ (1580) is higher than those in Q of other chondrites (e.g., Busemann et al., 2000), which is possibly due to radiogenic 4He incorporation into HF/HCl-resistant phases. Note that the recoil range of radiogenic 4He and of cosmogenic 3He is significantly larger (tens of lm; e.g., Farley et al., 1996; Huss et al., 2008; Ott et al., 2009) than that of cosmogenic Ne (2–3 lm; Ott and Begemann, 2000) so that (provided a rather uniform distribution of U and Th) redistribution by recoil is significantly more efficient for He than for Ne. Efficient redistribution is also indicated by the fact that the concentrations of 4He in the residue are virtually identical to those of the bulk meteorite samples (Table 1). Additionally, there may also be a 4HeHL contribution

84

Fig. 5. Xe three-isotope diagram of KR-1 and KR-2. The numerals near the data points refer to the extraction temperatures in 100 °C. Xe isotopic ratios of HL (130Xe/132Xe = 0.1524, 136Xe/132Xe = 0.6991)HL are from Huss and Lewis (1994a), and those of Xe–S (G component) (130Xe/132Xe = 0.53, 136Xe/132Xe  0)s from Ott et al. (1988).

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<11.1 17.7–25.1 12.6–13.2 3.29–3.49 0.709–10.9 29.5–61.8

KrEFQ (109 cm3/g)

132

XeEFQ (109 cm3/g)

132

XeHL

Diamondc (ppm)

Fractionated primordial noble gases in KLE 98300

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Fig. 6. Elemental ratio diagram of air-corrected 36Ar, 84Kr, and 132 Xe in KR-1 and KR-2. The 36Ar (but not 84Kr and 132Xe) abundances in the respective temperature fractions have been corrected also for the cosmogenic contribution. Total abundances of 36Ar, 84Kr, and 132Xe have been corrected for cosmogenic, HL, and G components and atmospheric noble gases (Appendix). Q noble gas elemental ratios of acid residues from carbonaceous and ordinary chondrites are shown for comparison as well as typical Q ratios (cf., Busemann et al., 2000). Elemental ratios of 36ArTrapped, 84 Kr, and 132Xe in E3 chondrites (Patzer and Schultz, 2002) are also shown for comparison. The dotted area represents a mixing area between total KR-1 and Air, while the shaded area represents a mixing area between total KR-2 and Air. The data points of estimated elemental ratios of the acid soluble fraction in the two bulk samples are located in the direction of the two black arrows (extending from the origin). The two arrows are less steep compared to the mixing areas (KR-1 to Air and KR-2 to Air), which means the data points of soluble fraction in the two bulk samples are plotted below the connecting areas.

(Appendix). The (20Ne/36Ar)EFQ ratio (Table 4) of KR-1 (0.02–0.10) is within (or exceeds) the range of those found for other acid residues (0.01–0.08; Busemann et al., 2000). Compared with the heavy elements, the lighter elements appear to be less fractionated. Such decoupled fractionation patterns were also seen in other trapped components including Q noble gases (Busemann et al., 2000; Ott, 2002). As suggested by Busemann et al. (2000), thermal metamorphism may have affected the lighter and heavier noble gases of the carrier phase differently (see the next section). 3.5. Release patterns of

36

Ar,

84

Kr, and

132

Xe

Fig. 7 shows release patterns of air-corrected 36Ar, 84Kr, and 132Xe in KR-1 (only 36Ar is corrected also for cosmogenic component), as well as release patterns of heavy noble gases in HF/HCl residues of Qingzhen (EH3) and Indarch (EH4) for comparison (Huss et al., 1996). In the case of normal Q noble gases, the release patterns of 36Ar, 84Kr, and 132Xe are nearly identical, and the release peaks appear at identical temperatures in the range 1000–1600 °C (Fig. 7b and c; e.g., Huss et al., 1996). The identical release patterns indicate that the noble gases are released by chemical reaction which destroys the noble gas carriers, not by

Fig. 7. Release patterns of air-corrected 36Ar, 84Kr, and 132Xe in KR-1 (a). The 36Ar (but not 84Kr and 132Xe) abundances have been also corrected for the cosmogenic contribution. The subscript “Cor” represents “corrected”. Only 36ArCor has variations in percentages at the respective temperatures, which is due to variable concentrations of 36ArCor (Appendix). The increase of 36ArCor in the 1700 °C fraction is most likely due to a contribution from atmospheric Ar (blank), because the Ar isotopic ratios at 1700 °C are close to those of air. Release patterns of heavy noble gases (no correction) in Qingzhen (EH3; (b)), Indarch (EH4; (c)), lunar rock 10057-20 (panel d) are shown for comparison (Huss et al., 1996; Hohenberg et al., 1970).

diffusion (Srinivasan et al., 1978; Huss et al., 1996). The primary release peaks between 1000 and 1600 °C are supposed to reflect the reaction of the Q gas carrier with oxide minerals in the residue such as chromite and spinel or with the Ta-foil surrounding the samples (Huss et al., 1996). In the case of KR-1, the release of 36Ar in KR-1 peaks at 900 °C before that of 132Xe (1200–1500 °C), and the release pattern of 84Kr is intermediate between those of 36Ar and 132 Xe. The observed trend that heavier noble gas has the higher release fraction at high temperature is indicative of diffusive noble gas release. The 84Kr and 132Xe release peak temperatures of 1200–1500 °C, which are within the range of release peak temperature of normal Q noble gases, may be explained by a reaction of the noble gas carrier with chromite (however not detected by SEM) and spinel, but not with the crucible, because the Pt-foil in which the

Fractionated primordial noble gases in KLE 98300

sample was the sample has a melting point of 1772 °C. The peak release temperature for 36Ar (900 °C) is slightly below the range of release peak temperature of normal Q noble gases, and may be attributed to diffusion rather than chemical reaction. Thus, it is suggested that the carrier phase of the EFQ noble gases differs from “normal” phase-Q. Since the diffusive noble gas release that we observe is characteristic for solar wind noble gas implanted species (e.g., Hohenberg et al., 1970; Fig. 7d), the carrier phase may have acquired the EFQ noble gases by similar low energy ion implantation (solar wind 1 keV/nuc; Walker, 1980). 4. DISCUSSION 4.1. Summary of results of noble gas analyses Noble gases in KR are isotopically Q-like but elementally fractionated favoring heavy elements relative to Q noble gases, indicative of elementally fractionated Q-type (EFQ) noble gases. In addition, the EFQ gases are different from normal Q noble gases also with respect to release patterns. Previous identifications of a “sub-Q” component relied on elemental ratios only (Patzer and Schultz, 2002), with no information about isotopic compositions, so our observations constitute the first proof of the occurrence of heavily fractionated Q-type noble gases in chondrites. They support the view that E3 chondrites in general contain “sub-Q” noble gases which are elementally fractionated relative to Q noble gases (Patzer and Schultz, 2002). The subQ noble gases identified by these authors from analyses of bulk E3 samples show higher Ar/Kr/Xe ratios than our new data from a HF/HCl residue of KLE 98300 (Fig. 6). An explanation may be contributions to the bulk samples of other noble gas components including terrestrial (as observed by us in our analysis of KLE 98300 bulk; see also discussion below). Accordingly, acid residues from other E3 chondrites may well have lower elemental compositions than those seen in bulk analyses. The 132Xe concentration in KR (KR-1 + KR-2) is 6.0  109 cm3/g. With a chemical yield of 0.91%, this corresponds to only 13–16% of the 132Xe concentration of the bulk meteorite samples, while KR carries only 2–3% of 36 Ar and 84Kr in the bulk meteorite samples. This indicates that most of the heavy noble gases in KLE 98300 are hosted by the acid soluble fraction. The 36Ar/84Kr/132Xe ratios in the soluble fraction are estimated as 114/3.2/1 (stepped pyrolysis) and 47/2.6/1 (total extraction). These elemental ratios plot below the mixing area defined by EFQ and Air (Fig. 6), indicative of incorporation of elementally fractionated atmospheric noble gases during weathering, as suggested earlier (Section 3.2). Thus, it is considered that heavy noble gases in the KLE 98300 bulk samples are dominated by fractionated atmospheric noble gases. Obviously this is the reason why KR accounts only for 2–16% of heavy noble gases in the bulk meteorite samples. 4.2. Formation process of the EFQ noble gases Assuming that the EFQ noble gases had formed from Q noble gases proper, there are two possibilities when the

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fractionation may have occurred: before accretion to the parent body and after that. In the latter case, possible processes include thermal metamorphism, shock heating, and late thermal event. A late thermal event in the history of EH3 chondrites has been suggested both, based on young Rb–Sr ages (2 Ga; Torigoye and Shima, 1993) and from mineralogical studies (El Goresy et al., 1988; Ehlers and El Goresy, 1988; El Goresy and Ehlers, 1989). A Rb-Sr age for KLE 98300 is not available, but given the meteorite is EH3 and has relatively young noble gas retention ages (U/Th–He age 3.0 Ga; K–Ar age 3.3 Ga; Nakashima et al., 2006), KLE 98300 may also have experienced this late thermal event. In the following we are going to discuss whether the Q-type noble gases having “non-identical” release peaks may have formed from Q noble gases proper by parent body processes such as thermal and shock metamorphism and the late thermal event. 4.2.1. “Non-identical” release peaks Thermal metamorphism can easily be ruled out as responsible for the non-identical release peaks, because the acid residue of Indarch (which is EH4 rather than EH3) shows identical release patterns of 36Ar, 84Kr, and 132 Xe with a peak at 1400–1600 °C (Fig. 7c; Huss et al., 1996). Shock metamorphism appears not to be a viable explanation for the difference in release pattern either. Nakamura et al. (1997) measured noble gases in artificially shocked samples of the Allende CV3 chondrite (up to 70 GPa). In their experiments, samples that experienced shock pressures P23 GPa showed an additional noble gas release peak at 800 °C. Similarly, the Leoville CV3 chondrite which experienced shock pressure 20 GPa (Nakamura et al., 1992) showed small peaks of 36Ar, 84Kr, and 132 Xe at 730 °C in addition to the main peaks at 1130 °C (Huss et al., 1996). In any case, the shock producing an additional noble gas release peak at lower temperature did so for Ar, Kr, and Xe simultaneously and thus is not an explanation for the non-identical release peaks in KR (Fig. 7a). Furthermore, as discussed earlier (Section 3.1), the maximum shock pressure experienced by KLE 98300 was much lower than in the above experiments. The putative late thermal event cannot be responsible for the nonidentical release peaks either. This is because in case of Qingzhen, which is suggested to have also experienced the late thermal event (e.g., Ehlers and El Goresy, 1988; Torigoye and Shima, 1993), the acid residue shows identical release patterns of 36Ar, 84Kr, and 132Xe with a peak at 1350 °C (Fig. 7b; Huss et al., 1996). Thus, parent body processes seem not be able to account for the non-identical release peaks observed in KR. Consequently, it is more likely that the Q-type noble gases having non-identical release peaks formed before accretion to the parent body. 4.2.2. Fractionation It may have been possible that the Q-type noble gases had normal Q-like elemental ratios (Fig. 6) originally and were fractionated by preferential loss of 36Ar and 84Kr induced by parent body processes. EH3 chondrites are believed to have experienced heating at a temperature of about 500 °C during thermal metamorphism according to

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Huss et al. (2006), while El Goresy and Ehlers (1989) estimated the reheating (late event) temperature in case of Qingzhen of less than 500 °C. Fractionation may thus have occurred during thermal metamorphism and/or the late thermal event. Although KLE 98300 is weakly shocked only (Section 3.1), there is no other example of acid residues from meteorites that contain fractionated noble gases having non-identical release peaks, hence shock-induced fractionation cannot be ruled out with certainty. In summary, in creating the EFQ noble gas pattern, all types of parent body processes considered may have played a role. In essence, while we cannot completely rule out a role for parent body processes in creating the fractionated elemental compositions, parent body processes in general seem not able to account for the non-identical release patterns in KR. The non-identical noble gas release patterns would have been established before accretion to the parent body (in the solar nebula or in the presolar molecular cloud), but at that time the Q-type noble gases may have been less fractionated than those we observe now (EFQ compositions), if indeed parent body processes were responsible for the fractionated elemental compositions. Identifying a process for forming the Q-type noble gases in the solar nebula or in the presolar molecular cloud is not straightforward. Ozima et al. (1998) suggested that atomic size and/or ionization potential could have played a role in Q noble gas formation (see Section 1), and the same may be true for the EFQ noble gases. Information may be contained in the non-identical release patterns of the EFQ noble gases (Fig. 7a). The situation is similar to the case of solar wind noble gases (Fig. 7d), which are acquired via implantation of low energy ions emitted from the Sun. Hence, a possible process to achieve non-identical release patterns plus elementally fractionated noble gas composition is noble gas ion implantation from a solar nebula plasma (with solar initial composition). Note that Ar-rich noble gases (36Ar/132Xe  200–400) in anhydrous carbonaceous and unequilibrated ordinary chondrites also show nonidentical release patterns (Miura et al., 2001; Nakamura et al., 2003), and that it was suggested that a possible origin of the Ar-rich noble gases in the Ningqiang carbonaceous chondrite is the noble gas ion implantation (Nakamura et al., 2003). Similarly, ion implantation from a plasma has been suggested earlier already for ureilite noble gases (Weber et al., 1971; Matsuda et al., 1991) and also for Q noble gases (Matsuda and Yoshida, 2001). If also true for the noble gases in KR, the fractionation pattern is determined by the ionization potentials of the elements and the electron temperature of the plasma (Elwert, 1952; Jokipii, 1964; Go¨bel et al., 1978; Matsuda et al., 1991; Nakamura et al., 2003; Rai et al., 2003). Significant isotopic fractionation is not induced, because the ionization potential for different isotopes of the same elements is the same. Note that in fact noble gas ion implantation experiments were able to reproduce the elemental fractionation patterns as well as isotopic fractionation relative to starting compositions favoring the heavy isotopes (Dziczkaniec et al., 1981; Bernatowicz and Hagee, 1987; Suzuki and Matsuda, 1990; Ponganis et al., 1997; Marrocchi et al., 2005). The isotopic fractionation observed in the experiments may be induced

by mass-dependent processes during implantation such as sputtering and diffusion (Bernatowicz and Hagee, 1987; Ponganis et al., 1997) or by a charge exchange accompanied by an isotopic exchange (Marrocchi et al., 2005). Given that relative to solar Q, Ar-rich, and ureilite noble gases all show slightly fractionated isotopic compositions favoring the heavy isotopes (cf., Ott, 2002; Nakamura et al., 2003) as observed in the ion implantation experiments (relative to the starting composition), ion implantation appears to be suitable process to produce primordial noble gases including the EFQ noble gases. The elemental ratios of the ionized noble gas fraction depend on electron temperature as given by the following equation (Elwert, 1952):  3 Hþ H01 n2 q1 E2 1 ¼     exp½ðE1  E2 Þ=kT : E1 Hþ H02 n1 q2 2 þ Here Hþ 1 =H2 is the abundance ratio of ionized elements 1 and 2, while H01 =H02 is the abundance ratio of the neutral elements (here: solar initial abundance; Anders and Grevesse, 1989), n is the principal quantum number, q the number of electrons in outermost shell of the neutral atom, E the ionization potential, k is Boltzmann’s constant, and T the electron temperature (Table 5). The equation means that lower electron temperature not only leads to less ionization but also results in a more fractionated elemental composition favoring the heavy (lower E) elements. The 36 Ar/84Kr/132Xe ratios of the EFQ noble gases (5.9–12/ 0.31/1 for KR-1 and 12–19/0.17/1 for KR-2) correspond to an electron temperature of about 5000 K (Fig. 8), which is lower than those inferred for ureilite noble gases (7000– 10,000 K; Go¨bel et al., 1978; Matsuda et al., 1991; Rai et al., 2003) and for Ar-rich noble gases in Ningqiang (8000 K; Nakamura et al., 2003). Due to the lower Ar/Kr/Xe ratios, it is also lower than calculated for Q noble gases (6000–7000 K; Rai et al., 2003). Note that the electron temperature may have been higher than 5000 K in case that the noble gases in KR had a less fractionated elemental compositions than those we observe (followed by fractionation on the parent body). Thus, if ion implantation in the early solar system indeed played a role in creating the abundance pattern of all these noble gas components, a corollary is that the electron temperature of the nebular plasma must have varied in the range P5000 to 10,000 K spatially or temporally. Rai et al. (2003) pointed out that at the lower electron temperatures more iodine compared to Xe was expected to be implanted in the carriers, which would have led to higher 129Xe/132Xe ratios. In fact, our KR data show 129 Xe/132Xe ratios (>1.10; Table 3) that are higher than in

Table 5 Parameters used for the calculation of electron temperatures. 36 a

Abundance ratio n q E (eV) a

84

Ar 4

6.84  10 3 8 15.8

Kr

20.6 4 8 14.0

Solar initial abundance (Anders and Grevesse, 1989).

132

Xe

=1 5 8 12.1

Fractionated primordial noble gases in KLE 98300

Fig. 8. Elemental abundance patterns of primordial noble gases observed in various meteorites and of noble gases at various electron temperatures in the nebular plasma. The patterns have been normalized to 132Xe and solar abundances (Anders and Grevesse, 1989). The dotted area corresponds to the range of elemental abundance patterns of noble gases in diamond from ureilites (Go¨bel et al., 1978; Rai et al., 2003), the gray area corresponds to the range of elemental abundance patterns of Arrich noble gases in Ningqiang (Nakamura et al., 2003), and the shaded area corresponds to the range of elemental abundance patterns of Q noble gases (Wieler et al., 1991, 1992; Busemann et al., 2000).

Xe-Q (1.042; Busemann et al., 2000), which is consistent with this suggestion. However, as noted above (Section 3) residual mineral(s) may also contribute to the enhanced 129 Xe/132Xe ratios. Therefore, the 129Xe/132Xe ratios of KR do not provide useful constraints on the timing of noble gas ion implantation (Rai et al., 2003). 4.3. Presolar grains Here we discuss abundances of presolar grains such as graphite, SiC, and diamond. Neon isotopic ratios suggest the presence of SiC (Fig. 2), while Xe is suggestive of the presence of diamond (Fig. 5). There is no evidence for the presence of graphite, which is possibly masked by other components. The SiC abundance in the bulk sample can be estimated as 0.007–0.009 ppm from the upper and lower limits for 22 NeE(H) in KR-1 (Table 4; Appendix), the 22NeE(H) concentration in SiC (“bulk” SiC: 1.65  104 cm3/g; Lewis et al., 1994), and the residue fraction (0.91%). This is clearly lower than the abundance reported for EH3 Qingzhen (1.53 ppm) and the Indarch EH4 chondrite (1.25 ppm); it is higher than that reported for EH5 Abee (0.00048 ppm; Huss and Lewis, 1995). Since presolar grain abundances tend to decrease with increasing petrologic type (Huss and Lewis, 1995), the low SiC abundance for Abee may be explained by parent body metamorphism. However, this explanation does not apply in the case of KLE 98300, which is less metamorphosed than Indarch. Given the shock pressure of KLE 98300 is as low as that of Qingzhen, shock metamorphism cannot be responsible for the low SiC abundance either. Even if KLE 98300 experienced a late thermal event like

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Qingzhen, this does not explain the low SiC abundance, given the higher SiC abundance in Qingzhen. Hence, we have no good explanation for the low SiC abundance. Possibly, KLE 98300 has acquired a smaller abundance of SiC compared to Qingzhen and Indarch during its formation. The 22NeA2/132XeHL ratio of KR-1 is calculated as 62 (Table 4), which despite its large uncertainty is clearly lower than in other chondrites (50; Huss and Lewis, 1994b). We estimate the diamond abundance based on the 132XeHL concentration. With a 132XeHL concentration in KR (KR1 + KR-2) of 4.0  1011 cm3/g (Table 4), 132XeHL concentration in diamond of 2  107 cm3/g (Huss and Lewis, 1994b), and the chemical yield of 0.91%, the diamond abundance in the bulk sample is 1.8 ppm, again clearly lower than in Qingzhen (73 ppm) and Indarch (43 ppm), but higher than in Abee (<0.4 ppm; based on Xe-HL abundance) (Huss and Lewis, 1995). Although Abee contains 100 ppm diamond, the diamond is free from Xe-HL and shows a d13C value distinct from that of presolar diamond (Russell et al., 1992). Again, considering the low shock pressure of KLE 98300, the low abundance of presolar diamond cannot be explained by shock metamorphism. The suggested late thermal event cannot be responsible for the low abundance of presolar diamond either, because of the higher presolar diamond abundance in Qingzhen. Like the case of SiC, KLE 98300 may have acquired a smaller abundance of diamond compared to Qingzhen and Indarch. 5. CONCLUSIONS Fractionated noble gases with low Ar/Kr/Xe ratio were identified in bulk samples and a HF/HCl residue from the KLE 98300 EH3 chondrite. The elemental fractionation of the residue gases relative to “normal” Q noble gases is even stronger than in the sub-Q composition reported by Patzer and Schultz (2002) based on analyses of bulk E3 chondrites. Noble gases in the acid residue are isotopically Q-like, which suggests they are derived from Q gases or from a common precursor. While we cannot completely rule out a role for parent body processes such as thermal and shock metamorphism or a late thermal event in creating the fractionated elemental compositions, parent body processes in general seem not able to account for the distinct release patterns from those of normal Q noble gases. Low energy ion implantation may account for the trend of noble gas release patterns that the heavier noble gas has a higher release fraction at high temperature. It was suggested that low energy ion implantation from a nebular plasma is the origin of A-rich noble gases in a carbonaceous chondrite which shows the noble gas release patterns similar to those we observed (Nakamura et al., 2003). The elementally fractionated Q-type noble gases in the KLE acid residue may have originated from noble gas ion implantation from a nebular plasma. If indeed ion implantation was responsible for creating the fractionated noble gas pattern, the electron temperature can be estimated as 5000 K. This is lower than electron temperatures inferred for other primordial noble gas components, which are in the range 6000–10,000 K (Go¨bel et al., 1978; Matsuda et al., 1991; Rai et al., 2003; Nakamura et al., 2003). If ion implantation

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in the early solar system was involved in case of all these primordial noble gas components, the electron temperature in the nebula must have varied spatially or temporally between P5000 and 10,000 K. Presolar grains such as SiC and diamond are depleted in KLE 98300 compared to Qingzhen and Indarch. KLE 98300 (EH3) is less metamorphosed than Indarch (EH4) and probably has experienced as similarly low shock pressure as Qingzhen. Even if KLE 98300 experienced also a late thermal event as Qingzhen did, this does not explain the low SiC abundance, given the higher SiC abundance in Qingzhen. We conclude that parent body processes are not responsible for the lower abundances of SiC and diamond, which may suggest that during its formation KLE 98300 acquired a lower abundance of presolar grains than Qingzhen and Indarch. ACKNOWLEDGMENTS The authors are grateful to Y. Marrocchi, J. Matsuda, and R. Wieler for their critical comments and helpful reviews and B. Marty for thoughtful comments and for handling this paper. The author also thank C. Sudek for support in the chemical treatments, S. Herrmann for technical supports during noble gas analyses, Y.N. Miura for helpful discussion, and the NASA Johnson Space Center for providing a sample of KLE 98300. This work was partly supported by the JSPS Research Fellowships for Young Scientists to D. Nakashima.

and the extension line to cosmogenic Ne is used. In the 1500 °C fraction, Ne appears to be a mixture of Ne-E(H), Ne-Q, and -cosmogenic (Fig. 2). Ne isotopic ratios in the 1700 °C fraction are close to Ne-Q, but also to Ne-Air within errors (Fig. 2). The lower limit of 20NeEFQ does not include 20Ne in this fraction. Cosmogenic corrections to 4He,

36

Ar,

84

Kr, and

132

Xe

The following cosmogenic and trapped isotopic ratios were used for deconvolution: Helium: 3He/4He = 0.2 (cosmogenic) and 1.23  104 (He-Q; cf., Ott, 2002); Argon: 38 Ar/36Ar = 1.54 (cosmogenic) and 0.188 (trapped). Cosmogenic 84Kr and 132Xe were corrected via 86Kr/83Kr and 130 Xe/126Xe, respectively. For the correction, we used the following isotopic compositions: Krypton: 86Kr/83Kr = 0.015 (cosmogenic; Marti and Lugmair, 1971) and 1.53 (Kr-Q; Busemann et al., 2000) together with cosmogenic 84 Kr/83Kr = 0.63 (Marti et al., 1966); Xenon: 130Xe/126Xe = 0.98 (cosmogenic; Hohenberg et al., 1981) and 39.9 (Xe-Q; Busemann et al., 2000) together with cosmogenic 132 Xe/126Xe = 0.83 (Hohenberg et al., 1981). The contributions of the cosmogenic component are 2.5% for 4He (only for KR-1), 1.1–2.5% for 36Ar, 0.11–0.20% for 84Kr, and less than 0.0012% for 132Xe. G component in SiC

Here we describe the deconvolution of the noble gases into various components in order to derive the abundance and abundance ratios of the “pure” elementally fractionated Q-type (EFQ) noble gases, and in order to estimate the abundances of presolar SiC and nanodiamond. The results are summarized in Table 4.

Since Ne isotopic ratios show the presence of Ne-E(H) in KR-1 (Fig. 2), 4He, 36Ar, 84Kr, and 132Xe in KR-1 would include 4HeG, 36ArG, as well as s-process 84Kr and s-process 132 Xe (G component). The four concentrations are estimated from the (4He/20Ne/36Ar/84Kr/132Xe)G ratios (=2326/1/0.026/0.0034/0.0072; cf., Ott, 2002) and the 20 NeE(H) concentration (Table 4) calculated from the upper limit to the 22NeE(H) concentration and (20Ne/22Ne)E(H) = 0.0827 (Lewis et al., 1994). The resulting contributions of the G component are less than 0.38% for 4He, 0.0005% for 36Ar, 0.0024% for 84Kr, and 0.0015% for 132Xe.

Neon

HL component in diamond

As shown in Fig. 2, Ne in KR-1 includes Ne-cosmogenic and Ne-E(H). Further possible trapped components are Ne-A2 and Ne-Q. Ne in the 500 and 900 °C fractions is essentially pure cosmogenic Ne, therefore Ne in the 1200– 1700 °C fractions only is used for calculating concentrations of 20NeEFQ, 22NeA2, and 22NeE(H) in KR-1. The concentrations are estimated as upper and lower limits, because the estimation depends on choice of the end members. Ne in the 1200 °C fraction can be described as a mixture of Q-E(H)-cosmogenic or A2-E(H)-cosmogenic. The former means trapped Ne consists of Ne-Q and Ne-E(H) (case 1), whereas the latter means the trapped Ne consists of Ne-A2 and Ne-E(H) (case 2). The cosmogenic Ne isotopic ratios are assumed to be the mean values of Ne isotopic ratios in the 500 and 900 °C fractions. In case 1, the trapped Ne isotopic ratios are determined by taking the intersection of the Q-E(H) line and an extension line to cosmogenic Ne, whereas in the case 2 the intersection of the A2-E(H) line

As shown in Fig. 5, Xe in KR-1 and KR-2 shows a minor contribution of Xe-HL as well as of atmospheric Xe. Xe isotopic ratios of “Q + Air” are determined by taking the intersection of the Q-Air line and an extension line to Xe-HL. The 132 XeHL concentrations (Table 4) were calculated based on the 136Xe/132Xe ratios of HL and “Q + Air” and the measured 132Xe concentrations. Concentrations of 4HeHL, 36 ArHL, and 84KrHL are estimated based on the so determined 132XeHL concentrations and (4He/36Ar/84Kr/ 132 Xe)HL = 300,000/50/0.48/1 (cf., Ott, 2002). The estimated 4 HeHL concentration is 1.2  105 cm3/g, which exceeds that of measured 4He (Table 1). Considering that the low 22 NeA2/132XeHL ratio (62; Table 4) suggests a larger depletion of 22NeA2 relative to 132XeHL, 4HeHL should also be depleted. For our calculation, we do not subtract 4HeHL from the corrected 4He. The 36ArHL contribution to the total 36 Ar is estimated as 1.4–3.1%, that of 84KrHL as 0.5–1.1%. Although the (36Ar/84Kr/132Xe)HL ratios might also be

APPENDIX A Deconvolution. of noble gas components

Fractionated primordial noble gases in KLE 98300

lowered, the HL contributions to the 36Ar and 84Kr concentrations do not affect the final 36Ar/84Kr/132Xe ratios derived for EFQ. Atmospheric noble gas correction For the atmospheric 132Xe correction, we used Xe/132Xe ratios of Q, Air, and “Q + Air” (determined at HL component calculation). Atmospheric 84Kr was corrected via 86Kr/84Kr, and for this correction we used the following 86Kr/84Kr ratios: 0.3052 (atmospheric; Basford et al., 1973) and 0.3095 (Q; Busemann et al., 2000). The atmospheric 84Kr contribution to total 84Kr is 13% for KR-1 and 68% for KR-2. The 38Ar/36Ar ratios of Q (0.187; Busemann et al., 2000) and Air (0.188; Nier, 1950) are almost identical, while the 40Ar/36Ar ratio of Air (299; Lee et al., 2006) is clearly higher than that of Q (mostly <3; e.g., Busemann et al., 2000). The 40Ar/36Ar ratio can thus serve as a monitor for atmospheric contamination. Assuming the measured 40Ar to be atmospheric in origin, atmospheric 36Ar contribution is calculated using 40Ar/36Ar ratios. However, given that high 40Ar/36Ar ratios can also be explained by a contribution of radiogenic 40Ar, the thus estimated atmospheric 36Ar concentration is an upper limit. The atmospheric 36Ar contribution to total 36Ar is <51% for KR-1 and <35% for KR-2. 130

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