Cosmic-ray produced ^3He, ^Ne, and ^Ar concentrations and the chemical composition of the Yamato-74357 lodranite have been determined. Concordant concentrations of ^3He and ^Ne have been obtained on both samples (Bern and Misasa). The concentration of cosmogenic ^Ar is twice as high in the Bern as in the Misasa sample. The ^3He, ^Ne and ^Ar data for both samples yield an average cosmic-ray exposure age of 5.8±2.0Ma. This age indicates that Yamato-74357 belongs to the lodranite group, which probably originates from a common break-up event about 5Ma ago. Radiogenic ^4He and ^Ar concentrations in the Bern sample are higher than those in the Misasa sample by more than a factor of two, suggesting that minerals enriched in U, Th and K are enriched in the finer grained fraction. The isotopic compositions of Kr and Xe are identical to those of the terrestrial atmosphere, except for the ^Xe abundance. The small ^Xe excess (^Xe/^Xe=1.15±0.04) is presumably due to the early formation of this meteorite

Eugster,Otto

Koeberl,Christian

Miura,Yayoi N.

Nagao,Keisuke

Takaoa,Nobuo

Weigel,Andreas

National Institute of Polar Research Repository

Proc. NIPR Symp. Antarct. Meteorites, 8, 297-303, 1995 NOBLE GASES, CHEMICAL COMPOSITION, AND COSMIC-RAY EXPOSURE AGE OF THE YAMAT0-74357 LODRANITE Keisuke NAGA0 1, Otto EuGSTER2, Andreas WEIGEL 2, Yayoi N. MIURA 3, Christian Ko EBERL 4 and N obuo TAKAOKA 5 1 Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori 682....()J 2 Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-30/2 Bern, Switzerland :,.Department of Earth and Planetary Physics, University o.f Tokyo, Bunkyo-ku, Tokyo I /3 4/nstitute of Geochemistry, University of Vienna, A-JOJO Vienna, Austria "Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 8/2 Abstract: Cosmic-ray produced :,.He, 21Ne, and '8Ar concentrations and the chemical composition of the Yamato-74357 lodranite have been determined. Concordant concentrations of 'He and 21 Ne have been obtained on both samples (Bern and Misasa). The concentration of cosmogenic 38 Ar is twice as high in the Bern as in the Misasa sample. The 3He, 21Ne and 38Ar data for both samples yield an average cosmic-ray exposure age of 5.8±2.0 Ma. This age indicates that Yamato-74357 belongs to the Iodranite group, which probably originates from a common break-up event about 5 Ma ago. Radiogenic 4He and 40Ar concentrations in the Bern sample are higher than those in the Misasa sample by more than a factor of two, sugg�sting that minerals enriched in U, Th and K are enriched in the finer grained fraction. The isotopic compositions of Kr and Xe arc identical to those of the terrestrial atmosphere, except for the 129Xc abundance. The small 129Xe excess ('29Xe/ mXe= 1.15±0.04) is presumably due to the early formation of this meteorite. 1. Introduction The Yamato-74357 lodranite was found in Antarctica as a smail (13.8 g) piece with a fusion crust on one side (YANAI and KOJIMA, 1987). Its texture, mineralogy, chemical composition, and oxygen isotopic ratio were reported elsewhere (NAGAHARA et al., 1990; CLAYTON et al., 1992; ToRIGOYE et al., 1993). Noble gas compositions of this meteorite, determined by the stepwise heating method, have been reported previously (TAKAOKA et al., 1993). Cosmic-ray exposure ages, based on the concentrations of cosmogenic 3He, 21Ne and 38Ar, showed some disagreement: 5.1 ±0.5, 10.1 ± 1.3, and 3.0±0.3 Ma, respectively, were obtained. This may suggest heterogeneity of the target element concentrations, e.g., high Mg and low Ca concentrations in the specimen studied by TAKAOKA et al. (1993). On the other hand, systematic noble gas studies of several lodranites indicated that the lodranites probably were ejected from their parent body by a single break-up event about 5 Ma ago (WEIGEL and EuGSTER, 1994; WEIGEL et al., 1994). Hence, a reliable cosmic-ray 297 298 K. NAGAO et al. exposure age of the Y-74357 lodranite is necessary to show whether it belongs to the lodranite group, with a common exposure age, or not. The objective of this paper is to report on our determination of the cosmic-ray exposure age of the Y-74357 lodranite. Here we discuss new noble gas data and an exposure age determined on a small sample prepared at the University of Bern from chips, which had remained from previous work in Misasa, Japan. 2. Experimental Procedures 2.1. Sample preparation Two small chips with a total weight of 18.07 mg were gently crushed in a stainless steel mortar to obtain homogeneous material of ::;300 µm grain size for analyses of He, Ne, and Ar in Bern and of the chemical composition at the University of Vienna. (The samples are designated hereafter as "Bern" and "Vienna", respectively). In the early stage of crushing several coarser grains ::;500 µm were separated for noble gas analyses in Misasa (sample designation is "Misasa"). This sample was not further crushed, to avoid atmospheric contamination of the heavy noble gases Kr and Xe. Metal grains (1.96 mg) were separated from the crushed silicates. They were cut into 20 pieces and put into the three silicate samples according to their weight proportions so as to represent the original chemical compositions. Thus, we obtained three samples for noble gas and chemical analyses: Bern 4.62 mg (He, Ne and Ar), Misasa 2.73 mg (He, Ne, Ar, Kr, and Xe) and Vienna 7.63 mg (chemical composition). A loss of 3.09 mg occurred during the procedure described above. Additional losses (10--15%) are due to loading the samples into the extraction line as indicated in Table la. 2.2. Analytical methods of noble gas and element concentrations 2.2.1. Bern sample ( 4.17 mg) He, Ne, and Ar isotopic ratios and concentrations were analyzed in Bern, employing a radio-frequency extraction line and two metal-tube mass spectrometers equipped with secondary electron multipliers. Details of the experimental procedures have been given elsewhere (e.g., EuGSTER, 1988; EuGSTER et al., 1993). Blank levels at 1700°C for the Bern sample were 4He= l 10, 20Ne=3.7 and 40Ar=7400 (10- 12cm3STP). Isotopic ratios of the blank were atmospheric (Table 2a). 2.2.2. Misasa sample (2.39 mg) All noble gases were analyzed using a modified VG5400 noble gas mass spectrometer, following a procedure that has been described elsewhere (e.g., NAGAO and MIURA, 1993; NAGAO et al., 1993; NAGAO, 1994). The sensitivity of the mass spectrometer, however, has recently been significantly improved by more than a factor of 10 by tuning of ion source parameters and flight tube geometry. With the higher sensitivity, 132Xe of 1.5 x 10- 16cm3STP can now be measured with a count rate of 1 cps, and the detection limit is less than lxl0- 16cm3STP. Corrections for 40Ar++ and CO2++ at the 20Ne and 22Ne peaks were 7 and 2%, respectively. Cosmic-Ray Exposure Age of Y-74357 Lodranite 299 2.2.3. Vienna sample (6.42 mg) This coarse-grained sample (with some metal) was analyzed for major and trace element concentrations, using electron microprobe analyses and instrumental neutron activation analysis (INAA). First, the bulk sample was sealed into a polyethylene vial and then, together with a set of synthetic standards, irradiated for 8 hours at a flux of about 2x 1012 n cm- 2s- 1, using the TRIGA Mark II  reactor of the Atominstitut der Osterreichischen Universitaten in Vienna. The accuracy of the data was evaluated by also analyzing international geological standard materials. For details of the analytical procedures, see KoEBERL ( 1993). After completion of the measurements and following an additional cooling period, the irradiated sample was fused under an Ar atmosphere to form a fused bead. This bead was then mounted and polished, and analyzed for major element composition with a Cameca Camebax electron microprobe, using standard Cameca data evaluation procedures. The bulk major element composition was obtained by averaging 15 individual analyses of 20x20 µm areas. Some elemental concentrations are reported in the footnote to Table 3; of those elements, Fe, K, Cr, and Ni were measured by INAA, and the others by fused bead electron microprobe analysis. 3. Noble Gas Isotopic Compositions and Cosmic-ray Exposure Age Noble gas isotopic ratios and concentrations are listed in Tables la and 1 b. The noble gas data from TAKAOKA et al. (1993) are also included for comparison. Because the samples used in this study were very small, we present abundances and isotopic ratios measured in blank runs in Tables 2a and 2b. The noble gas data in Tables la and lb have been blank corrected. The blanks of 4He, 20Ne, and 40Ar were 1.2, 3.6, and 21 % of the measured amounts (i.e. of the amounts before blank subtraction) of the respective isotopes for the Bern sample, and 9, 13, and 6% for those of the Misasa sample. The blanks of 84Kr and 132Xe were 10 and 7% for the Misasa sample. The high 8°Kr/84Kr ratio of 0.20 in the blank is presumably due to an interference of 40 Ar ions at the 8°Kr peak. The concentrations of 3He, 21Ne, and 36Ar for the Bern and Misasa samples agree well, while the concentrations of radiogenic 4He and 40Ar, as well as the ratios 4He/3He and 40Ar!36Ar of the Bern sample, are two times higher than those of the Misasa sample. As the Bern sample consisted of crushed material :::;300 µm, while for the Misasa sample grain size was :::;500 µm, a possible explanation is that minerals enriched in U, Th and K, the parent nuclides of 4He and 40Ar, respectively, are concentrated in the finer grain size fractions. The 40Ar/36 Ar ratio of the Misasa sample is identical to the atmospheric value (296), but this agreement clearly is fortuitous, because of the unambiguous detection of cosmogenic 38 Ar. There is also evidence for the presence of a small amount [ca. (1.3±0.3) x 10-8cm3STP/g] of trapped 20Ne in the Bern sample. The Kr and Xe isotopic ratios in the Misasa sample agree, within the respective errors, with those measured by TAKAOKA et al. (1993), and are indistinguishable from the terrestrial atmospheric ratios, except for an excess of 129Xe, i.e., 129Xe/132Xe = 1.12-1.15. The presence of radiogenic 129Xe from extinct 1 291 indicates early formation of the Y-74357 lodranite. On the other hand, the K-Ar age calculated using the 40Ar and K concentrations of Table la. 'He 4He 'He Sample 4Hc Bern 9.!� 214 0.0459 (4.17 mg) ±.5 ±11 ±. 0009 Misasa 10.9 122 0. 0895 (2.39 mg) ±I.I ±12 ± .0038 He, Ne and Ar abundances and isotopic compositions of Y-74357. 73. 211Nc 21Ne 22Ne 20Ne 21Nc 22Ne ·'6Ar ,xAr 22Ne 22Nc _:,1Ne 2.38 0.977 1.515 1.573 0.645 1.551 0.837 0.334 ±.14 ± .160 ±.150 ±.129 ±.084 ±.203 ±.074 ±.035 1.31 1.12 1.50 0.874 0. 747 1.339 0. 750 0.232 ±.13 ±.11 ±.15 ±.044 ±.014 ±.026 ±.075 ± .023 40Ar ,xAr 40Ar ·'6Ar .v'Ar - --- - -- - - ------- - -668 0.398 798 ±36 ±.024 ±55 222 0.3087 295.9 ±22 ±.0024 ± 1.0 - - -- - -----TA KAO KA et al. 8. 29 239 lJ. 0347 4.30 4.59 4.94 0.870 0.929 1. ()76 0.291 0.184 (38.7 mg) ±.0003 ±.009 ±.010 ±.012 (Step heating) Concentrations arc given in units of 10-x cm'STP/g. Table I b. Kr and Xe abundances and isotopic compositions of Y-74357, 73. Sample X4Kr 7xKr xuKr x2Kr - - -·- -- ··-·-1'4Kr=100 Misasa 202 0. 598 3.75 20.05 ±20 ±.088 ±.29 ±.42 TAKAOKA 95.1 0. 71 4.45 20.2 et al. ±.16 ±.35 ±1.l (1993) "3Kr X6Kr u2xc 124Xc ------- - - -20.02 30.51 48. 1 n. d. ±.63 ± l .11 ±4. 8 20.7 30.4 53.3 (0.44) ±. 9 ± 1. 0 Concentrations arc given in units of 10-12 cm·'STP/g. n.g.: not determined. 12<>xe n.d. (0.48) 12xxe 7.45 ±.57 ----i2,1Xc 130Xc 132Xc = 100 114.5 15.7 ±3.5 ±.4 - - - - ---- - -- - - --- ·--7.32 112.2 15.8 ±.63 ±4.0 ±I.I uixc 79.4 ±3.6 79.7 ±3.9 1027 0.632 ± .004 u4Xe 13<,xe ---------- - --39.1 32.7 ±I.I ± 1.4 --- --- --39.3 32.7 ±2.1 ±1.6 3528 ±54 v.) C C ;;::: z ;i, Cl ;i, C � r2-Bern 1700°C Misasa 1800°C Cosmic-Ray Exposure Age of Y-74357 Lodranite Table 2a. He, Ne and Ar abundances and isotopic compositions of blank run. n.d. 110 n.d. 290 3He 4He n.d. <0.00003 3.7 4. 8 a a a (l.050 0.55 8.7 ±.6 Jo Ar 38Ar a a a 0.09 1.17 0. 257 ±.03 7400 337 JHAr .,1,Ar a 0.2195 ±. 0030 301 a 288.0 ±.4 -- -- - --- --- - -- -" ----·-- - --- - - - -----·--- - ------ - ---- -�--- -·- ---- �-�--- --· ----- �-�- - -- - -- -·---- ---- ---- - ·-- · --- �- - ·-· - ---� Concentrations are given in units of 10 · 12 cm3STP/g. a: atmospheric isotopic compositions. n.d.: not determined. Table 2b. Kr and Xe abundances and isotopic compositions of blank run. Misasa 52 1800°C 0.94 20.0 20.9 20.2 30.7 8.6 n.d. n.d. ±.40 ±.9 ±1.3 ±1. 7 ±2. 2 Concentrations are given in units of 10- 15 cm3STP/g. n.d.: not determined. 132Xe= 100 8.3 98 15.7 80.4 42.3 35. 8 ±3.3 ± 11 ±3.9 ±6.3 ±3.2 ±3.7 668X 10-8cm3STP/g (Bern) and 150 ppm (Vienna) is 3.6 Ga, which is much younger than the age of the solar system. This may reflect a difference in retentivity for radiogenic 129Xe and 40Ar in the meteorite. The concentrations of cosmogenic 3He, 21Ne, and 38Ar (Table 3) were calculated based on the isotope ratios of cosmogenic and trapped components given by EuGSTER et al. (1993). Concentrations of 3He and 2 1Ne determined for the Bern and Misasa samples agree well, while the 38Ar concentrations disagree, i.e., (0.20±0.03) x 10- 8cm3STP/g for Bern and (0.103±0.010) x 10- 8cm3STP/g for Misasa. The disagreement is likely due to a low Ca concentration of the Misasa sample compared with that of the Bern sample. Cosmogenic 22Ne/21Ne ratios of 1.401±0.187 and 1.326±0.026 for the two samples indicate a small preatmospheric size of this meteorite, in accordance with the small recovered mass (13.8 g). To determine the cosmic-ray exposure age for this meteorite, production rates, P, were calculated using the formulas presented by EuGSTER and MICHEL (1995). This procedure was particularly derived for achondritic compositions. The target element composition as measured for the Vienna sample (Table 3) was used in the calculation. The shielding of the sample within the meteoroid was taken into account for P3 and P2 i , using the formulas for diogenites (EuGSTER and MICHEL, 1995), as the chemical composition for the relevant elements of Y-74357 and diogenites is quite similar. The resulting production rates and exposure ages are given in Table 3. For the Bern and Misasa samples, a good agreement of the T 3 and T 21 ages is observed. The T 38 ages are significantly lower than the T3 and T21 ages, perhaps due to inhomogeneities of 302 K. NAGAO et al. Table 3. Concentrations of cosmogenic 3He, 2 1 Ne, and ·'8 Ar, cosmogenic 22Nei2 1 Ne ratio, production rates, and cosmic-ray exposure ages. 38Ar P., P:,x T_, T 2 1 T .18 Tave rage Sample 10·- 10 cm3 STP/g 10-10 cm3 STP/gMa Ma Bern 980± 60 97±16 20 ±3 1. 401±0. 187 145 Misasa 1090± 110 112± 11 10. 3± 1.0 1. 326±0. 026 150 13.0 4. 51 6.8 7. 5 4. 4 6. 2± 1. 6 16.9 4. 51 7. 3 6. 6 2. 3 5. 4±2. 7 Concentrations of targe t e lements (wt%) used for the calculation of the production rate s (see text): Si (22. 8), Ti (0. 12), Al (0. 21), Fe (21. 5), Mn (0. 37), Mg ( 18. 3), Ca (0. 84), K(0. 015), Cr ( 1.45), and Ni (0. 96). the distribution of the main target elements for the 38 Arc production among the different samples used for the noble gas and target element determinations. In conclusion, we adopt as a cosmic-ray exposure age of the Yamato-74357 lodranite the average value of 5.8±2.0 Ma (la) for the Bern and Misasa samples. Consequently, this lodranite belongs to the lodranite group, which originated in a single break-up event that occurred 5±1 Ma ago (WEIGEL and EuGSTER, 1994; WEIGEL et al., 1994). Acknowledgments The authors thank Ors. I. KusHIRO and H. NAGAHARA for their effort to organize the consortium study of unique meteorites from Antarctica. They are also grateful to Ors. K. YANAI and H. KOJIMA for the sample. We are greatly indebted to M. ZUBER, P. GuGGISBERG, and A. SCHALLER for their support in the sample preparation and the noble gas analysis. C. KoEBERL thanks L. LE and G. McKAY (NASA Johnson Space Center, Houston) for permission to use, and help with, the Cameca Camebax electron microprobe. The manuscript has been improved by the critical reading and comments of Ors. U. Orr and J.-1. MATSUDA. This work was supported by the Swiss National Science Foundation. References CLAYTON, R. N. , MAYEDA, T. K. and NAGAHARA, H. (1992): Oxyge n isotope re lationship among primitive achondrites. Lunar and Planetary Science XXIII. Houston. Lunar Planet. Inst. , 231-232. EuGSTER, 0. (1988): Cosmic-ray production rates for 3He , 21Ne , 38Ar, 83Kr, and 126Xe in chondrite s based on 8 1Kr-Kr exposure ages. Geochim. Cosmochim. Acta, 52, 1649-1662. EuGSTER, 0. and MICHEL, TH. ( 1995): Common asteroid break-up e vents of eucrite s, diogenites, and howardites and cosmic-ray production rate s for noble gase s in achondrite s. Ge ochim. Cosmochim. Acta, 59, 177-199. EUGSTER, 0. , MICHEL, TH. , NIEDERMANN, S. , WANG, D. and YI, w. (1993): The re cord of cosmogenic, radiogenic, fissiogenic, and trapped noble gases in recently re cove red Chinese and other chondrites. Ge ochim. Cosmochim. Acta, 57, 1115-1142. KoEBERL, C. (1993): Instrumental neutron activation analysis of geochemical and cosmochemical sample s: A fast and re liable method for small sample analy sis. J. Radioanal. Nucl. Chem. , 168, 47-60. NAGAHARA, H. , FUKUOKA, T. , KANEOKA. I. , KIMURA, M. , KOJIMA, H. et al. (1990): Petrology of unique Cosmic-Ray Exposure Age of Y-74357 Lodranite 303 meteorites , Y-74063 , Y-74357, Y-75261 ,  Y-75274, Y-75300, Y-75305 , A-77081 ,  A-78230, and Y-8002. Papers Presented to the 1 5th Symposium on Antarctic Meteorites , May 30-June 1 ,  1 990. Tokyo , Natl Inst. Polar Res. , 92-94. NAGAO , K .  ( 1994) : Noble gases in host and inclusions from Yamato-75097(L6) , -79324 1 (L6) and -794046( H5) .  Proc . NIPR Symp. Antarct . Meteorites, 7,  197-216. NAGAO , K. and MIURA , Y. ( 1993) :  Noble gases and 81 Kr-terrestrial age of Asuka-88 1 757 lunar meteorite. Proc. NIPR Symp. Antarct. Meteorites , 6, 76-87 . NAGAO , K. , MIURA, Y. N .  and SHIMA, Masako ( 1993) :  Noble gases in Japanese chondrites: Their gas retention and cosmic-ray exposure histories. J. Mass Spectrom.  Soc . Jpn. , 4 1 ,  1 91-209. TAKAOKA, N . ,  NAGAO , K. and MIURA , Y .  ( 1993) :  Noble gases in the unique meteorites Yamato-74063 and -74357. Proc. NIPR Symp. Antarct . Meteorites, 6, 1 20- 134. ToRIGOYE, N. , YAMAMOTO , K . , MISAWA , K. and NAKAMURA , N. ( 1993) :  Compositions of REE , K,  Rb , Sr, Ba, Mg, Ca, Fe , and Sr isotopes in Antarctic "unique" meteorites. Proc. NIPR Symp . Antarct . Meteorites , 6, 100-1 1 9. WEIGEL, A. and EuGSTER, 0. ( 1994) :  Primitive trapped Xe in Lodran minerals and further evidence from EET84302 and Gibson for break-up of the Lodranite parent asteroid 4 Ma ago. Lunar and Planetary Science XXV. Houston , Lunar Planet. Inst . ,  1479- 1480 . WEIGEL, A . ,  EUGSTER, 0. , KoEBERL, C. and KRAHENBUHL, u. ( 1 994) :  The Lodranite class : Asteroid break-up and chemical composition. Meteoritics , 29, 548. YANAI, K. and KoJIMA, H. comp. ( 1 987) :  Photographic Catalog of the Antarctic Meteorites. Tokyo , Natl Inst . Polar Res. , 298p. (Received September 16, 1994; Revised manuscript received January 10, 1995) 

Noble gases, chemical composition, and cosmic-ray exposure age of the Yamato-74357 lodranite

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Noble gases, chemical composition, and cosmic-ray exposure age of the Yamato-74357 lodranite

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