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Chinese Journal of Polar Science, Vol. 18, No. 1, 27235, June 2007

Exposure ages and radiogen ic ages of ure ilite( GRV 024516) and ordinary ( GRV 024517) from Antarctica

W ang Daode (王道德 ) 1 , M iao B ingkui(缪秉魁 ) 2, 3 and L in Yangting(林杨挺 ) 2 1. Key L aboratory of Isotope Geochronology and Geochem istry, Guangzhou Institute of Geochem istry , Chinese Acad2 em y of Science, Guangzhou 510640, China 2. Key L aboratory of L ithosphere Tectonic Evolution, Institute of Geology and Geophsics , Chinese Academ y of Sci2 ence, B eijing 100029, China 3. D epartm ent of Resoures and Envirom ental Engineering, Guiling Institute of Technology, Guiling, Guangxi 541004, China

Received January 5, 2007

Abstract The GRV 024516 and GRV 024517 samp les collected from Grove Montains, Antactica are and H5 , respectively. Based on the study of m ineralogy2petrology , the cosm ic2ray exposure ages and gas retention ages of these two were determ inated and calculated. Their cosm ic2ray expo2 sure ages are 33. 3 Ma , 51. 7 Ma, and gas retention ages are 1936. 8 Ma and 3720 Ma, respectively. The ureilite contains , and amorphous C, which are mainly carrier of noble gases indicating obviously effects, which induced 40 A r partial loss. The H5 chondrite indicates thermalmetamorphism of , its gas retention age fall the range between 3220 Ma and 4510 Ma of the least shocked H5 . Key words meteorite, , noble gases, cosmogenic nuclide.

1 In troduction

Cosm ic2ray p roduced nuclides p rovide important constraints on meteorite origin, orbital evolution , and parent body histories. Especially, understanding origin of diamond in ureili2 tes have important significance. U reilites show signs of both igneous crystallization and p rim itive nebular condensation. On the one hand. , in the aspects of m ineralogy, texture, lithophile element chem istry, and Sm2Nd systematics, they appear as highly fractionated rocks: either magmatic cumulates[ 1, 2 ] or partial melt residues[ 3 ] , and thus the p roduct of p lanetary differentiation p rocesses. On the other hand, they contain high abundance of car2 bon which contains large amount of fractionated p rimordial noble gases, and metal with high abundance of trace siderophile elements, both of which are typ ical of undifferentiated chon2 dritic material[ 4 ] . In addition , contain the oxygen isotope signature of p rim itive chondrites[ 5 ] . Recently, Rai et al. [ 6 ] investigated abundances and isotop ic compositions of Ne, A r, Kr 82 W ang Daode et al. and Xe in 6 monom ict, 3 polym ict, and the diamond2free ureilite ALH78019 and their acid2 resistant, C2rich residues. 21 Ne2based cosm ic2ray exposure ages for these 10 ureilites stud2 ied range from 0. 1 Ma to 47 Ma indicating that no single consp icuous event ejected all the ureilites from their parent bodies. D iamond and amorphous are main noble gas car2 ried. In addition, p roduction rates of He, Ne and A r for H chondrites are also all attention. Laya et al. [ 7, 8 ] investigated p roduction rates of cosmogenic nuclides in H chondrites and other stony meteorites and suggested a purely physical model for the calculation of dep th2 and size dependent p roduction rates of cosmogenic nuclides by galactic cosm ic2ray parti2 cles. Based on the concentrations and isotop ic compositions of He, Ne, and A r for nonmag2 netic fraction and bulk samp les of 17 H chondrites and their 36 Cl236 A r exposure ages, 21 Ne p roduction rates as a function of 22 Ne /21 Ne and mean 38 A r p roduction rates are determ ined. They gave p roduction rate ratios P (38 A r from Ca) /P (38 A r from Fe). The radio varies be2 tween 10 and 77, and is not correlated with the absolute 38 A r p roduction or with 22 Ne /21 Ne. Because of 3 He deficits in the metal phase ismuch more p ronounced than that in the silicate m inerals. Eugster and Lorenzetti[ 9 ] systematically studied p roduction rates of cosmogenic nu2 clides of , especially of , and (HED meteorite———4 Vesta) , and the break2up events of . They also concludes experience formula for calculating p roduction rates. Recently, Eugster and Lorenzetti[ 10 ] determ ined the He, Ne and A r isotop ic abundances and cosm ic ray exposure ages in two differentiated and . They suggested evidence for a two2layer structure of the / parent . The acapulcoites correspond to the H3 and H4 chondrites and originated from exterior, whereas the lodranites sim ilar to H5 and H6 chondrites, rep resent the inner regions of their parent body. W ang et al. [ 11, 12 ] studied netron cap ture effects and p re2atmos2 pheric sizes of and determ inated the cosm ic ray exposure history of two Antarctic meteorites. GRV 024516 is ureilitte with weighing 24. 7 g; GRV 024517 is H5 ordinary chondrite with weighing 40. 5 g. They were collected in the Grove Mountains region, Antarctica by the 19 th Chinese Antarctic Expedition Team, on Jan. 2003. U reilites are the rare type in achon2 drites and consist of olivine and p igeonite (123 mm) with interstitial and vein material that contains a nuber of carbon polymorphs[ 13 ] . GRV 024516 ( ureilite) composed mainly of p i2 geonite ( En77. 1 ± 0. 4Wo9. 3 ± 0. 1 Fs13. 6 ± 0. 4 ) and olive ( Fo84. 0 ± 0. 4 ). The chem ical compositions of olivine and p igeonite are homogeneous. In the reduced rim of olivine grains there are inclu2 sions of N i2poor N i metal and sulfide. L imonite veins are common and shock stage is S2 / S3[ 13 ] . GRV 024517 (H5 chondrite) consist mainly of olivine [ (16. 0 ± 0. 4) mol% Fa ] and pyroxene [ (16. 0 ± 0. 4) mol% Fs]. In this paper, on the bases of the studing m ineralogic2petrographic features of GRV 024516 ( ureilite) and GRV 024517 (H5 chondrite) we report cosm ic2ray exposure ages and retention ages of these two meteorites, and discuss their cosm ic ray exposure history.

2 Exper im en ta l procedure

Crushed meteorite samp leswere heated in vacuum at 90 ℃ for about 10 days to remove atmospheric gases. Noble gases extraction and mass spectrometric measurements were per2 formed using our standard p rocedure[ 14 ] . Two different system of extraction and mass spec2 Exposure ages and radiogenic ages of ureilite ( GRV 024516) and ordinary ⋯ 92 trometer were used. System A contains two 60 ℃ sector2extraction mass spectrometers with glass tubes. The spectrometer for analyses of He, Ne and A r has a Faraday collector, whereas the Kr/Xe spectrometer is equipped with an additional secondary electron multip lier. System B has a different gas extraction line and two metal2tube mass spectrometers equipped with secondary electron multip liers. One of the mass spectrometers was used for analyses of He and Ne, and the other for A r. The detailed analytical p rocedure, including background and blank correction , is referred to refs[ 15, 16 ] . Analysis errors correspond to a 95% confidence level. Isotope analyses of noble gases were conducted in the Institute of Physics of the Uni2 versity of Bern.

3 Ca lcula tion of cosm ic2ray exposure age and reten tion age

One of the main goals of analysis and determ ination of cosm ic ray irradiation p roduced nuclides is calculation of cosm ic2ray exposure age. W ith concentrations of cosmogenic nu2 clides of meteorites, the cosm ic2ray exposure ages can be calculated based on below equa2 s s s - 8 3 tion: Ts = C /P , Ts for the age (Ma) , C for concentrations (10 cm STP /g) of stable cosmogenic nuclides (3 He, 21 Ne, 38 A r) , and Ps for p roduction rates of the nuclides (10 - 8 cm3 STP /g· Ma). The concentrations of cosmogenic ( c) , trapped ( tr) , and radiogenic ( r) nuclides can be determ ined from the analyses of the noble gases, using some experien2 tial isotop ic ratios. Calculated method of ordinary chondrites referred to refs. [ 11 ] . The calculation of concentrations of cosmogenic ( c ) , trapped ( tr) , and radiogenic ( r) nuclides determ ined from the analyses of the noble gases for ure2 ilites is somewhat different, their calculated methods are given in table 1.

Table 1. Methods for calculating concentrations of cosmogenic ( c) , , trapped ( tr) and radiogenic ( r) nuclids and p roduction rates. Calculated formula of concentrations of of cosmogenic ( c) , trapped ( tr) and radiogenic ( r) isotopes nuclides

22 22 22 20 22 20 22 20 22 20 22 Nec , (1) Nec = Nem × [ 1 - ( Ne / Ne) m / ( Ne / Ne) tr ] / [ 1 - ( Ne / Ne) c / ( Ne / 21 20 Nec , Netr Ne) tr ] 38 36 21 21 21 22 22 22 A rc , A rtr (2) Nec = Nem - ( Ne / Ne) tr [ Nem - Nec ] 20 20 22 20 22 T40 (3) Netr = Nem - Nec ( Ne / Ne) c 20 36 m is measured concentratios of He, Ne, A r and their ratios. If ① ( Ne / A r) tr < 1, as2 20 22 21 22 22 sump tion: ( Ne / Ne) tr = 8. 46, ( Ne / Ne) tr = 0. 035, ( 20Ne / Ne) c = 0. 8; If ②Ne, 20 22 21 22 20 22 assump tion: ( Ne / Ne) tr = 9. 80, ( Ne / Ne) tr = 0. 0290, ( Ne / Ne) c = 0. 8; If ③ 20 36 20 22 21 22 20 22 ( Ne / A r) tr > 1, assump tion: ( Ne / Ne) tr = 12. 4, ( Ne / Ne) tr = 0. 0310, ( Ne /

Ne) c = 0. 8。 22 21 20 Nec , Nec and Netr are obtained from (1) , (2) and (3). 38 36 (4) calculation of A rc and A rtr : 36 38 38 38 36 38 36 38 ( A r/ A r) c = 0. 65, A rc = A rm [ 1. 1392 - 0. 2141 ( A r/ A r) m ]; A rtr = 0. 65 × A rc 36 38 38 38 36 38 36 38 ( A r/ A r) c = 0. 63, A rc = A rm [ 1. 1343 - 0. 2132 ( A r/ A r) m ]; A rtr = 0. 63 × A rc 40 (5) T40 = 1. 805 ln[ A rr /0. 701 × K + 1 ] , K = ppm

Production rates (p) are a function of shielding parameter (22Ne /21Ne) c and target ele2 isotopes ments(HED meteorites and ureilites) 03 W ang Daode et al.

3 He (6) P’3 = 1. 15 [ Ti + Cr +Mn + Fe +N i] + 1. 75{100 - [ Ti + Cr +Mn + Fe +N i] } - 10 3 [ x] is element(wt% ) , P’3 = 10 cm STP / ( g,Ma) ] 3 22 21 (7) P ( He) vs. ( Ne / Ne) c 22 21 P3 = 0. 62 P’3 [ 2. 09 - 0. 43 ( Ne / Ne) ] - 10 3 [ x] is element(wt% ) , P3 = 10 cm STP / ( g,Ma) ]

21 Ne (8) P’21 = 1. 63 [Mg] + 0. 6 [A l] + 0. 32 [ Si] + 0. 22 [ S] + 0. 07 [ Ca ] + 0. 021 [ Fe +N i] - 10 3 [ x] is element(wt% ) , P’21 = 10 cm STP / ( g,Ma) ] 21 22 21 P ( Ne) vs. ( Ne / Ne) c 22 21 - 1 P21 = 4. 8P’21 [ 25. 7 ( Ne / Ne) - 23. 7 ] - 10 3 [ x] is element(wt% ) , P21 = 10 cm STP / ( g,Ma) ]

38 A r (9) P’38 = 1. 81 [ Ca ] + 0. 098 [ Fe +N i] + 0. 38 [ Ti + Cr +Mn ] + 2. 9 [ K ] = [ x ] is ele2 - 10 3 ment(wt% ) , P’38 = 10 cm STP / ( g,Ma) ] Calculation of p roduction rate of GRV 024516 ( ureilite) is using - 10 3 P’38 = 3. 0 × 10 cm STP / ( g,Ma) and mean p roduction rate (0. 0440) of H5 chondrite, but the concentrations of these elements in ureilites are about a factor of 1. 9 lower than that in H2chondrites its p roduction rate of 38 A rc is 0. 0440 /1. 9 = 0. 0232 × 10 - 8 cm3 STP / ( g, Ma)

( ) (22 21 ) ( isotopes Production rates p are a function of shielding parameter Ne / Ne c H, L and LL chondrites)

3 21 22 21 He, Ne, (10) P3 = F[ 2. 09 - 0. 43 ( Ne / Ne) c ] FH = 0. 98; FL, LL = 1. 00 38 22 21 - 1 A r (11) P21 = 1. 61F[ 21. 77 ( Ne / Ne) c - 19. 32 ] FH = 0. 93; FL, LL = 1. 00 22 21 (12) P38 = F[ 0. 125 - 0. 071 ( Ne / Ne) c ] FH = 1. 08; FL, LL = 1. 00 - 8 3 F is chem ical correction factor; P3 , P21 , P38 = 10 cm STP / ( g,Ma)

The mean chem ical compositions of ureilites are compared with mean chem ical com2 positons of Changde (H5) and M ianchi (H5) chondrites , and are given in table 2.

Table 2. Mean elemental compositios(wt. % ) by calculation of cosm ic2ray exposure ages for ureilites. meteorite Mg A l Si S Ca Fe Cr Ti Mn N i

ureilite ( n = 9) [ 6 ] 21. 81 0. 25 18. 60 0. 330 0. 71 14. 62 0. 508 0. 07 0. 290 0. 131

Changde chondrite [ 16 ] 13. 79 1. 15 17. 58 2. 04 1. 37 28. 21 0. 36 0. 05 0. 139 1. 8 (H5)

M ianchi chondrite [ 16 ] 14. 03 1. 11 18. 08 2. 01 1. 28 28. 38 0. 32 0. 08 0. 271 1. 69 (H5)

Mean chem ical Com2 position of the 8 14. 1 1. 15 - - 1. 26 27. 4 0. 36 - 0. 23 1. 64 H5chondrites[ 16 ]

It must point out, the determ ination of the p roduction rate for 38 A r in the ureilite is very difficult. Up to date, there yet no data of the p roduction rate for 38 A r in ureilites . Rai et [ 6 ] 3 21 al. only obtain Hec ( T3 ) and Nec ( T21 ) cosm ic2ray exposure ages of 10 ureilites. Eug2 ster and M ichel[ 17 ] investigated p roduction rates of cosmogenic nuclides of HED meteorites and they also suggested that although the 22 Ne /21 Ne ratio of are variable the ob2 Exposure ages and radiogenic ages of ureilite ( GRV 024516) and ordinary ⋯ 13 served 38 A r p roduction rates are consistant with the shielding independent formula ( table 1). Considering the p resent p recision p roduction rate calibration it is not justified to invoke 38 38 a shielding dependency for A r. But on the graph of A r p roduction rate P38 , calculated from cosmogenic 38 A r and 81 Kr2Kr exposure age, versus as calculated from chem ical abun2 dances[ 17 ] all data, allowing for the experiment errors, fall on the correlation line. They, 38 38 therefore, estimate the 2σ error for A r to be about 10%. W hen A r p roduction rate P38 of - 10 3 GRV 024516 ( ureilite) is calculated, if P3'8 = 3. 0 × 10 cm STP / ( g,Ma) chosen, then 38 38 38 A rc ( T38 ) age is 41. 6Ma ( A rc /P38 = 125. 05 /3. 0 = 41. 6 Ma). But the average A r p roduction rate for H2chondrites is (0. 0440 ± 0. 0022) × 10 - 8 cm3 STP / ( g,Ma) [ 8 ] and its shielding parameter (22 Ne /21 Ne = 1. 15) is close to GRV 024516 (22 Ne /21 Ne = 1. 16). In stoy meteorites 38 A r is p roduced exclusively by reaction on Ca, Fe , and N i. The concentra2 tions of these elements in ureilites are about a factor of 1. 9 lower than those in H2chon2 drites, therefore, the p roduction rate for 38 A r. In ureilites must also be a factor of 1. 9 lower than in H2chondrites. Therefore P (38 A r) = 0. 0440 /1. 9 = 0. 0232 ± 0. 0012 × 10 - 8 cm3 STP / ( g,Ma) , which gives exposure age of T(38 A r) = 52. 7 ± 5. 8Ma ( E2mail communication). The retention age of 40 A r ( radiogenic 40

A r age) is calculated from T40 = 1. 805 ln [ 40A rr /0. 701 × K (ppm ) + 1 ]. For H5 ordinary chondrite ( GRV 024517) , K 900 × 10 - 6 ; for ureilite ( GRV 024516) , K 80 × 10 - 6 [ 18 ] .

4 Results and d iscussion

The analyses of isotope of He, Ne and A r and their ratios of ureilite ( GRV 024516) and H25 chondrite ( GRV 024517) are listed in table 3. Table 4 shows their calculated con2 centrations of the cosmogenic, trapped, and radiogenic nuclides, and table 5 shows the deter2 3 21 38 m ined p roduction rates ( P3 , P21 , P38 ) of cosmogenic He, Ne and A r, their exposure a2 ges, and the gas retention ages.

Table 3. Results of He, Ne and A r measurements(10 - 8 cm3 STP /g) for ureilite ( GRV 024516) and ordinary chondrite ( GRV 024517).

Samp le 4 20 22 36 40 4 3 20 40 He Ne Ne A r A r 36 meteorites weight He He Ne A r 3 22 21 38 36 A r (mg) He Ne Ne A r A r

GRV 024516 303 ± 54. 5 ± 10. 7 ± 108 ± 1. 09 ± 1. 20 ± 5. 27 ± 605 ± 110. 77 5. 5596 0. 1785 ( ureilite) 13 2. 4 0. 4 5 0. 02 0. 02 0. 22 31

GRV 024517 75. 9 ± 19. 2 ± 4330 ± 1. 01 ± 1. 12 ± 1. 92 ± 3. 64 ± (ordinary 177. 36 n. d 3. 2 0. 8 180 0. 02 0. 02 0. 08 0. 17 chondrite, H5)

The most widely used shielding indicator is ( 22 Ne /21 Ne) c, which correlates with (3 He /21 Ne) c, with least squares fit for 138 chondrites been caculated and the aquation re2 sulted as following: 3 21 22 21 [ 19 ] ( He / Ne) c = 21. 77 ( Ne / Ne) c - 19. 32 22 21 This correlation is valid in the range for the ( Ne / Ne) c 23 W ang Daode et al.

Table 4. Cosmogenic, trapped and radiogenic noble gases( concentrations in 10 - 8 cm3 STP /g) for ureilite ( GRV 024516) and ordinary chondrite ( GRV 024517)

22 3 21 22 38 Ne 20 36 4 40 meteorites Hec Nec Nec A rc 21 Netr A rtr Her A rr Ne

GRV 024516 1) 54. 5 ± 2. 4 8. 17 ± 0. 17 9. 445 1. 224 ± 0. 12 1. 16 3. 144 0. 8128 83. 0 ( ureilite) 30. 5

GRV 024517 4330 - (ordinary - 16. 9 ± 0. 4 18. 46 1. 374 ± 0. 01 1. 094 4. 416 0. 899 - 0. 28 = chondrite, H5) 4329. 722) 3 4 [ 6 ] 4 4 Note: 1) If take ( He / He) c = 0. 2 , then Hec = 54. 5 /0. 2 = 272. 5; Her = 303 - 272. 5 = 30. 5 40 38 2) If take ( A r/ A r) c = 0. 2; then40A rc = 1. 374 × 0. 2 = 0. 275.

- 8 3 Table 5. Cosm ic2ray exposure ages and gas retention ages , and p roduction rates( P3 , P21 , P38 = 10 cm STP / g,Ma) for ureilite ( GRV 024516) and ordinary chondrite ( GRV 024517)

T3 T21 T38 T40 meteorites P3 P21 P38 (Ma) (Ma) (Ma) (Ma)

[ 6 ] [ 6 ] [ 6 ] [ 6 ] [ 6 ] 0. 282. 0. 030. 29. 40. 8. GRV 024516 1. 639. [ 6 ] ( P’21 = 0. 421) 0. 0232 ± 33. 8 32. 7 ± 52. 7 ± 1936. 8 ( ureilite) ( P’ = 1. 633) [ 7, 8 ] [ 7, 8 ] [ 7, 8 ] 3 0. 25[ 7, 8 ] 0. 0012 6. 5 5. 8

GRV 024517 0. 328 ± 0. 0440 ± 51. 5 ± 31. 2 ± (ordinary chon2 - [ 7, 8 ] [ 7, 8 ] [ 7, 8 ] [ 7, 8 ] 3720 drite, H5) 0. 045 0. 0022 7. 4 2. 0

ratio between about 1. 06 and 1. 30. It may used for calculation of the cosm ic2ray exposure ages and allows to recognize meteoriteswhich m ight have suffered 3 He diffusion loss, as they fall below the correlation line. The (22 Ne /21 Ne) c ratio is 1. 16 for GRV 024516 ( ureilite 3 21 ) and it has no diffusion loss. The Hec ( T3 ) age is 33. 8 Ma consisting with Nec ( T21 ) age 38 3 21 ( 32. 7 Ma) , but its A rc ( T38 ) age (40. 8 Ma or 52. 7 Ma) is different from Hec and Nec ages. The p robably errors caused by heterogeneous of concentrations of target elements. In 21 38 38 general, N ec ( T21 ) age is more reliable than the A rc ( T38 ) age ( possible trapped A r = 0. 15 × 10 - 6 STP /g) . Therefore, the average exposure age of this ureilite is 33. 3 M a. 3 4 There no diffusion loss for Hec in the GRV 024517 (H5 chondrite) , but He content 3 exceed detection lim it, so the Hec cosm ic2ray exposure agecan not be caculated without da2 4 22 21 22 21 ta of He. The determ ined ( Ne / Ne) ratio is 1. 12 closed to cosmogenic ( Ne / Ne) c ratio (1. 094). Thus, the assump tion whether the trapped component is of atmospheric or 21 p lanetary compositions does not change the results. The Nec ( T21 ) age of GRV 024517 21 38 (H5 chondrite) is 51. 5 ± 7. 4 Ma. But ages determ ined via Nec and A rc disagree with it, one p robable reason is, that 38 A r is not homogeneously distributed in meteorites. In addi2 tion, gas retention age falls beween 3220 Ma and 4510 Ma of the least shocked H5 chon2 drites. The relative abundance pattern of noble gases in ureilites is of the fractionated p laneta2 ry type, the same as in carbonaceous chondrites[ 20, 21 ] , though gas content vary considerable Exposure ages and radiogenic ages of ureilite ( GRV 024516) and ordinary ⋯ 33

( for examp le, Xe content vary by a factor of ～100 ) in bulk samp les. Analysis of the carbo2 naceous matrix or vein material showed the noble gases to be enriched at least 600 fold rela2 tive to the silicates, and indicated that the gases are largely contained in carbon. Rai et al. [ 6 ] point out, in diamond2bearing ureilites, diamond is the p rincipal gas carrier and graphite is virtually free of trapped gases[ 22 ] . The diamond 2free ureilite has also trapped no2 ble gases , which are carried by fine2grained carbon whose structural state is unknown. The distribution of cosm ic2ray exposure ages of 28 ureilites are as follows[ 6 ] . Excep t for two small clusters as at 1 ± 1 Ma ( n = 6) and 10 ± 1 Ma ( n = 5) , for the rest the exposure ages are distributed between 1Ma and 47Ma, indicating no obvious single event thar all the ureili2 tes ejected from their parent bodies. W hereas the diamond2free monom ict ureilite (ALH78019) has the lowest exposure age (0. 1 Ma) , but the polym ict ureilite which is also diamond2free has the largest exposure age of 47 Ma. About 60% of ureilites are having cos2 m ic2ray exposure ages less than 10 Ma. This indicats that ureilites, in general, has low cos2 m ic2ray exposure ages. The cosm ic2ray exposure ages of four polym ict ureilites ( EET83309: 47 Ma; EET87720: 8. 9 Ma, N ilpena: 9. 8 Ma; DaG319: 21. 5 Ma) are dif2 ferent. And reveal that they were ejected from their parent body. [ 6 ] in separate events. The most of the Ne is cosmogenic in origin. The small amount of trapped Ne could be mostly due to the Ne component from diamond and , , the main noble gas carriers. Thus, the trapped Ne content and its variable amount among bulk ureilites m ight be a direct reflection of fractional abundance of the carbon. GRV 024516 is a ureilite with diamond and amorphous carbon. Graphite, diamond and amorphous carbon are a nebular condensate under reduced condition. O livine and clinoau2 gite are a directly condensed from gas phase at same location or the different location and at same time, but their condensations are later than that of carbon. A ll three carbon phases have been p roduced from the gas phase in the nebula and later have been incorporated into ureilites. That is to say, the diamond formation is unrelated to parent body p rocesses, and hence that the diamond is not p roduced in situ[ 23 ] . In addition, the composition of nitrogen in diamond is very sim ilar to other cases of diamond2bearing ureilites. Thus, mere p resence of diamond in ALH82130 of low shock grade and in the almost unshock ureilite DaG 868[ 24 ] p rovides anather agrumem t against the role of shock in the formation of diamond. Since diamond discovered in ureilites, its origin has been puzzle. Many of the features of ureilites indicate that the diamond was p roduced in situ by shock conversion of graph2 ite[ 23 ] . A lthough most ureilites show evidence of shock, it is not clear whether the shock is really responsible for diamond formation or whether the diamond was already p resent when ureilites underwent the shock event. Fukunaga andMatsuda[ 25 ] have interp reted the diamond as a nebular origin for it can be p roduced by chem ical vapor deposition in an artificial noble gas atmosphere under low2p ressure p lasma condition. U reilites contain coarse (m illimeter2size) olivine and pyroxene grains that either crys2 tallized from or equilibrated with melts at high temperatures. Cohenite [ ( Fe, N i) 3 C ] is p resent inside metallic spherules which occur within olivine and p igeonite grains in several ureilites[ 26 ] , indicating that the melts from which ureilites formed were carbon rich. It dem2 onstrates that carbon was indigenous to the ureilite parent body[ 27 ] . The carbon polymorphs are p resent in many ureilites, Haver«contains chaoite (C) along with diamond and lonsda2 43 W ang Daode et al.

leite (Vdovykin 1972). A ll three carbon polymorphs were formed by shock[ 28, 29 ] . Shock p ressure of at least 100 GPa appear to have been involved (Carter et al. 1968). H igh con2 centrations of p lanetary2type noble gases are p resent in chondrites, but not in basaltic a2 chondrites. The high concentrations of p lanetary2type noble gases in ureilites[ 21, 30, 31, ] are consistent with very brief heating ep isodes and rap id burial m inim izing the time available for noble gases to escape[ 27 ] . A s can be seen, The diamond is major carrier of noble gases. Is origin has been a top2 ic of great debate which is still not settled. The are p robably either p roduced from gas2rich amorphous carbon ( nebular condensation origin) or converted from graphite into diamond phase by shock effect.

Acknowledgem en ts W e are very grateful to Prof. Ingo. Leya and his group in the Institu2 te of Physics of the University of Bern, for help s in noble gas isotope measurements. This work was supported by the National Natural Science Foundation of China ( Grant No. 40473037).

References

[ 1 ] Berkley JL, B rown HG, Keil K et al. (1976) : The Kenna ureilite: An ultramafic rock with evidence for igneous, metamorphic, and shock origin. Geochim Cosmochim Acta, 1976, 40: 142921437. [ 2 ] Goodrich CA, Jones JH and Berkely JL ( 1987) : O rigin and evolution of the ureilite parent magmas: Multi2stage igneous activity on a large parent body. Geochim Cosmochim Acta, 1987, 51: 225522273. [ 3 ] Scott ERD, Taylor G and Keil K(1993) : O rigin of ureilite meteorites and imp lications for p lanetary ac2 cretion. Geophys Res. Lett. 20: 4152418. [ 4 ] Sp itz AH and Boynton WV (1991) : Trace element analysis of ureilites: New constraints on their petro2 genesis. Geochim Cosmochim Acta, 55: 341723430. [ 5 ] Clayton RN and Mayeda TK(1988) : Formation of ureilites by nebular p rocesses. Geochim Cosmochim Acta, 52: 131321318. [ 6 ] Rai VK, Murty SVS and O tt U ( 2003) : Noble gases in ureilites: Cosmogenic, radiogenic, and trapped components. Geochim Cosmochim Acta, 67: 443524456. [ 7 ] Leya I, Lange HJ, Neumann S, W ieler R and M ichel R (2000) : The p roduction of cosmogenic nuclides in stony meteoroids by galactic cosm ic2ray particles. Meteorit Planet Sci, 2000, 35: 2592286. [ 8 ] Leya I, Graf TH, N ishiizum i K and W ieler S(2001) : Cosm ic2ray p roduction rates of helium, neon and argon isotopes in H chondrites based on chlorine236 /argon236 ages. . Meteorit Planet Sci, 36: 9632973. [ 9 ] EugsterO and Lorenzetti S(2005) : Codm ic2ray exposure ages of four acapulcoites and two differentiated achondrites and evidence for a two2layer structure of the acapulcoite / lodranite parent asteroid. Geochim Cosmochim Acta, 69: 267522685. [ 10 ] Eugster O and Lorenzetti S(2005) : Codm ic2ray exposure ages of four acapulcoites and two differentiated achondrites and evidence for a two2layer structure of the acapulcoite / lodranite parent asteroid. Geochim Cosmochim Acta, 69: 267522685. [ 11 ] W ang DD, L in YT, L iu XH and Ju YT(2003) : Cosm ic2ray exposure histories of two Antarctic meteor2 ites from Chinese collections and the Guangm ingshan and Zhuanghe chondrites. Science in china ( Series D) , 46 (12) : 123621240. [ 12 ] W ang DD and L in YT(2003) : Neutron cap ture effects and p re2atmospheric sizes of meteoroids. Chinese Science Bulletin, 48 (20) : 223622240. [ 13 ] Goodrich CA (2004) : Scott Edward R. D and Fioretti A M. U reilitic : clues to the petrologic structure and impact disrup tion of the ureilite parent asteroid. Chem ie der Erde, 64: 2832327. [ 14 ] Eugster O, M ichel T, N iedermann S et al. (1993) : The record of cosmogenic, radiogenic, fissiogenic, Exposure ages and radiogenic ages of ureilite ( GRV 024516) and ordinary ⋯ 53

and trapped noble gases in recently recovered Chinese and other chondrites. Geochim Cosmochim Acta, 57: 111521142. [ 15 ] Eberhardt P, Geiss J, Graf H et al. (1970) : Trapped solar wind noble gases, exposure age and K/A r2 age in Apollo 11 lunar fine material. Proc Apollo 11 Lunar Sci Conf, 103721070. [ 16 ] W ang, D, ChenY, L i Z et al. ( 1993) : Introduction of Chimese Meteorites ( in Chinese) , Beijing: Sci2 ence Press, 505. [ 17 ] Eugster O andM ichel TH (1995) : Common asteroid break2up events of eucrites, , and howard2 ites and cosm ic2ray p roduction rates for noble gases in achondrites. Geochim Cosmochim Acta, 59: 1772 199. [ 18 ] Mason B (1979) : Geochem istry Part 12Meteorites. In Data of Geochem istry ( ed. . F. Fleischer) , B1～ B132. Geological Survey Professional Paper 4402B21,W ashington DC, USA. [ 19 ] Eugster O (1988) : Cosm ic2ray p roduction rates for 3 He, 21 Ne, 38 A r, 83 Kr and 126 Xe in chondrites based on 81Kr2Kr exposure ages. Geochim. Cosmochim. Acta, 52 (6). [ 20 ] W eber HW (1971) : H intenberger H, and Begemann F. Noble gases in the Haver«ureilite. Earth Planet Sci. Lett. , 13: 2052209. [ 21 ] Gobel R ( 1978) : O tt U, and Begemann F. On trapped noble gases in ureilites. J. GeophysRes. , 83: 8552867. [ 22 ] Rai VK, Murty SVS, and O tt U ( 2002) : N itrogen in diamond2free ureilite A llan H ills 78019: Clues to the origin of diamond in ureilites. . Meteorit Planet Sci, 37: 104521055. [ 23 ] Rai VK, Murty SV and O tt U (2003) : N itrogen components in ureilites Geochim. Cosmochim. Acta, 67 (12) : 221322237. [ 24 ] Takeda H (1987) : M ineralogy of Antarctic ureilites and a working hypothesis for their origin and evolu2 tion. Earth Planet Sci. Lett. , 81: 3582370. [ 25 ] Fukunaga K and Matsuda J I( 1997) : Vapor2growth carbon and the origin of carbonaceous material in ureilites. Geochim J. , 31: 2632273. [ 26 ] Goodrich CA and Berkley JL ( 1986) : Primary magmatic carbon in ureilites: Evidence from cohenite2 bearing spherules. Geochim Cosmochim Acta, 50: 6812691. [ 27 ] Rubin AE (2005) : Shock and annealing ureilites. 68 th AnnualMeteoritical SocietyMeeting, 5007. [ 28 ] Vdovykin GP (1970) : U reilites. Space Sci. Rev. , 10: 4832510. [ 29 ] Vdovykin GP (1976) : Haver«meteorite. Space Sci. Rev. , 18: 7492776. [ 30 ] W aker JF (1986) : Noble gases in the diamond2free ureilite, ALH78019: The roles of shock and nebular p rocesses. Geochim Cosmochim Acta, 50: 6332642. [ 31 ] W eber HW (1971) : H intenberger H, and Begemann F. Noble gases in the Haver«ureilite. Earth Planet Sci. Lett. , 13: 2052209.