6) chondrite: Chemistry, petrography, noble gases and nuclear tracks

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Kaprada L(5/6) chondrite: Chemistry, petrography, noble gases and nuclear tracks N. Bhandari a,b, S.V.S. Murty a,, R.R. Mahajan a, G. Parthasarathy d, P.N. Shukla a, M.S. Sisodia c, V.K. Rai a a

Physical Research Laboratory, Navrangpura, Ahmedabad 380009, India Basic Sciences Research Institute, Navrangpura, Ahmedabad 380009, India c Department of Geology, J.N.V. University, Jodhpur 342005, India d National Geophysical Research Institute, Council of Scientific and Industrial Research, Uppal Road, Hyderabad 500007 India b

a r t i c l e in f o

a b s t r a c t

Article history: Received 16 February 2009 Received in revised form 1 September 2009 Accepted 7 September 2009 Available online 12 September 2009

A single stone weighing about 1.6 kg fell in Kaprada village of south Gujarat, India in October, 2004. It has been studied for mineralogy, petrography, chemical and isotopic composition and cosmogenic effects. The olivine is 23.7% fayalite. The petrography, bulk chemistry and oxygen isotopic composition indicate that it belongs to L(5/6) group of chondrites. The cosmic ray exposure age of the meteorite is estimated to be 11.4 Ma based on He, Ne and Ar isotopes. A trapped nitrogen amount of 0.25 ppm with d15N = 4.7 7 0.3% is typical of ordinary chondrites of higher metamorphic grade. The gas retention ages, based on U/Th–4He and K–40Ar are calculated to be 2.6 and 4.1 Ga, respectively. Cosmic ray track data indicate that the pre-atmospheric radius of the meteorite was about 7 cm, and about 75% mass was ablated during its journey through the Earth’s atmosphere. & 2009 Elsevier Ltd. All rights reserved.

Keywords: Kaparada meteorite Noble gases Exposure age Pre-atmospheric size

1. Introduction Meteorites are broken pieces of asteroids and provide valuable information about early solar system processes. A new meteorite fall or find needs to be classified to ascertain its importance for more focused investigations. Despite the fact that the meteorite finds from cold and hot deserts have increased the world meteorite collection several fold (Grady, 2000), each meteorite fall has its special importance because it preserves its extraterrestrial characteristics which, in finds, get altered because of terrestrial weathering. We have therefore carried out a series of studies related to mineralogical, petrographical, chemical and isotopic characterization, in addition to effects of cosmic rays and determination of atmospheric ablation. The results are described here with a view to document the petrograph, chemical and isotopic characteristics of the Kaprada meteorite fall.

2. The fall of the meteorite A fully crusted stone weighing about 1.6 kg fell in the farm of Kashiram Bhikabhai Diva in Nandgam village of Kaprada Taluka (201200 20.9600 N, 731130 23.8600 E) of Valsad district in South Gujarat, India at about 16:30 h Indian Standard Time on 28th October, 2004 (Weisberg et al., 2008). The meteorite was  Corresponding author. Tel.: + 9179 26314408; fax: + 9179 26314407.

E-mail address: [email protected] (S.V.S. Murty). 0032-0633/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2009.09.008

promptly brought to Physical Research Laboratory, Ahmedabad by Mr. Manoj Pai, an amateur astronomer of Ahmedabad. The stone has approximately conical shape, the convex face containing apex having been smoothed due to ablation whereas the concave base appears rough (see Fig. 1).

3. Microscopic, chemical and mineral studies Microscopic examination of thin sections indicate that presence of abundant chondrules in the fine-grained matrix of the meteorite which is composed of olivines, clino- and orthopyroxenes and some metallic opaques as major minerals with occasional plagioclase and apatites. One thin section shows 50% (by volume) olivines, 28% pyroxenes, about 10% feldspars, 8% opaques and  1% apatites. Most of the chondrules are deformed or fragmented and generally the chondrule margins are diffuse, tending to merge with the granular groundmass (Fig. 2). A variety of chondrule types is present, the most common being granular olivine, olivine pyroxene and radiating fine-grained pyroxene. The mean composition of olivine, high and low calcium pyroxenes was determined by electron probe micro analysis (EPMA). The results of duplicate analysis are given in Table 1a–c. The olivine data indicate that it is Mg rich (Fo75.7 Fa23.7 Tp0.55). The composition of Ca- rich pyroxenes is Wo39.92 En43.3 Fs16.79 and of calcium poor pyroxenes is Wo0.7 En76.3 Fs22. Occasional sodiumrich feldspars having composition An12 Ab86 can be observed. Opaques are mostly troilite or metal.

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Fig. 1. Smooth front and rough back face of the Kaprada meteorite indicate its oriented entry into the earth’s atmosphere. The longest dimension of the stone is about 13 cm.

Fig. 2. Photomicrographs of thin section of Kaprada. Some chondrules are discernible but margins in many cases are poorly defined and show integration with the granular matrix. Crossed nicols, photograph (width 1.25 mm).

Table 1b Compositional data of Ca-rich pyroxenes. Table 1a Compositional analyses of olivines. Oxides (wt%)

Analysis 1 (N = 5)

Analysis 2 (N= 5)

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO NiO CaO

38.12 0.00 0.10 0.026 21.83 0.5 39.12 0.07 0.04

38.20 0.00 0.00 0.020 21.20 0.420 39.38 0.04 0.06

Total

99.716

99.32

End members Fo Fa Tp

75.74 23.71 0.55

76.45 23.08 0.46

Concentrations of some major elements were determined by the X-ray fluorescence spectroscopy (XRF) and K and Co were determined by the atomic absorption spectroscopic (AAS) technique. The results are given in Table 2 Based on the concentration of iron (21.5%), the fayalite composition (24–21 mol%), Ca poor pyroxene composition (Fs19.5–23.4 mol%) and microscopic studies, the meteorite is classified as belonging to L(5/6) group of ordinary chondrites (Weisberg et al., 2008). The homogeneity of olivine and low Ca pyroxene compositions indicate that the meteorite belongs to petrologic type 5 or 6.The presence of lower content of monoclinic pyroxene compared to orthorhombic

Oxides (wt%)

Analysis 1 (N =5)

Analysis 2 (N = 5)

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O

52.85 0.30 0.62 0.82 10.23 0.25 15.20 19.50 0.00 0.00

52.35 0.20 0.62 0.78 8.02 0.22 17.95 19.50 0.45 0.04

Total

99.77

100.13

End members in mol % Wo En Fs Ac

39.92 43.30 16.79 0.00

37.75 48.36 12.31 1.58

pyroxene and also the absence of hydrous component confirm that the meteorite is L(5/6) type.

4. Oxygen isotopes To further confirm the classification of this stone as L chondrite, the oxygen isotopic measurements were made at the University of California, San Diego, by fluorination technique, using a CO2 laser. Nearly 2–5 mg of powdered meteorite samples (in a batch of 2–3) were loaded in stainless steel boat along with a

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Table 1c Compositional data of Low Ca- pyroxenes. Oxides (wt%)

Analysis 1 (N = 3)

SiO2 Al2O3 FeO MnO MgO CaO Na2O

56.35 0.30 12.60 0.45 29.80 0.42 0.08

Total

100.0

End members (mol %) Wo En Fs Ac

0.80 79.39 19.52 0.28

Analysis 2 (N = 5)

Analysis 3 (N= 4)

Analysis 4 (N = 3)

55.95 0.25 13.65 0.03 29.58 0.50 0.00

55.00 0.25 14.50 0.21 29.34 0.430 0.00

54.80 0.45 15.50 0.22 28.30 0.58 0.00

99.96

99.73

99.85

0.96 78.65 20.39 0.00

0.82 77.49 21.70 0.00

1.11 75.46 23.43 0.00

Table 2 Concentration of some elements in Kaparada meteorite, determined by the XRF, except for K and Co (by AES). Element

Concentration (wt%)

Element

Concentration (wt%)

Si Fe Mg Al Ni Ca Mn

18.12 21.5 15.3 1.2 1.27 1.33 0.24

Na Cr P S Ti K ( ppm) Co (ppm)

0.7 0.3 0.14 2.27 0.07 877 600

Table 3 Oxygen isotopic composition of Kaprada. Sample

d17OSMOW (%)

d18OSMOW (%)

D17O(%)

Kaprada H4,5,6 L4,5,6 LL4,5,6

4.173 70.106 2.85 70.15 3.52 70.14 3.88 70.16

5.8357 0.061 4.08 7 0.22 4.70 7 0.24 5.04 7 0.24

1.139 0.73 70.09 1.07 70.09 1.26 70.12

Mean values for H, L and LL (Clayton et al., 1991) are given for comparison.

D17O are calculated as D17O= d17O  0.52d18O.

couple of NBS-28 samples. The lasing chamber was pumped to high vacuum and subsequently degassed by heating tape at 100 1C overnight. Before lasing, samples and chamber were preetched with 30 torr of BrF5 for an hour to remove any remaining absorbed water. Subsequently, the samples were laser fluorinated at  30 torr of pre-cleaned BrF5, by pumping off any noncondensable matter at liquid nitrogen temperature for 20 min. The evolved oxygen was collected on molecular sieve through two U traps at liquid nitrogen temperature. Oxygen isotopic compositions were measured off-line by a Finnigan MAT 251 mass spectrometer. The results of the measurement are given in Table 3. In the standard 3-isotope oxygen plot (d17O vs d18O, not shown here), the data points fall on the L chondrite fractionation line, confirming the classification based on chemical and mineral composition.

5. Noble gases and nitrogen A clean chip of the meteorite, part of which was used for chemical analysis, was analyzed for noble gases and nitrogen. The sample was wrapped in Al foil and loaded into the extraction system of the noble gas mass spectrometer. All noble gases and nitrogen were analyzed by stepwise pyrolysis, after an initial

combustion at 400 1C in 2 torr O2 using standard procedures described earlier (Murty, 1997; Murty et al., 1998). The data reported here have been corrected for blanks, interferences and instrumental mass discrimination. Blanks at all temperatures are r5% of the signal and have, within errors, near atmospheric isotopic composition. The noble gas concentrations and isotopic ratios are given in Table 4. He and Ne consist of almost pure cosmogenic and radiogenic (4He) components, while Ar is a mixture of trapped, cosmogenic and radiogenic components. Using the end member compositions suggested by Eugster (1988) for trapped and cosmogenic components in ordinary chondrites, we have derived cosmogenic (3He, 21Ne, 38Ar) and radiogenic (4He, 40Ar, 129Xe) components. These components are given in Table 5. The release pattern of N components is shown in Fig. 3. N release up to 1000 1C is dominated by trapped N with d15N= 4.7%, while in the melting step the release of cosmogenic N steeply raises the d15N. Oxygen is the principal target for the production of cosmogenic N, and the release of cosmogenic N in the melting step clearly shows its high retentivity in the main silicate phases (olivine and pyroxene). The total N content of 0.25 ppm and the composition of trapped component (4.7%) represented by the release up to 1000 1C is typical of metamorphosed ordinary chondrites (Hashizume and Sugiura, 1995).

5.1. Cosmogenic components and exposure ages Cosmogenic (22Ne/21Ne)c has a value of 1.20370.003, indicating shallow depth for the sample analyzed for noble gases. For the chemical composition of Kaprada and the shielding parameter as given by (22Ne/21Ne)c, production rates are calculated for 3He and 21 Ne following the procedure suggested by Eugster (1988). In case of 38Ar the modified procedure of Marti and Graf (1992) has been used. The exposure ages obtained for various noble gas isotopes (Table 6) are (in Ma) T3 = 10.9, T21 = 12.0 and T38 = 11.4. All these ages are comparable within the experimental uncertainties of 715% and we take the mean of these 11.471.7 Ma as the exposure age of Kaprada in the interplanetary space, before it fell on the earth. This exposure age does not coincide with the three peaks at 5, 28 and 40 Ma identified for the L chondrite exposure ages (Marti and Graf, 1992), but falls within a small cluster of 10 L chondrites, which have exposure ages of 10–12 Ma (Eugster et al., 2006). Agreement of the estimated shielding based on rare gases and tracks (discussed below) indicate that the meteorite had a simple one-stage exposure history without fragmentation in space.

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Table 4 He, Ne, Ar and N concentrations and isotopic ratios in Kaprada chondrite. 4

Temp. (1C)

22

He

Ne

36

N (ppm)

d15N (%)

3

20

Ar

He/4He (10  4)

Ne/22Ne

21

Ne/22Ne

38

Ar/36Ar

40

Ar/36Ar

(10  8 cm3STP/g) 400

430

0.103

0.064

BL

–

133 7 11

4.781 .020

0.4981 .0012

0.5192 .0158

39475 320

1000

476

1.81

0.119

0.183

4.45

227 19

1.113 .004

0.8010 .0017

1.060 .003

22753 204

331 28

1.088 .003

0.8115 .0027

0.6154 .0012

185 2

185 15

1.208 .004

0.7971 .0021

0.6791 .0027

7262 62

7 0.33 1600

19.7

1.64

0.551

0.066

101.2

3.55

0.734

0.249

30.4

7 0.1 Total

926

7 0.3

Errors in concentrations are 7 10%. Errors in isotopic composition represent 95% C.L.; BL: Blank level.

Table 5 Cosmogenic, radiogenic and trapped components (cm3 STP/g). Cosmogenic 3

He

Radiogenic

21

38

2.82

0.41

Ne

10  8 17.1

100

Ar

4

40

10  8 835

5330

He

Ar

Trapped 129

Xe

36

84

10  8 0.459

10  12 112

Ar

10  12 31

132

Kr

Xe

90

N (ppm) = 0.249 δ15N (‰) = 30.4 ± 0.3

1600°C

δ15N (‰)

75

50

Kaprada, we derive nominal U, Th–4He (T4) and K–Ar (T40) ages of 2.6 and 4.1 Ga. (Table 6). Many L chondrites have lost radiogenic 40 Ar and 4He (leading to low U–Th–4He ages compared to formation ages) and most of these belong to exposure age clusters of 5 and 28 Ma (Eugster et al., 2006). Though loss of radiogenic 4 He and 40Ar is indicated for Kaprada, the event that led to the gas loss is milder as compared to the cluster belonging to U–Th–4He age of  0.4 Ga, and/or occurred much earlier (  2.6 Ga ago) in its history. The measured ratio 129Xe/132Xe (1.12570.006) is higher than the trapped chondritic value indicating presence of radiogenic 129Xe* (from decay of 129I). The radiogenic 129Xe* (8.6  10 12 ccSTP/g) is, however, very small, indicating that the parent body attained the Xe retention temperature late in the history of the meteorite. 5.3. Trapped component

25 1000°C 0 0

25 50 75 Cumulative N release (%)

100

Fig. 3. Release pattern of nitrogen from Kaprada. A trapped component (d15N = 4.75%) is released up to 1000 1C and the cosmogenic nitrogen comes in the melting step.

Table 6 Cosmic ray exposure ages (Ma) and gas retention ages (Ga) of the Kaprada chondrite (Errors in ages are 7 15%). Cosmic ray exposure ages

Trapped He and Ne in Kaprada are negligible, while about 60% of 36Ar and 495% of 84Kr and 132Xe are of trapped origin (Tables 4 and 5). Trapped 84Kr and 132Xe fall in the range of values expected for petrologic type 6 (Schultz et al., 1990). The elemental ratios (36Ar/132Xe) =51.6 and (84Kr/132Xe)= 1.24 fall towards the lower end of the range observed for ordinary chondrites. The release patterns of the cosmogenic (21Ne, 38Ar and 15N) trapped gases (noble gases and N2) have been compared in Fig. 4. While the cosmogenic and trapped components of light noble gases (Ne, Ar) follow similar pattern (peak release at 1000 1C), those of cosmogenic N and trapped Kr and Xe have peak release at melting step, mainly reflecting the diffusion characteristics of these gas species. The trapped N on the other hand has a peak release at 1000 1C, indicating that the host phase of trapped N is a minor phase, which is more labile.

Gas retention ages

T3

T21

T38

T4

T40

10.9

12.0

11.4

2.6

4.1

5.2. Radiogenic components and gas retention ages From the radiogenic 4He and 40Ar (Table 5) and the average L chondrite concentrations (Wasson and Kallemeyn, 1988) for U (18 ppb), Th (42 ppb) and the measured K (877 ppm) content for

6. Particle tracks Olivine and pyroxene grains selected from two spots from the farthest points of the recovered specimen were polished and etched for revealing tracks using suitable etchants following the procedure of Bhandari et al. (1980). Track density in olivines were found to be about half of those in pyroxenes as expected and both the samples (KI and KII) showed comparable track density (Table 7). Using the exposure ages of 11.4 Ma, based on neon isotopes discussed above, the shielding depth is estimated to be 472 cm on either face, consistent with (22Ne/21Ne)c value

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7. Summary and conclusions 21Ne

100

c

Based on bulk and mineral chemical composition and oxygen isotopic composition, Kaprada is classified as L Chondrite. The diffuse chondrule-matrix boundaries and the trapped noble gas contents suggest the petrographic type 5/6. Mass ablation of 75%, based on nuclear track density is lower than the usual 90–95% observed for ordinary chondrites. Cosmogenic gases and track density suggest a simple one-stage exposure history for this meteorite in space. Cosmic ray exposure age of 11.4 Ma does not coincide with the well-recognized age clusters observed at 5, 28 and 40 Ma for L chondrites.

38Ar

c

15N

80

c

60 40

% Release

20 0

Acknowledgements 22Ne

80

36Ar t

t

We are grateful to Shri Manoj Pai of the Federation of Amateur Astronomers, Ahmedabad and his team for quick recovery of the meteorite and making it available for our study. Our appreciation is due to the local administration of Gujarat Government, specially Shri J.R. Patel for cooperation in collecting the stone from a remote area. We thank Lui Folco for very useful comments, which has helped improve the manuscript. We dedicate this paper to our colleague K.M. Suthar who carried out track density analysis but did not live to see the publication of this work. We also thank the Atomic Minerals Directorate, Hyderabad for making their analytical facilities available for our work. One of us (N.B.) acknowledges the Indian National Science Academy, Delhi for Honorary Scientist grant.

N2

84Kr t 132Xe

60

t

40

20

0 400

800 1200 Temperature (°C)

1600

Fig. 4. Release patterns of (a) cosmogenic and (b) trapped noble gas and nitrogen components from Kaprada.

Table 7 Nuclear tracks in olivine and pyroxene in surface samples of Kaprada. Sample #

Track density (cm  2) (# tracks) Olivine

Track density (cm  2) (# tracks) Pyroxenes

K-I K-II

2.69  106(683) 2.75  106(635)

4.65  106(574) 5.04  106(995)

discussed above. The pre-atmospheric radius, under the assumption of spherical shape of the meteoroid and bulk density of 3.5 g cm  3 (Brit and Consolmagno, 2003), is estimated to be about 7 cm, equivalent to about 5 kg. The mass ablation is estimated to be about 7575%, much lower than the usual 90–95% observed in most chondrites, based on nuclear tracks (Bhandari et al., 1980). The low ablation indicates geo-centric velocity of entry of the meteoroid in the Earth’s atmosphere of  21 kmsec  1 (Potdar, 1981).

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