A stony meteorite fell in the village Devgaon, Bastar district, Chattisgarh, Central India on 2001 February 12. The meteorite was fully covered with fusion crust and weighed about 12 kg. Chemical composition, cosmogenic nuclear tracks, radionuclides and noble gases have been studied to determine its classification, the preatmospheric size and the irradiation history

Bhandari, N.

Murty, S. V. S.

Rai, V. K.

Shukla, A. D.

Shukla, P. N.

Srinivasan, G.

Suthar, K. M.

English

Publications of the IAS Fellows

COSMOGENIC RECORDS IN THE RECENTLY FALLEN
DEVGAON (H4) CHONDRITE. S.V.S. Murty*, V.K. Rai, A. D.
Shukla, G. Srinivasan, P.N. Shukla, K.M. Suthar and N. Bhandari,
Physical Research Laboratory, Navrangpura, Ahmedabad 380009,
India. (*e-mail: murty@prl.ernet.in).
     A stony meteorite fell in the village Devgaon, Bastar district,
Chattisgarh, Central India on 2001 February 12. The meteorite was
fully covered with fusion crust and weighed about 12 kg. Chemical
composition, cosmogenic nuclear tracks, radionuclides and noble
gases have been studied to determine its classification, the
preatmospheric size and the irradiation history.
     Chemical composition and petrography: Several major, minor
and trace elements were measured using ICPAES, AAS and INAA
techniques. Based on elemental concentrations of siderophile
elements Fe (26.65%), Ni (1.75%), Co (821 ppm), Ir (824 ppb), Os
(824 ppb) and Au (170 ppb), the meteorite is classified as H group
chondrite[1]. Other elements such as Mg, Al and K also lie within
the range observed for H chondrites[1]. The meteorite contains
numerous chondrules varying in size between 0.2 to 1 mm.  Many
chondrules have metallic rim, some of which are degraded,
indicating metamorphic grade 4. Irregular fractures have been
noticed within the olivine grains inside the chondrules suggesting
S2 stage of shock.
     Noble gases: He and Ne are mainly cosmogenic with
(22Ne/21Ne) c = 1.182, indicative of small shielding.  (
3He/21Ne)c =
4.5 and (21Ne/38Ar)c = 5.3 indicating partial loss of He and Ne.
Using the chemical composition of Devgaon and adopting the
production rates [2, 3] we obtain the following cosmic ray exposure
ages (Ma): T3 (43.4), T21 (63.8) and T38 (69.4). Exposure ages based
on spallogenic 83Kr, and 126Xe yield T83 = 103.5 Ma and T126 = 111
Ma. Excesses at 80Kr, 82Kr and 128Xe accompanied by high
129Xe/132Xe values have been observed suggesting neutron
produced contributions from Br and I respectively. Assuming
normal chondritic ratio of 35Cl/81Br ~ 3000 [4], we estimated the
36ArCl from 
82KrBr and recalculated the value of T38 to be 88.4 Ma in
agreement with T83 and T126, within experimental uncertainities.
The average of T38, T83 and T126 of 101 Ma may represent the
integrated cosmic ray exposure age of Devgaon meteorite. Very
few chondrites have such large exposure ages. The elemental ratios
of trapped noble gases 84Kr/132Xe = 0.85 and 36Ar/132Xe = 53.6 are
in the range observed for ordinary chondrites, and the absolute
amount of 132Xe = 1180x10-12 ccSTP/g is consistent with its
metamorphic class as H4[5].
     Radionuclides and nuclear tracks: Twelve radionuclides
including short-lived 52Mn (5.3 d) were measured using their
characteristic γ-rays on a large volume hyper-pure Ge-detector.
60Co activity is low (≤0.5 dpm/kg) indicating low thermal neutron
fluxes and hence it can be inferred that the meteoroid was a small
object during the terminal phase of its cosmic ray exposure.
Olivines from five spot samples below the crust were studied for
nuclear tracks. The track density varies between (4.6 to 9) x106
cm2, showing a small gradient within the meteorite and ablation of
about 4.8 to 6.8 cm, if we assume single stage exposure of 101 Ma.
However, low 60Co, high (22Ne/21Ne)c coupled with large neutron
produced excesses at 80Kr, 82Kr and 128Xe suggest a complex
exposure history possibly involving a long duration shielded
exposure followed by a shorter duration recent exposure as a small
object. Therefore the estimated cosmic ray exposure age may
represent an integrated exposure during various stages.
     References: [1] Wasson J.T. and Kallemeyn G.W. (1988) Phil.
Trans. R. Soc. Lond. A 325, 535-544. [2] Eugster O. (1988) GCA
52, 1649-1662. [3] Marti K and Graf Th. (1992) Ann. Rev. Earth
Planet. Sci. 20, 221-243. [4] Mason B. (1979) USGS Prof. Paper
440-B-1. [5] Schultz L. et al., (1990) Meteoritics 25, 405-406.
65th Annual Meteoritical Society Meeting (2002) 5045.pdf


Cosmogenic records in the recently fallen Devgaon (H4) chondrite

Indian Academy of Sciences

http://repository.ias.ac.in/89264/1/30-Pub.pdf

Cosmogenic records in the recently fallen Devgaon (H4) chondrite

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