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Fossil track studies of the Omolon pallasite crystals
Radiation Measurements, Vol. 28. Nos I-6, pp. 337-340, 1997
Pergamon
P l l : S 1350-4487(97)00095-4
FOSSIL
TRACK
O 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1350-4487/97 $17.00 + 000
STUDIES OF THE OMOLON CRYSTALS
PALLASITE
J. V. BONDAR, I V. P. PERELYGIN 2 AND R. I. PETROVA 2 t Institute of Geochemistry and Physics of Minerals, Kiev, Ukraina 2 Joint Institute for Nuclear Research, 141980, Dubna, Russia ABSTRACT The olivine and stanfildite crystals from Omolon pallasite meteorite were selected from some residue and from locations of an about 150 g fragment of that meteorite in the Meteoritic Committee, Moscow, Russia. The radiation age of Amnion pallasite was determined to be about 78 MY The olivine crystals were mounted in epoxy resin, polished and etched in Krishnaswami solution, modified by adding 30 g per liter oxalic acid. The etching of the olivine crystals was performed in hermetically closed vessels during 36-72 hours at T = 100 o C. The measured Fe- group track density varies fi'om 3 x 10 4 up to 2.6 x 10 5 track, cm"2. The measure in 12 Fe-group track rich olivine 43 track due to VVH nuclei were found. It means that the depth of ablation of Omolon meteorite does not exceed (8.2 :t 2. I) cm out of preatmospheric surface for the most favourable locations. KEYWORDS Crystal track detectors; meteorites; cosmic ray nuclei; fossil tracks.
INTRODUCTION The nonconducting crystals of extraterrestrial origin are able to register and to store during many million years the tracks, due to galactic cosmic rays nuclei with Z > 20 ( M. Maurette et al, 1964 ). In our previous study it has been shown that certain meteorites - pallasites, which contain about 50 % of large transparent and homogeneous olivine crystals could be used for detailed investigations of galactic cosmic ray nuclei with 22< Z _<92 ( Perelygin, 1995 ). The track studies of meteorite- pallasites with different radiation age allows one to get the unique information about the history of galactic cosmic ray- the variations of its chemical composition and energy variations in the f'me interval up to a few hundred million years ago. The other source of fossil tracks in nonconducting pallasite matrix crystals, in particularly- phosphates, which, contrary to olivine crystals, originally contains also spontaneously fissioning actinide is the spontaneous fission of nuclei 23s U and now extinct 244 Pu. / The investigation of these tracks provides useful information about the track age of the parent body of pallasites (Bondar and Semenenko, 1994 ). The fall down ofpallasite Omolon was observed in 1981 in Magadan region, Russia. It has been found next year in the basin of river Kegaly, tributary of Omolon river in deserted region of tundra. The total mass ofOmolon pallasite is about 250 kg, the main mass of this meteorite now is in collection of Magadan museum of natural history. The preliminary study of olivine crystals shows that the meteorite belongs to main group of pallasites with rather low concentration of fajalite about 12,3 %. The irradiation age of this meteorite was determined to be equal to (78:1:7 ) M. Y.-close to that of other stony-iron meteorites - Patwar, Thiel Mountain, Springwater, Admire (Shukoljukov et al., 1992). By studying the rare gas isotopic
i. z,:,16~
337
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
338
composition Shukoljukov and his colleagues have also concluded that preatmospheric body of Omolon pallasite was rather big - with diameter about 1.5 - 2 meters and with mass 500 - 100 tons. It means that the main mass of this meteorite has been lost due to the processes of breakdown of its origin body in upper layers of atmosphere and subsequent ablation of a smaller fragments. EXPERIMENTAL
The olivine crystals were taken from small fragments of Omolon meteorite in Committee of Meteorites, Vernadcky Institute, Moscow and nonlocated 120 g portion of these crystals, Magadan museum collection. The portion of about 5 g. olivine crystals was mounted in epoxy resin, grinded and polished. These crystals were than etched in order to get the Fe- group tracks with diameter 3-4 lam. In our study we have used Krishnaswarni etchant ( Krishnaswami et al., 1971) modified by adding 30 g. of oxalic acid per 100 cm 3 of solution ( Perelygin, 1995 ). The pH of the solution was 7.9 + 0.05; the etching of olivine crystals was performed in hermetically closed volume in order to prevent the evaporation of original solution at 100 °C during 24 - 72 hours. Such a procedure provides a rather high etching rate along the heavy nuclei tracks V~/Vb --- 120, and the reproductivity of the etching process. The Fe- group track density measured in 120 olivine crystals varies from 3 x I 0 4 up to 2,6 x 10 5 tracks per cm 2. We have measured also the volume etchable track length of about 200 tracks of Fe- group nuclei in these crystals by using TINT and TINCLE method. The Fe- group track spectrum in olivine crystals of Omolon pallasite is presented at Fig. 1.
3025m
2015 z
10
' I
0
r-n
0
IHn !
4
8
12 Track length, lan
i
16
ml-"l !
20
Fig. 1. The volume etchable track length. (VETL) of Fe- group galactic cosmic ray nuclei in olivine crystals of Omolon pallasite meteorite. The mean track length of these tracks (about 8 gin) was compared with the measured earlier Fe- group tracks in olivines from pallasites Marjalahti - 7.8 gin, Eagle Station - 10.3 pan, Lipovsky Khutor - 9.2 gm (Perelygin and Kashkarova, 1979 ), olivine crystals from Moon surface - 9.6 gm. The mean etchable track length of accelerated S4Cr and 56Fe nuclei was determined to be equal 9.5 and 13.6 gm. correspondingly. It means that the partial annealing of fossil Fe- group tracks in Omolon pallasite olivines was more significant, than in other extraterrestrial olivine crystals with exception of Marialahti pallasite. This effect can be explained by periodical heating of meteorite Omolon at perigee near the Sun. We have found also tracks with length > 40 gm in 12 track- rich olivine crystals ( Fe-group track density > 2 • 10 ~ tracks cm "2). These tracks were produced by slowing down to rest o f Z > 36 galactic cosmic
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
339
ray nuclei (VVH group of cosmic ray nuclei).The total number of measured VVH tracks was 43. The ratio of track densities due to W H and VH groups of cosmic ray nuclei was determined to be N VVH/ N VH = ( 2.1 ± 0.5 )" 10"4. In our previous study we have measured the experimental dependence of the ratio of Nvv,/Nv, track densities with the depth in preatmospheric body of pallasite type of meteorites. (Perelygin and Stetsenko, 1981).
10-3
;Z Z
104 0
5
10
15
20
Depth, cm
Fig. 2. The dependence of ratio of track densities due to VVN and VH galactic cosmic ray nuclei N w'H / N vH on the depth in pallasite type of meteorites for preatmospheric surface. As one can see from Fig. 2 the obtained value (2.1 + 0.5) .10 -4 corresponds to the depth 8.9 + 2.1 cm to the preatmospheric surface ofpallasite Omolon. CONCLUSION Taking in account rather high radiation age of this meteorite - ( 78 + 7 ) M. Y. one can conclude that this meteorite can be used for fossil track studies of 50 _50 groups of galactic cosmic ray nuclei. These variations obviously take place when the Solar system crosses the spiral arm of our Galaxy about 80 M.Y. ago, due to the nearby explosions of Supernova stars (about 3-4 explosions in close system, see Perelygin, 1995). These explosions can provide the additional irradiation of Omolon meteorite crystals with freshly formed and rather short-lived nuclides (Tt/2 > 10a years).
340
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
Acknowledgments: The authors wish to express their thanks to prof. Yu. A. Shukoljukov for his help, V. P. Semenenko for useful advises and G.P. Knjazeva for proper etching and scanning of olivine crystals. The authors are thankful to International Science Foundation and Russian Foundation of Fundamental Research for financial support of our work during 1994-1994 years.
REFERENCES Bondar Yu. V. and Semenenko V. P. (1994) The thermal history of pallasites basing on mineralogical and geochemical analysis. Abstracts of XXII Meteoritic Conference, 6-8 December, 1994, Chemogolovka, pp. 20-2 i. Krishnaswami S., Prabhu N., Tamhane A.S. and Lal D. (1971) Olivine revelation of tracks of charged particles. Science 74, 287-29 I. Maurette M., Pellas P. and Walker R. M.(1964) Cosmic ray induced particle tracks in a meteorite. Nature 204, 821-823. Perelygin V. P. and Kashkarova V. T. (1979) The thermal history of pallasites Marjalahti, Lipovsl
Pergamon
P l l : S 1350-4487(97)00095-4
FOSSIL
TRACK
O 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1350-4487/97 $17.00 + 000
STUDIES OF THE OMOLON CRYSTALS
PALLASITE
J. V. BONDAR, I V. P. PERELYGIN 2 AND R. I. PETROVA 2 t Institute of Geochemistry and Physics of Minerals, Kiev, Ukraina 2 Joint Institute for Nuclear Research, 141980, Dubna, Russia ABSTRACT The olivine and stanfildite crystals from Omolon pallasite meteorite were selected from some residue and from locations of an about 150 g fragment of that meteorite in the Meteoritic Committee, Moscow, Russia. The radiation age of Amnion pallasite was determined to be about 78 MY The olivine crystals were mounted in epoxy resin, polished and etched in Krishnaswami solution, modified by adding 30 g per liter oxalic acid. The etching of the olivine crystals was performed in hermetically closed vessels during 36-72 hours at T = 100 o C. The measured Fe- group track density varies fi'om 3 x 10 4 up to 2.6 x 10 5 track, cm"2. The measure in 12 Fe-group track rich olivine 43 track due to VVH nuclei were found. It means that the depth of ablation of Omolon meteorite does not exceed (8.2 :t 2. I) cm out of preatmospheric surface for the most favourable locations. KEYWORDS Crystal track detectors; meteorites; cosmic ray nuclei; fossil tracks.
INTRODUCTION The nonconducting crystals of extraterrestrial origin are able to register and to store during many million years the tracks, due to galactic cosmic rays nuclei with Z > 20 ( M. Maurette et al, 1964 ). In our previous study it has been shown that certain meteorites - pallasites, which contain about 50 % of large transparent and homogeneous olivine crystals could be used for detailed investigations of galactic cosmic ray nuclei with 22< Z _<92 ( Perelygin, 1995 ). The track studies of meteorite- pallasites with different radiation age allows one to get the unique information about the history of galactic cosmic ray- the variations of its chemical composition and energy variations in the f'me interval up to a few hundred million years ago. The other source of fossil tracks in nonconducting pallasite matrix crystals, in particularly- phosphates, which, contrary to olivine crystals, originally contains also spontaneously fissioning actinide is the spontaneous fission of nuclei 23s U and now extinct 244 Pu. / The investigation of these tracks provides useful information about the track age of the parent body of pallasites (Bondar and Semenenko, 1994 ). The fall down ofpallasite Omolon was observed in 1981 in Magadan region, Russia. It has been found next year in the basin of river Kegaly, tributary of Omolon river in deserted region of tundra. The total mass ofOmolon pallasite is about 250 kg, the main mass of this meteorite now is in collection of Magadan museum of natural history. The preliminary study of olivine crystals shows that the meteorite belongs to main group of pallasites with rather low concentration of fajalite about 12,3 %. The irradiation age of this meteorite was determined to be equal to (78:1:7 ) M. Y.-close to that of other stony-iron meteorites - Patwar, Thiel Mountain, Springwater, Admire (Shukoljukov et al., 1992). By studying the rare gas isotopic
i. z,:,16~
337
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
338
composition Shukoljukov and his colleagues have also concluded that preatmospheric body of Omolon pallasite was rather big - with diameter about 1.5 - 2 meters and with mass 500 - 100 tons. It means that the main mass of this meteorite has been lost due to the processes of breakdown of its origin body in upper layers of atmosphere and subsequent ablation of a smaller fragments. EXPERIMENTAL
The olivine crystals were taken from small fragments of Omolon meteorite in Committee of Meteorites, Vernadcky Institute, Moscow and nonlocated 120 g portion of these crystals, Magadan museum collection. The portion of about 5 g. olivine crystals was mounted in epoxy resin, grinded and polished. These crystals were than etched in order to get the Fe- group tracks with diameter 3-4 lam. In our study we have used Krishnaswarni etchant ( Krishnaswami et al., 1971) modified by adding 30 g. of oxalic acid per 100 cm 3 of solution ( Perelygin, 1995 ). The pH of the solution was 7.9 + 0.05; the etching of olivine crystals was performed in hermetically closed volume in order to prevent the evaporation of original solution at 100 °C during 24 - 72 hours. Such a procedure provides a rather high etching rate along the heavy nuclei tracks V~/Vb --- 120, and the reproductivity of the etching process. The Fe- group track density measured in 120 olivine crystals varies from 3 x I 0 4 up to 2,6 x 10 5 tracks per cm 2. We have measured also the volume etchable track length of about 200 tracks of Fe- group nuclei in these crystals by using TINT and TINCLE method. The Fe- group track spectrum in olivine crystals of Omolon pallasite is presented at Fig. 1.
3025m
2015 z
10
' I
0
r-n
0
IHn !
4
8
12 Track length, lan
i
16
ml-"l !
20
Fig. 1. The volume etchable track length. (VETL) of Fe- group galactic cosmic ray nuclei in olivine crystals of Omolon pallasite meteorite. The mean track length of these tracks (about 8 gin) was compared with the measured earlier Fe- group tracks in olivines from pallasites Marjalahti - 7.8 gin, Eagle Station - 10.3 pan, Lipovsky Khutor - 9.2 gm (Perelygin and Kashkarova, 1979 ), olivine crystals from Moon surface - 9.6 gm. The mean etchable track length of accelerated S4Cr and 56Fe nuclei was determined to be equal 9.5 and 13.6 gm. correspondingly. It means that the partial annealing of fossil Fe- group tracks in Omolon pallasite olivines was more significant, than in other extraterrestrial olivine crystals with exception of Marialahti pallasite. This effect can be explained by periodical heating of meteorite Omolon at perigee near the Sun. We have found also tracks with length > 40 gm in 12 track- rich olivine crystals ( Fe-group track density > 2 • 10 ~ tracks cm "2). These tracks were produced by slowing down to rest o f Z > 36 galactic cosmic
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
339
ray nuclei (VVH group of cosmic ray nuclei).The total number of measured VVH tracks was 43. The ratio of track densities due to W H and VH groups of cosmic ray nuclei was determined to be N VVH/ N VH = ( 2.1 ± 0.5 )" 10"4. In our previous study we have measured the experimental dependence of the ratio of Nvv,/Nv, track densities with the depth in preatmospheric body of pallasite type of meteorites. (Perelygin and Stetsenko, 1981).
10-3
;Z Z
104 0
5
10
15
20
Depth, cm
Fig. 2. The dependence of ratio of track densities due to VVN and VH galactic cosmic ray nuclei N w'H / N vH on the depth in pallasite type of meteorites for preatmospheric surface. As one can see from Fig. 2 the obtained value (2.1 + 0.5) .10 -4 corresponds to the depth 8.9 + 2.1 cm to the preatmospheric surface ofpallasite Omolon. CONCLUSION Taking in account rather high radiation age of this meteorite - ( 78 + 7 ) M. Y. one can conclude that this meteorite can be used for fossil track studies of 50 _
340
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
Acknowledgments: The authors wish to express their thanks to prof. Yu. A. Shukoljukov for his help, V. P. Semenenko for useful advises and G.P. Knjazeva for proper etching and scanning of olivine crystals. The authors are thankful to International Science Foundation and Russian Foundation of Fundamental Research for financial support of our work during 1994-1994 years.
REFERENCES Bondar Yu. V. and Semenenko V. P. (1994) The thermal history of pallasites basing on mineralogical and geochemical analysis. Abstracts of XXII Meteoritic Conference, 6-8 December, 1994, Chemogolovka, pp. 20-2 i. Krishnaswami S., Prabhu N., Tamhane A.S. and Lal D. (1971) Olivine revelation of tracks of charged particles. Science 74, 287-29 I. Maurette M., Pellas P. and Walker R. M.(1964) Cosmic ray induced particle tracks in a meteorite. Nature 204, 821-823. Perelygin V. P. and Kashkarova V. T. (1979) The thermal history of pallasites Marjalahti, Lipovsl