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Increasing uranium migration in chondritic meteorites with annealing temperature
Radiation Measurements, Vol. 25, Nos I-4, pp. 523-524, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1350-4487/95 $9.50 + .00
Pergamon
1350.4487(95)00145-X
INCREASING URANIUM MIGRATION IN CHONDRITIC METEORITES WITH ANNEALING TEMPERATURE
RUIYING HU,* SHI-LUN GUO,~" XIU-HONG HAO? and YU-LAN WANGt
*Institute of Geochemistry,AcademicSinica,Guangzhou 510640,China; and l'China Institute of AtomicEnergy, P.O. Box 275(96),Beijing 102413,China
ABSTRACT Uranium in two new chondrites--Yanzhuang Meteorite and Sixiangkou Meteorite in C h i n o - as well as in jilin Meteorite has been mapped by the fission track technique after different annealing temperatures from 700 to 1300~ for 1 hour. Uranium concentration and microscopic distribution have been obtained by counting tracks of induced fissions of m U in the meteorites. It shows that: (1) Uranium concentration in chondrules decreases with increasing of annealing temperature while the total uranium concentration in chondrules and their surrounding materials in each chondrite is nearly constant. This means that uranium migrates from inside to outside of chondrules at rate which increases as temperature increases. (2)Total Uranium concentration is about 20 ppb in Sixiangkou Meteorite and about 14 ppb in both Yanzhuang Meteorite and Jilin Meteorite. These results confirm that Uranium concentration in L group of meteorites is higher than that in H group. (3)It seems that the irr[ormations of Uranium concentration and its microscopic distribution can be used to study the processes of formation and evolution of chondrites.
KEYWORDS Yanzhuang Meteorite, Sixiangkou meteorite, Jilin Meteorite, Uranium concentration, Uranium distribution, temperature effect, Uranium migration. INTRODUCTION Our study was focused on three chondritic meteorites--Jilin(HS), Yanzhuang(H6), and Sixiangkou (L6) chondrites, Yanzhuang and Sixiangkou meteorites fell at Yanzhuang village north of Wen--yuan Country, Guangzhou province, and at Sixlangkou town in Jiangsu province respectively. In order to ascertain the relationship between uranium concentration and chemical--petrologic type or thermal metamorphism of the meteorites, we investigated their uranium concentrations by fission track techniques and mapped their microscopic distributions of uranium. Over 30 polish sections were examinated, and olivine, pyroxene and so on minerals were selected to measure their fission track densities under microscopy. A summary is presented in the following paragraph. EXPERIMENTAL METHOD First, A Jilinmeteorite specimen was split into pieces which weigh around 10g each. Each sample was put into porcelain crucible, and then was heated by thermocouple pyrometric furnace for 60 minutes at temperature ranging from 700 to 1300"C, and then annealed slowly, Each heated sample was made into more than 2 polish sections for the purpose of m[nimiz~ug statisticderivation. The observed dimension of each polish section was about I square centimetre. Heated up to 1000~C Chondrule texture in Jilinmeteorite was obscured. Jilinmeteorite sample was molten at 1280"C wholly. As a result, we have to measure the Uranium concentrations of whole rock and local region of different texture.
523
524
RUIYING HU et al.
RESULTS The results of measurements are listed in table I. Table 1 Uranium concentration of heated and/or natural samples of chondritic meteorites No. of heated heated U concentration(ppb) samples meteorite temperature( "C ) time(rain) chondrules whole rock 91--1
Jilin
700
60
5.52+0. 68
14. 394-0. 21
91-2
Jilin
800
60
4. 124-0. 65
15. 184-0. 23
91-3
Jilin
900
60
2. 974-0. 42
13. 394-0. 28
91--4
Jilin
1000
60
--
14. 434-0. 31
91--5
Jilin
1100
60
-
12. 834-0. 18
91--6
Jilin
1280
60
-
12. 524-0. 24
91--7
Jilin
unheated
6. 74-4-0. 83
12.59+0. 21
91--8
Sixiangkou
unheated
--
19. 144-0. 24
01--9
Yanzhuang
unheated
--
13. 524-0. 17
To classify chemical group and petrologic type based on the Uranium concentration and microdistribution, we heated artificiallyJilin meteorite samples and then determined their uranium concentration by means of fission track detector, and mapped their microdistributions. DISCUSSION AND CONCLUSION
(1)By means of fission track observation for a variety of samples, it was found that the uranium concentration of chondrules decrease with the increasing temperature. It is obviously illustrated from Table 1 that uranium concentration of pyroxecn and olivine chondrules range from 3 to 7 ppb. Fission track densities of matrix and chondrule rims in jilinmeteorite increase with temperature. Maybe the reason is that uranium, a much more active element, especially the adsorption-state uranium is extremely easy to diffuse from chondrules when samples were heated. W e defined as self--purified phenomenon. This was early reported in 1980 while we investigated fission tracks of Jilin meteorite. W e discovered that fission tracks distributed around chondrules or along cleavages and cracks of minerals. This study confirmed further that the self--purified process is extremely relative to temperature. (2)Bulk uranium concentrations of Jilin meteorite we determined, both heated and natural samples, their average uranium contents vary from 12 to 15 ppb. It was inferred that the bulk uraniu m concentration in meteorite was not extremely pertaining to its heated temperature. However, it was observed that the fission track density increased with temperature. Nevertheless, it is premature to believe that uranium concentration of uranium--bearing phrase is relative to thermal metamorphism intensity. (3) Bulk uranium concentration is 14 ppb Jilin(H5), 20 ppb in Sixiangkou (L6), and 14ppb in Yanzhuang (H6)meteorite. These data are consistant with our former studies. This confirmed further that u r a m u m in ordinary chondrites increase from H, L, LL. (4)Uranium concentration of black veins in Yanzhuang meteorite varies from 25 to 40 ppb while that of grey breccia ranges from 13 to 16 ppb. Although their mineral compositions are similar basically, their fission track distributions appears bo be different each other. The fission track density of black veins increases greatly compared with grey breccia. W e believe that it is caused by different formation age or thermal metamorphism they experienced. In conclusion, the study on uranium concentration and distribution in meteorites has great significances. They can suggest us to tap some useful information on formation and evolution of meteorites. REFERENCE 1. Wang Daode et al. , 1993. An introduction to chinese Meteorites. Science Press, Beijing. 2. Hu Ruiying and Klans Thiel, 1986. A study of microdistribution in drill core samples of Jilin meteorite. Chinese Journal of Geochemistry.
Pergamon
1350.4487(95)00145-X
INCREASING URANIUM MIGRATION IN CHONDRITIC METEORITES WITH ANNEALING TEMPERATURE
RUIYING HU,* SHI-LUN GUO,~" XIU-HONG HAO? and YU-LAN WANGt
*Institute of Geochemistry,AcademicSinica,Guangzhou 510640,China; and l'China Institute of AtomicEnergy, P.O. Box 275(96),Beijing 102413,China
ABSTRACT Uranium in two new chondrites--Yanzhuang Meteorite and Sixiangkou Meteorite in C h i n o - as well as in jilin Meteorite has been mapped by the fission track technique after different annealing temperatures from 700 to 1300~ for 1 hour. Uranium concentration and microscopic distribution have been obtained by counting tracks of induced fissions of m U in the meteorites. It shows that: (1) Uranium concentration in chondrules decreases with increasing of annealing temperature while the total uranium concentration in chondrules and their surrounding materials in each chondrite is nearly constant. This means that uranium migrates from inside to outside of chondrules at rate which increases as temperature increases. (2)Total Uranium concentration is about 20 ppb in Sixiangkou Meteorite and about 14 ppb in both Yanzhuang Meteorite and Jilin Meteorite. These results confirm that Uranium concentration in L group of meteorites is higher than that in H group. (3)It seems that the irr[ormations of Uranium concentration and its microscopic distribution can be used to study the processes of formation and evolution of chondrites.
KEYWORDS Yanzhuang Meteorite, Sixiangkou meteorite, Jilin Meteorite, Uranium concentration, Uranium distribution, temperature effect, Uranium migration. INTRODUCTION Our study was focused on three chondritic meteorites--Jilin(HS), Yanzhuang(H6), and Sixiangkou (L6) chondrites, Yanzhuang and Sixiangkou meteorites fell at Yanzhuang village north of Wen--yuan Country, Guangzhou province, and at Sixlangkou town in Jiangsu province respectively. In order to ascertain the relationship between uranium concentration and chemical--petrologic type or thermal metamorphism of the meteorites, we investigated their uranium concentrations by fission track techniques and mapped their microscopic distributions of uranium. Over 30 polish sections were examinated, and olivine, pyroxene and so on minerals were selected to measure their fission track densities under microscopy. A summary is presented in the following paragraph. EXPERIMENTAL METHOD First, A Jilinmeteorite specimen was split into pieces which weigh around 10g each. Each sample was put into porcelain crucible, and then was heated by thermocouple pyrometric furnace for 60 minutes at temperature ranging from 700 to 1300"C, and then annealed slowly, Each heated sample was made into more than 2 polish sections for the purpose of m[nimiz~ug statisticderivation. The observed dimension of each polish section was about I square centimetre. Heated up to 1000~C Chondrule texture in Jilinmeteorite was obscured. Jilinmeteorite sample was molten at 1280"C wholly. As a result, we have to measure the Uranium concentrations of whole rock and local region of different texture.
523
524
RUIYING HU et al.
RESULTS The results of measurements are listed in table I. Table 1 Uranium concentration of heated and/or natural samples of chondritic meteorites No. of heated heated U concentration(ppb) samples meteorite temperature( "C ) time(rain) chondrules whole rock 91--1
Jilin
700
60
5.52+0. 68
14. 394-0. 21
91-2
Jilin
800
60
4. 124-0. 65
15. 184-0. 23
91-3
Jilin
900
60
2. 974-0. 42
13. 394-0. 28
91--4
Jilin
1000
60
--
14. 434-0. 31
91--5
Jilin
1100
60
-
12. 834-0. 18
91--6
Jilin
1280
60
-
12. 524-0. 24
91--7
Jilin
unheated
6. 74-4-0. 83
12.59+0. 21
91--8
Sixiangkou
unheated
--
19. 144-0. 24
01--9
Yanzhuang
unheated
--
13. 524-0. 17
To classify chemical group and petrologic type based on the Uranium concentration and microdistribution, we heated artificiallyJilin meteorite samples and then determined their uranium concentration by means of fission track detector, and mapped their microdistributions. DISCUSSION AND CONCLUSION
(1)By means of fission track observation for a variety of samples, it was found that the uranium concentration of chondrules decrease with the increasing temperature. It is obviously illustrated from Table 1 that uranium concentration of pyroxecn and olivine chondrules range from 3 to 7 ppb. Fission track densities of matrix and chondrule rims in jilinmeteorite increase with temperature. Maybe the reason is that uranium, a much more active element, especially the adsorption-state uranium is extremely easy to diffuse from chondrules when samples were heated. W e defined as self--purified phenomenon. This was early reported in 1980 while we investigated fission tracks of Jilin meteorite. W e discovered that fission tracks distributed around chondrules or along cleavages and cracks of minerals. This study confirmed further that the self--purified process is extremely relative to temperature. (2)Bulk uranium concentrations of Jilin meteorite we determined, both heated and natural samples, their average uranium contents vary from 12 to 15 ppb. It was inferred that the bulk uraniu m concentration in meteorite was not extremely pertaining to its heated temperature. However, it was observed that the fission track density increased with temperature. Nevertheless, it is premature to believe that uranium concentration of uranium--bearing phrase is relative to thermal metamorphism intensity. (3) Bulk uranium concentration is 14 ppb Jilin(H5), 20 ppb in Sixiangkou (L6), and 14ppb in Yanzhuang (H6)meteorite. These data are consistant with our former studies. This confirmed further that u r a m u m in ordinary chondrites increase from H, L, LL. (4)Uranium concentration of black veins in Yanzhuang meteorite varies from 25 to 40 ppb while that of grey breccia ranges from 13 to 16 ppb. Although their mineral compositions are similar basically, their fission track distributions appears bo be different each other. The fission track density of black veins increases greatly compared with grey breccia. W e believe that it is caused by different formation age or thermal metamorphism they experienced. In conclusion, the study on uranium concentration and distribution in meteorites has great significances. They can suggest us to tap some useful information on formation and evolution of meteorites. REFERENCE 1. Wang Daode et al. , 1993. An introduction to chinese Meteorites. Science Press, Beijing. 2. Hu Ruiying and Klans Thiel, 1986. A study of microdistribution in drill core samples of Jilin meteorite. Chinese Journal of Geochemistry.