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Antarctic micrometeorites collected by the Japanese antarctic research expedition teams during 1996–1999
Antarctic micrometeorites collected by the Japanese Antarctic Research Expedition teams during 1996 - 1999 T. Noguchi a, H. Yano b, K. Terada e, N. Imae d, T. Yada e, T. Nakamura e and H. Kojima f a Department of Materials and Biological Science, Ibaraki University, 2-1-1, Bunkyo, Mito, Ibaraki, 310-8512, Japan b Planetary Science Division, Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan c Department of Earth and Planetary System Science, Hiroshima University, HigashiHiroshima, Hiroshima, 739-8526, Japan 't National Institute of Polar Research, 1-9-10 Kaga, Itabashi, Tokyo, 173-8515, Japan. e Department of Earth and Planetary Sciences, Kyushu University, 33, Hakozaki, Fukuoka, 812-8581 Japan The Japanese Antarctic Research Expedition (JARE) teams have started collection of unmelted and melted micrometeorites (MMs) in Antarctica since 1996. Some results of the consortium studies are: (1) relatively common occurrence of magnesiowtistite (MW) in unmelted MMs, (2) coexistence of MW with low-Ca pyroxene in moderately heated MMs, and (3) evidence that MMs were formed as small particles, rather than fragments of larger bodies, within < a few Ma. 1. RECOVERY OF M I C R O M E T E O R I T E S (MMs) IN THIS STUDY 1.1.
Recovery of MM candidates from the dust samples collected at the Dome Fuji Station in 1996 and 1997 In 1996 and 1997, the JARE teams collected MMs at the Dome Fuji Station, located at Queen Maud Land [1]. Fine-grained deposits in a water tank at the station were processed to enhance the abundance of MMs by filtration, separation by differences of sedimentation rate, and in some cases, magnetic separation [1,2]. The MM candidates were handpicked and investigated by SEM. More than 230 MMs were identified from the deposits collected in 1996 [2]. EDS data show that they have mostly undifferentiated chondritic elemental abundance. As heated during atmospheric entry to variable extents, they represent a wide range of mineralogy from phyllosilicate-dominated to barred olivine-dominated types [ 1, 2]. 1.2. MM collection in 1998-1999 at Yamato Mountains From November 1998 to January 1999, MMs were collected in bare ice areas of the Yamato Mountains. Thirty-six tons of ice were melted and filtered by a system to melt ice and filter the melted water [3]. After transferring the samples to Japan, captured particles on the sieves were removed to small petri dishes carefully by distilled water or acetone in a clean room. To understand the basic characteristicss of the samples, 5 among 24 sampling sites were selected to pick up all extraterrestrial dust particles. For example, a size fraction of 40-100 #m
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Antarctic micrometeorites collected by the JARE teams during 1996-1999
contained about 200 MMs. The number of MMs of this size fraction in the 24 sites is estimated as ---4800. Allocation of these samples to qualified researchers in the world began in the middle of November 2000. 2. A BRIEF SUMMARY OF A COMBINED STUDY ON MMs COLLECTED IN 1996 230 MMs collected in 1996 were investigated by the consortium [e.g., 1, 2, 6-10]. Here we will introduce some of the interesting results and also refer to some preliminary results of MMs collected in 1999. 2.1 Bulk mineralogy Bulk mineralogy of 28 individual unmelted to partially melted MMs was determined by Xray diffraction using Gandolfi camera. Four mineralogical types of MMs were identified. They are characterized as follows: (1) dehydrated-smectite dominant, olivine and low-Ca pyroxene being absent, while primary Fe-sulfide and magnetite present, (2) phyllosilicate absent while olivine in low crystallinity, and Fe-sulfide and magnetite present, (3) presence of variable amounts of olivine, low-Ca pyroxene, Fe-sulfide, and magnetite, and (4) abundant olivine and magnetite with small amounts of low-Ca pyroxene and Fe.sulfide. It has already been suggested that anhydrous mineral assemblages are often seen in unmelted MMs due to dehydration of phyllosilicates by heating [4, 5]. However, our study revealed that the species and relative abundance of anhydrous minerals formed by dehydration of phyllosilicates are governed by the extent of heating. Olivine and magnetite increase as heating proceeds, while once low-Ca pyroxene first increases and then decreases as heating proceeds. This study also revealed that type 3 (that is, low-Ca pyroxene abundant) MMs often
Figure 1. Backscattered electron images of MMs with XRD charts of them. These MMs show four types of typical mineral assemblages in MMs. Abbreviations: O1: olivine, Px: pyroxene, Mt: magnetite, Sul: Fe-sulfide.
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T. Noguchi et aL
contain Mg- and Fe-oxide (magnesiowtistite); the first discovery of magnesiowtistite in MMs. 2.2 TEM observation of MMs Only one phyllosilicate-bearing MM (type 1) was found among 83 MMs collected in 1996 [7]. It contains abundant saponite. Its mineralogy is similar to the phyllosilicate-bearing MM previously reported [4]. However, this MM contains abundant aggregates of magnesiowtistite (hereafter, MW) and Fe sulfide (Figure 2). The aggregates are surrounded by bundles of saponite. Most of the MW grains in such aggregates are < 20 nm across. During the preliminary study of MMs collected in 1998-1999, another saponite-rich MM was found. It also contains MW-bearing aggregates. As well as the absence of serpentine, MW-bearing aggregates may be also an unique feature of saponite-rich MMs. Although whether such aggregates are primary constituents or not, it is more plausible that they were formed from minerals that decomposed during weak atmospheric entry heating. MW in the recently found MM contains Mn as well as Mg and Fe. Its composition suggests that MW in this MM was formed from Mg, Fe, Mn-bearing minerals such as Mg, Fe, Mn-bearing carbonates found in CI chondrites [10]. As described in 2.1, MW is often found in MMs that experienced relatively weak heating (type 3). TEM observation revealed that MW coexists with low-Ca pyroxene intimately (Figure 3). These minerals have typical morphology of recrystallization. Because these minerals are absent in the peripheries of MMs, where abundant anhydrous minerals are olivine and magnetite, mixture of MW and low-Ca pyroxene may have been formed from phyllosilicates under slightly reducing conditions during atmospheric entry.
Figure 2. (A) A TEM photograph of a MW and Fe-sulfide aggregate in a smectite-rich type 1 MM. (B) MW contains only Mg and Fe peaks. (Cu peaks from the supporting grid) (C) SAED pattern showing diffraction tings from MW.
Figure 3. A TEM photomicrograph of MW in a moderately heated type 3 MM.. MW in them coexists with low-Ca pyroxene. Grain boundaries of these minerals show typical texture of recrystallization. Their EDS spectrum and SAED pattern indicate that the crystals are MW.
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Antarctic micrometeorites collected by the JARE teams during 1996-1999
2.3 Noble gas analysis Noble gas isotopic compositions in fine particles including individual MMs were analyzed by stepped pyrolysis [ 10]. Figure 4 shows a three-isotope diagram of Ne in MMs collected in 1996. The high ratio of solar/cosmogenic gas in MMs suggests that they had been small particles in space to have wide surface areas exposed to solar winds, being consistent with the results of previous works [9-14]. These MMs are not particles produced by breaking-up of hydrous carbonaceous chondrites during the atmospheric entry; otherwise a set of such particles must have Ne isotopic ratio close to an average value of hydrous carbonaceous chondrites enriched in cosmogenic gases. Based on these noblegas compositions, they emerged into interplanetary space less than a few million years ago as individual particles and came to the earth in 1950-1980.
1~ ta 13~ ]
SWff]
O MMs 70-200 pm (total melting) @ MMs < 70 lam (Stepped heating): r CI and CR chondrites * CM chondrites [ [~] Solar system reservoirs
.a..7oo*c 12-] 1000"C'~=m. 1300"C 41,
0
.
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Air
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.
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.
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.*
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9
oo
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.
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Cosmogenic. 6~ . . . . , . . . . . 0.02
0.03
....
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0.06
0.07
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Figure 4. Three isotope diagram of Ne. 3. CONCLUSIONS The JARE teams have been collecting MMs in Antarctica since 1996. The consortium studies of these MMs revealed that MW is common among unmelted MMs and that mixtures of MW and low-Ca pyroxene is characteristic of MMs that have experienced moderate atmospheric entry heating. In addition, noble-gas compositions revealed that MMs were in interplanetary space for < a few Ma as individual particles and came to the Earth in 1950-80. REFERENCES 1. 2. 3. 4.
T. Nakamura et al., Antarct. Meteorite Res. 12 (1999) 183. T. Noguchi et al., Antarct. Meteorite Res. 13 (2000) 270. T. Yada and H. Kojima, Antarct. Meteorite Res. 13 (2000) 9. W. K16ck and F.J. Stadermann, in Analysis of Interplanetary Dust (eds. M.E. Zolensky, T.L. Wilson, F.J.M. Rietmeijer and G.J. Flynn, AIP Press (1994) 51.. 5. A. Gresheke et al., Meteoritics Planet. Sci. 33 (1998), 267. 6. T. Nakamura et al., Papers presented to the 24 th Symp. Antarct. Meteorites (1999) 125. 7. T. Noguchi and T. Nakamura, Antarct. Meteorite Res. 13 (2000) 285. 8. I. Nakai et al., Antarct. Meteorite Res. 13 (2000) 285. 9. T. Osawa et al., Antarct. Meteorite Res. 13 (2000) 322. 10. T. Nakamura and N. Takaoka, Antarct. Meteorite Res. 13 (2000) 311. 11. M. Zolensky and H.Y. McSween, in Meteorites and the Early Solar System (1987) 114. 12. C.T. Olinger, M. Maurette, R.M. Walker and C.M. Hohenberg, Earth Planet Sci. Lett. 100 (1990) 77. 13. A. Nier and D.J. Schlutter, Meteoritics 25 (1990) 263. 14. M. Maurette, C. Olinger, M.C. Michellevy, G. Kurat, M. Pourchet, F. Brandstatter and M. Bourotdenise, Nature 351 (1991) 44.
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Recovery of MM candidates from the dust samples collected at the Dome Fuji Station in 1996 and 1997 In 1996 and 1997, the JARE teams collected MMs at the Dome Fuji Station, located at Queen Maud Land [1]. Fine-grained deposits in a water tank at the station were processed to enhance the abundance of MMs by filtration, separation by differences of sedimentation rate, and in some cases, magnetic separation [1,2]. The MM candidates were handpicked and investigated by SEM. More than 230 MMs were identified from the deposits collected in 1996 [2]. EDS data show that they have mostly undifferentiated chondritic elemental abundance. As heated during atmospheric entry to variable extents, they represent a wide range of mineralogy from phyllosilicate-dominated to barred olivine-dominated types [ 1, 2]. 1.2. MM collection in 1998-1999 at Yamato Mountains From November 1998 to January 1999, MMs were collected in bare ice areas of the Yamato Mountains. Thirty-six tons of ice were melted and filtered by a system to melt ice and filter the melted water [3]. After transferring the samples to Japan, captured particles on the sieves were removed to small petri dishes carefully by distilled water or acetone in a clean room. To understand the basic characteristicss of the samples, 5 among 24 sampling sites were selected to pick up all extraterrestrial dust particles. For example, a size fraction of 40-100 #m
- 392 -
Antarctic micrometeorites collected by the JARE teams during 1996-1999
contained about 200 MMs. The number of MMs of this size fraction in the 24 sites is estimated as ---4800. Allocation of these samples to qualified researchers in the world began in the middle of November 2000. 2. A BRIEF SUMMARY OF A COMBINED STUDY ON MMs COLLECTED IN 1996 230 MMs collected in 1996 were investigated by the consortium [e.g., 1, 2, 6-10]. Here we will introduce some of the interesting results and also refer to some preliminary results of MMs collected in 1999. 2.1 Bulk mineralogy Bulk mineralogy of 28 individual unmelted to partially melted MMs was determined by Xray diffraction using Gandolfi camera. Four mineralogical types of MMs were identified. They are characterized as follows: (1) dehydrated-smectite dominant, olivine and low-Ca pyroxene being absent, while primary Fe-sulfide and magnetite present, (2) phyllosilicate absent while olivine in low crystallinity, and Fe-sulfide and magnetite present, (3) presence of variable amounts of olivine, low-Ca pyroxene, Fe-sulfide, and magnetite, and (4) abundant olivine and magnetite with small amounts of low-Ca pyroxene and Fe.sulfide. It has already been suggested that anhydrous mineral assemblages are often seen in unmelted MMs due to dehydration of phyllosilicates by heating [4, 5]. However, our study revealed that the species and relative abundance of anhydrous minerals formed by dehydration of phyllosilicates are governed by the extent of heating. Olivine and magnetite increase as heating proceeds, while once low-Ca pyroxene first increases and then decreases as heating proceeds. This study also revealed that type 3 (that is, low-Ca pyroxene abundant) MMs often
Figure 1. Backscattered electron images of MMs with XRD charts of them. These MMs show four types of typical mineral assemblages in MMs. Abbreviations: O1: olivine, Px: pyroxene, Mt: magnetite, Sul: Fe-sulfide.
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T. Noguchi et aL
contain Mg- and Fe-oxide (magnesiowtistite); the first discovery of magnesiowtistite in MMs. 2.2 TEM observation of MMs Only one phyllosilicate-bearing MM (type 1) was found among 83 MMs collected in 1996 [7]. It contains abundant saponite. Its mineralogy is similar to the phyllosilicate-bearing MM previously reported [4]. However, this MM contains abundant aggregates of magnesiowtistite (hereafter, MW) and Fe sulfide (Figure 2). The aggregates are surrounded by bundles of saponite. Most of the MW grains in such aggregates are < 20 nm across. During the preliminary study of MMs collected in 1998-1999, another saponite-rich MM was found. It also contains MW-bearing aggregates. As well as the absence of serpentine, MW-bearing aggregates may be also an unique feature of saponite-rich MMs. Although whether such aggregates are primary constituents or not, it is more plausible that they were formed from minerals that decomposed during weak atmospheric entry heating. MW in the recently found MM contains Mn as well as Mg and Fe. Its composition suggests that MW in this MM was formed from Mg, Fe, Mn-bearing minerals such as Mg, Fe, Mn-bearing carbonates found in CI chondrites [10]. As described in 2.1, MW is often found in MMs that experienced relatively weak heating (type 3). TEM observation revealed that MW coexists with low-Ca pyroxene intimately (Figure 3). These minerals have typical morphology of recrystallization. Because these minerals are absent in the peripheries of MMs, where abundant anhydrous minerals are olivine and magnetite, mixture of MW and low-Ca pyroxene may have been formed from phyllosilicates under slightly reducing conditions during atmospheric entry.
Figure 2. (A) A TEM photograph of a MW and Fe-sulfide aggregate in a smectite-rich type 1 MM. (B) MW contains only Mg and Fe peaks. (Cu peaks from the supporting grid) (C) SAED pattern showing diffraction tings from MW.
Figure 3. A TEM photomicrograph of MW in a moderately heated type 3 MM.. MW in them coexists with low-Ca pyroxene. Grain boundaries of these minerals show typical texture of recrystallization. Their EDS spectrum and SAED pattern indicate that the crystals are MW.
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Antarctic micrometeorites collected by the JARE teams during 1996-1999
2.3 Noble gas analysis Noble gas isotopic compositions in fine particles including individual MMs were analyzed by stepped pyrolysis [ 10]. Figure 4 shows a three-isotope diagram of Ne in MMs collected in 1996. The high ratio of solar/cosmogenic gas in MMs suggests that they had been small particles in space to have wide surface areas exposed to solar winds, being consistent with the results of previous works [9-14]. These MMs are not particles produced by breaking-up of hydrous carbonaceous chondrites during the atmospheric entry; otherwise a set of such particles must have Ne isotopic ratio close to an average value of hydrous carbonaceous chondrites enriched in cosmogenic gases. Based on these noblegas compositions, they emerged into interplanetary space less than a few million years ago as individual particles and came to the earth in 1950-1980.
1~ ta 13~ ]
SWff]
O MMs 70-200 pm (total melting) @ MMs < 70 lam (Stepped heating): r CI and CR chondrites * CM chondrites [ [~] Solar system reservoirs
.a..7oo*c 12-] 1000"C'~=m. 1300"C 41,
0
.
7101 t"
Air
. .
.
*
117oo'c
.
*
..
.*
*% , t , , t . ~
9
oo
*
1
.
i
Cosmogenic. 6~ . . . . , . . . . . 0.02
0.03
....
0.04
0.05
0.06
0.07
o.oe
0.09
0.1
21Ne/2ZNe
Figure 4. Three isotope diagram of Ne. 3. CONCLUSIONS The JARE teams have been collecting MMs in Antarctica since 1996. The consortium studies of these MMs revealed that MW is common among unmelted MMs and that mixtures of MW and low-Ca pyroxene is characteristic of MMs that have experienced moderate atmospheric entry heating. In addition, noble-gas compositions revealed that MMs were in interplanetary space for < a few Ma as individual particles and came to the Earth in 1950-80. REFERENCES 1. 2. 3. 4.
T. Nakamura et al., Antarct. Meteorite Res. 12 (1999) 183. T. Noguchi et al., Antarct. Meteorite Res. 13 (2000) 270. T. Yada and H. Kojima, Antarct. Meteorite Res. 13 (2000) 9. W. K16ck and F.J. Stadermann, in Analysis of Interplanetary Dust (eds. M.E. Zolensky, T.L. Wilson, F.J.M. Rietmeijer and G.J. Flynn, AIP Press (1994) 51.. 5. A. Gresheke et al., Meteoritics Planet. Sci. 33 (1998), 267. 6. T. Nakamura et al., Papers presented to the 24 th Symp. Antarct. Meteorites (1999) 125. 7. T. Noguchi and T. Nakamura, Antarct. Meteorite Res. 13 (2000) 285. 8. I. Nakai et al., Antarct. Meteorite Res. 13 (2000) 285. 9. T. Osawa et al., Antarct. Meteorite Res. 13 (2000) 322. 10. T. Nakamura and N. Takaoka, Antarct. Meteorite Res. 13 (2000) 311. 11. M. Zolensky and H.Y. McSween, in Meteorites and the Early Solar System (1987) 114. 12. C.T. Olinger, M. Maurette, R.M. Walker and C.M. Hohenberg, Earth Planet Sci. Lett. 100 (1990) 77. 13. A. Nier and D.J. Schlutter, Meteoritics 25 (1990) 263. 14. M. Maurette, C. Olinger, M.C. Michellevy, G. Kurat, M. Pourchet, F. Brandstatter and M. Bourotdenise, Nature 351 (1991) 44.
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