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Antarctic micrometeorites collected by the Japanese Antarctic Research Expedition teams during 1996 - 1999

T. Noguchia, H. Yano b, K. Teradac, N. Imaed, T. Yadae, T. Nakamurae and H. Kojimaf 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, Higashi-Hiroshima, Hiroshima, 739-8526, Japan d 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 magnesiowiistite (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 MICROMETEORITES (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 oli vine-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 ¡im 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 X-ray 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: Ol: olivine, Px: pyroxene, Mt: magnetite, Sul: Fe-sulfide.

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