Mineralogy and Oxygen Isotopes of Unmelted Antarctic Micrometeorites

Introduction: Cosmic spherules and micrometeor-ites account for the major fraction of extraterrestrial material accreting on the Earth. The amount of mi-crometeorites arrived on the terrestrial surface is up to 16000 ton [1], which is around 10 times as much as that of meteorites. The JARE-39 expedition recovered great amounts of micrometeorites from Kuwagata and Tottuki areas in Antarctica in 1998 [2]. We have collected unmelted Antarctic micrometeorites (AMMs) from the 40-100 µm fractions of these collections. The Kuwagata AMM collection catalog is available in http://dust.cc.gakushuin.ac.jp. In order to know the origin and history of AMMs, we started to investigate them one by one in respects of oxygen isotopes, rare earth element abundances and noble gas isotopes, as well as petrologic and minera-logical studies. A new sample preparation has been developed to carry out these multidisciplinary analyses. Here we report mineralogical characteristics and the preliminary results of oxygen isotope analysis. Experimental: Kuwagata AMMs were embedded in crystal bond and then polished. The polished AMMs were investigated with a scanning electron microscope at the Kyushu Univ. Chemical compositions were determined using an electron microprobe analyzer at the Kyushu Univ.. Oxygen isotope analysis is now in progress. For the present, three Kuwagata AMMs were measured with a cameca 6f ion microprobe at the Kyushu Univ. The 0.6nA Cs + primary ion beam was focused in aperture-illumination mode to produce a ~20µm spot. The secondary mass spectrometer was operated at 9.5kV with a mass resolving power of ~5500 and an 80eV energy window. The normal-incident electron flood gun was used for charge compensation. San Carlos olivine grains were used for standardization. Results and Discussion: Kuwagata AMM of Y98K11KS201 (abbreviated as KS201) consists of fine-grained minerals and the SEM image suggest that the constituent minerals appear to be homogeneously distributed within this AMM. Thus the spot analysis on this AMM would represent its bulk composition. Fig. 1 is a Si-Mg-Fe ternary diagram. The composition of KS201 is close to the CI matrix composition [3]. Y98K11KS212 consists of two lithologies: one is fine-grained porous matrix with many vesicles, and another is a compact clast (enclosed by a white dotted line in Fig. 2). The compact clast of KS212 contains