Photon Factory Activity Report 2019 #37 (2020)

BL-3A/2019G610

X-ray diffraction analysis of dark inclusions in a meteorite from asteroid 4 Vesta
Miki Takahashi*, Tomoki Nakamura, Shota Shimizu,
Katsumi Ohgiya, Megumi Matsumoto, Yoshiki Kosaka and Yuri Fujioka
Division of Earth Sciences, Graduate School of Science,
Tohoku University, Sendai 980-8576, Miyagi, Japan
1 Introduction
Dark inclusions (DIs) are micron- to cm-size dark rockfragments in meteorites and they have mineralogical andpetrological features different from the host meteorites.
Some DIs show unique petrographic properties, and they
may contain material information never found inmeteorites. DIs have escaped high-speed impacts uponaccretion on asteroids, because gravitation of asteroids ismuch smaller than the earth. When meteorites including
DIs come to the earth, DIs do not experience hightemperature heating and high pressure upon atmosphericentry because they are enveloped within the host meteorite.
In order to understand the formation and accretion processof DIs in a meteorite from 4 Vesta, we performed
synchrotron X-ray diffraction analysis of individual smallDIs separated from the EET 15101 eucrite.
2 Experiments
HED (Howardite, Eucrite, and Diogenite) meteorites,coming from asteroid 4 Vesta, commonly contain DIs [1].
The EET 15101 eucrite contains many small DIs andparticles in the size range of 100-200 μm were taken out
from each DI. Individual single-particle was exposed to Xrays at a wavelength of 2.16 Å and the obtained X-ray
powder diffraction patterns was analyzed for the mineralcomposition of each DI. Then the DI was embedded in
epoxy resin and coated by carbon deposition to beobserved by FE-SEM/EDS and FE-EPMA/WDS.
3 Results and Discussion
Synchrotron X-ray diffraction analyses proved that theDIs consist mainly of hydrous silicates and are classified
into two types (Fig. 1): CM chondrite (CM) and Tagish
Lake or CI chondrite (TL/CI) type. Hydrous silicates inboth types show the absence of (or presence of small) basal
001 reflection and the presence of prism reflections,indicating that they were heated and partially decomposed.
The bulk mineralogy of TL/CI-type DI (Fig. 1) is similarto Tagish Lake and CI chondrite that are experimentallyheated at 400-600°C [2,3], while that of the CM-type DI issimilar to the 400-600°C heated Murchison. Murchison is
known to form poorly-crystalline magnetite-like material
at about 500°C [4] and thus the CM type has experienced
heating at about 500°C.
Electron microscope observation of a polished crosssection of the CM-type DI showed abundant occurrence ofpure tochilinite (Fig. 2), which suggests the low degree of
aqueous alteration, because tochilinite was decomposed bypervasive alteration. The 500°C-heating decomposed anddehydrated the tochilinite, because it was not identified in
the XRD pattern (Fig.1 ).

Both types of DIs came from primitive hydrated asteroids
that formed at outer solar nebula beyond the snow line
because they show bulk mineralogy similar to meteorites
from these asteroids. The DIs might have been heatedduring or after incorporation to asteroid 4 Vesta, because
all DIs showed partial dehydration of hydrous silicates.
Acknowledgement
We thank Dr. Nakao for technical support during X-ray
diffraction analysis.

Fig. 1. X-ray powder diffraction pattern of the DIs in theEET 15101.

Fig. 2. SEM/BSE image at high magnification of the CMtype DI. It contains serpentine (Ser), tochilinite (Toch), achondrule rim (Rim), and a chondrule.
References[1] Zolensky et al. (1996) Meteoritics & Planetary Science31, 518-537.
[2] Mogi et al. (2017) 80th Annual Meeting of the
Meteoritical Society, Abstract #6225.
[3] Nozaki et al. (2005) Meteoritics & Planetary Science41, 1095-1114.
[4] Yamashita et al. (2015) 78th Annual Meeting of the
Meteoritical Society, Abstract. #5154.
*miki.takahashi.p4@dc.tohoku.ac.jp