Photon Factory Activity Report 2016 #34 (2017)

BL-3A/2016G123

X-ray diffraction analysis of primitive micrometeorites
recovered from Antarctic surface snow
Rina Oike*, Tomoki Nakamura, Yuma Enokido, Miki Takahashi,
Masayuki Hatakeyama and Kazuki Kikuchi
Division of Earth Sciences, Graduate School of Science,
Tohoku University, Sendai 980-8576, Miyagi, Japan
1 Introduction
Micrometeorites (MMs) are extraterrestrial materials
smaller than 1mm in size, that retain information aboutprimitive bodies formed in the early solar system. MMs
recovered from the Antarctic surface snow are much lessaffected by terrestrial weathering than those from
Antarctic blue ice fields [1], and it has been reported thatthe snow-MMs include samples that originate fromcomets [2]. In order to investigate bulk mineralogy ofsnow-MMs, we performed synchrotron X-ray diffraction
analysis on each MMs.

heating when entered the atmosphere. We conclude that
ROP23O10 was originated from a small body made ofprimitive material similar to CM chondrites whoseserpentine was dehydrated and decomposed by heating
above 900 ° C when entered the atmosphere.

2 ExperimentsSurface snow (~700kg) were collected at around Talos
Dome in Antarctica by the Korea Polar Research Institutein November 2015. After melting and filtering of thesnow, we identified MMs which have chemicalcomposition close to the sun by using SEM/EDS. Thenthe MMs were attached to the top of carbon fiber (5umdiameter) with glycol phthalate. Irradiating to synchrotron
X-ray for one hour at beamline 3A, we obtained X-ray
diffraction pattern and identified the mineral compositionof each MM. Then the MMs were embedded in epoxy
resin and sliced to 70nm thick ultra-tin sections by
microtome for FE-TEM/EDS analysis, and remaining
MMs on epoxy surface were coated by carbon deposition
to be observed by FE-SEM/EDS.

Fig.1: X-ray diffraction pattern of ROP23O10.

3 Results and Discussion
X-ray diffraction pattern of ROP23O10 shows that thisMM consists of crystalline olivine and pyroxene (Fig.1).
The relatively large crystal located at the top of
ROP23O10 is pure forsterite and the small crystal at
central part is augite (Fig.2). On the other hand, finegrained portions show chemical composition close toserpentine, while serpentine was not identified in the Xray diffraction pattern. Therefore, original serpentine wasdehydrated and decomposed to olivine by heating.
Quantitative elemental analysis by FE-SEM/EDS showsthat FeO contents of fine-grained olivines are low(Fe/(Mg+Fe)=0.25~0.30)
andcorrespondwithcompositional range of CM chondrite matrix. (Fig. 2).
According to heating experiments of Murchison CM
chondirte [3], the temperature at which matrix serpentine
decomposes to well-crystalline secondary olivine isestimated to be 900℃ to 1000℃. Consequently, there is apossibility that ROP23O10 also experienced similar

Fig.2: FE-SEM backscatter electron image of ROP23O10.
All the fibrous bright crystals in this sample are low-FeO
olivine. Fo = Forsterite, Au = Augite.
Acknowledgement
We thank Dr. Nakao for technical support during X-ray
diffraction analysis.
References[1] Duprat et al. (2007) Advance in Space Research 39,605-611.
[2] Noguchi et al. (2015) Earth and Planetary Science
Letters 410, 1-11.
[3] Nozaki et al. (2006) Meteoritic and Planetary Science41, 7, 1095-1114.
* rina.ooike.s7@dc.tohoku.ac.jp