Photon Factry Activity Report 2017 #35
BL-3A/2017G632

X-ray diffraction analysis of primitive micrometeorites
recovered from Antarctic surface snow
Shota Shimizu*, Tomoki Nakamura, Rina Oike,
Miki Takahashi and Yuma Enokido
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 are coming to the Earthfrom primitive bodies formed in the early solar system.
MMs recovered from the Antarctic surface snow aremuch less affected by terrestrial weathering than thosefrom Antarctic blue ice fields [1] and other fields on theearth. It has been reported that the snow-MMs include
samples that originate from comets [2]. In order toinvestigate bulk mineralogy of snow-MMs, we performed
synchrotron X-ray diffraction analysis of each MMs.
2 Experiments
Surface snow was collected at around Talos Dome andDome C in Antarctica by the KOPRI (Korea PolarResearch Institute), in November 2015, and the IPEV(Institut Polaire Francais Paul Emile Victor), in January
2016, respectively. After melting and filtering of the snow,
by using SEM/EDS, we identified MMs which havechemical composition close to solar elemental abundance.
Then each MM was attached to the top of carbon fiber(5um in diameter) with glycol phthalate. Irradiating it tosynchrotron X-rays at beamline 3A, we obtained X-ray
diffraction pattern of each MM and identified the mineralcomposition. Then the MM was embedded in epoxy resinand sliced to 70nm thick ultra-thin sections by ultramicrotome for FE-TEM/EDS analysis, and remaining
portion of the MM exposed on epoxy surface was coatedby carbon deposition to be observed by FE-SEM/EDS.
3 Results and Discussion
X-ray diffraction pattern of SSP3Q4 shows (002)reflection of tochilinite and many sharp peaks ofserpentine (Fig.1). From the BSE images taken by FESEM/EDS, SSP3Q4 (Fig.2) shows bright and dark fibroustexture. The chemical composition of some points in thissample overlaps that of CM chondrite matrix.
Composition of the dark fibrous region is similar toserpentine and that of bright one is similar to a mixture ofserpentine and TCI (tochilinite-cronstedtite intergrowth).
These analysis shows SSP3Q4 containing TCI.
TCI is produced during aqueous alteration in parent
asteroid and decomposed at low temperature ~300°C. It ispeculiarly observed in CM chondrites but has never beenrecognized in CM-chondritic MMs probably because it
was decomposed by heating during atmospheric entry.
This is the first MM that retains TCI that survived theentry heating.

Comparing diffraction pattern of Fig.1 with that ofheated CM chondrite [3], this MM suffered <300°C
heating. Average FeO/SiO2 value in this TCI is 2.3,
indicating low degrees of aqueous alteration [4].
We suppose that primitive MMs can reach to the earth
that escaped even weak heating in the atmosphere.

Fig.1 X-ray diffraction pattern of SSP3Q4. The value
below the mineral name is D value of each peak.
amorphousrimchondrule rim likematrixbright fibrous
regionamorphousrim

dark fibrous
regionFig.2 Fe-SEM BSE image of a part of SSP3Q4. In
addition to two fibrous regions, overall texture andmineralogy are similar to CM chondrites.
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.
[4] Rubin et al. (2007) Geochimica et Cosmochimica
Acta, 71(9), 2361-2382.
*shota.shimizu.q4@dc.tohoku.ac.jp