Photon Factory Activity Report 2018 #35 (2019)

X-ray diffraction analysis of primitive micrometeorites
recovered from Antarctic surface snow

BL-3A/2017G632

Shota Shimizu*, Tomoki Nakamura, Megumi Matsumoto,Katsumi Ohgiya, 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 material,
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 other fields on the earth. It has beenreported that the snow-MMs include samples that originate
from comets [2]. In order to investigate bulk mineralogy ofsnow-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 and identified the mineral compositionof each MM. 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
SSP4E5 is an unmelted MM and has S-rich composition
relative to solar abundance. X-ray diffraction pattern ofSSP4E5 shows broad (001) reflection of saponite andmany sharp peaks of pyrrhotite, FeS-Fe7S8 (Fig.1). The
mineral composition differs from any type of carbonaceouschondrites. Framboidal aggregates of pyrrhotite are foundin this MM (Fig. 2) and this is the first report of this typeof aggregates in meteorites. Selected-area electron
diffraction by TEM indicates that the pyrrhotite grains aremostly hexagonal structure (Fig. 3). The aggregates appear
to have formed during aqueous alteration, similar to theframboid aggregates of magnetite [3], but formation
conditions and mechanism are not identified so far. This
MM is a part of aqueously-altered primitive body with Srich solution, which are responsible for the S-excess of thebulk MM and abundant aggregates of pyrrhotite.

Fig.1 X-ray diffraction pattern of SSP4E5. The value below“sap” is D value of each peak.

Fig.2 FE-SEM BSE image of a part of SSP4E5, showing
framboidal iron sulphides (FeS-Fe7S8).

Fig.3 Diffraction pattern of pyrrhotite inSSP4E5. This grainshows hexagonal structure.

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] Kimura et al. (2013) Nature communications, 4, 2649.
*shota.shimizu.q4@dc.tohoku.ac.jp