TY - JOUR
T1 - Bulk mineralogy of individual micrometeorites determined by X-ray diffraction analysis and transmission electron microscopy
AU - Nakamura, Tomoki
AU - Noguchi, Takaaki
AU - Yada, Toru
AU - Nakamuta, Yoshihiro
AU - Takaoka, Nobuo
N1 - Funding Information:
We thank Drs. Maurette and Yano for giving us the opportunity to study the micrometeorites of the EUROMET collection; the National Institute of Polar Research for providing micrometeorite samples; Dr. Imae for providing compositional data of one micrometeorite; Dr. Tachibana for kindly giving us orthoenstatite powder used for standard samples; and Messrs. Nozaki, Yamada, and Shimada for technical support during the course of this study. Extensive reviews by Drs. Engrand and Koeberl and two anonymous referees significantly improved the quality of the article. This work has been supported by the grant-in-aid of the Japan Ministry of Education, Science and Culture to TN (grants 11740303 and 13740318).
PY - 2001
Y1 - 2001
N2 - Bulk mineralogy of individual fine-grained micrometeorites from 50 to 200 μm in diameter was determined on the basis of the powder X-ray diffraction patterns and the observation of internal textures by a transmission electron microscope (TEM). X-ray diffraction analysis of 56 micrometeorites indicated that 42, 11, and 3 samples are olivine-rich, pyroxene-rich, and phyllosilicate-rich micrometeorites, respectively. Among the phyllosilicate-rich micrometeorites, one contains saponite and other two contain serpentine. No samples contain both saponite and serpentine. We found that saponite-rich micrometeorite was weakly heated, which results in shrinkage of 001 basal spacing of saponite down to 9.7 Å, and that cronstedtite, which is commonly contained in CM chondrites, occurs in serpentine-rich micrometeorites. Micrometeorites that consist entirely of anhydrous minerals and amorphous phases are predominant in the samples studied. The major phases of such micrometeorites are olivine, low-Ca pyroxene, magnetite, and Fe-sulfide and the average abundances are 65, 17, 11, and 7 wt%, respectively, when the total abundance of the four minerals are normalized to 100 wt%. The relative mineral abundance varies greatly between samples: low-Ca pyroxene/olivine ratios range from 0 to 3.5, with a mean of 0.3. TEM observations of inner portions of some micrometeorites revealed that they are aggregates of very small equigranular grains (~ 100 nm) of olivine + magnetite, or low-Ca pyroxene + olivine + magnesiowüstite. The textures are very similar to those of hydrous carbonaceous chondrite that was experimentally heated to temperature below melting point, thus suggesting that the micrometeorites had been hydrous particles but were decomposed by the brief heating upon atmospheric entry. It is newly found that magnesiowüstite was formed in micrometeorites instead of magnetite as a product of phyllosilicate decomposition under low oxygen fugacity. The decomposed hydrous micrometeorites gave two types of characteristic X-ray diffraction patterns: (1) broad olivine and magnetite reflections or (2) variable intensities of magnesiowüstite reflections together with magnetite, low-Ca pyroxene, and olivine reflections. Twenty-nine olivine- or pyroxene-rich micrometeorites showed such diffraction patterns, thus suggesting that more than half of micrometeorites investigated must be decomposed hydrous particles. The results confirmed that hydrous dust particles are much more abundant in the interplanetary space than in the micrometeorites recovered on the Earth.
AB - Bulk mineralogy of individual fine-grained micrometeorites from 50 to 200 μm in diameter was determined on the basis of the powder X-ray diffraction patterns and the observation of internal textures by a transmission electron microscope (TEM). X-ray diffraction analysis of 56 micrometeorites indicated that 42, 11, and 3 samples are olivine-rich, pyroxene-rich, and phyllosilicate-rich micrometeorites, respectively. Among the phyllosilicate-rich micrometeorites, one contains saponite and other two contain serpentine. No samples contain both saponite and serpentine. We found that saponite-rich micrometeorite was weakly heated, which results in shrinkage of 001 basal spacing of saponite down to 9.7 Å, and that cronstedtite, which is commonly contained in CM chondrites, occurs in serpentine-rich micrometeorites. Micrometeorites that consist entirely of anhydrous minerals and amorphous phases are predominant in the samples studied. The major phases of such micrometeorites are olivine, low-Ca pyroxene, magnetite, and Fe-sulfide and the average abundances are 65, 17, 11, and 7 wt%, respectively, when the total abundance of the four minerals are normalized to 100 wt%. The relative mineral abundance varies greatly between samples: low-Ca pyroxene/olivine ratios range from 0 to 3.5, with a mean of 0.3. TEM observations of inner portions of some micrometeorites revealed that they are aggregates of very small equigranular grains (~ 100 nm) of olivine + magnetite, or low-Ca pyroxene + olivine + magnesiowüstite. The textures are very similar to those of hydrous carbonaceous chondrite that was experimentally heated to temperature below melting point, thus suggesting that the micrometeorites had been hydrous particles but were decomposed by the brief heating upon atmospheric entry. It is newly found that magnesiowüstite was formed in micrometeorites instead of magnetite as a product of phyllosilicate decomposition under low oxygen fugacity. The decomposed hydrous micrometeorites gave two types of characteristic X-ray diffraction patterns: (1) broad olivine and magnetite reflections or (2) variable intensities of magnesiowüstite reflections together with magnetite, low-Ca pyroxene, and olivine reflections. Twenty-nine olivine- or pyroxene-rich micrometeorites showed such diffraction patterns, thus suggesting that more than half of micrometeorites investigated must be decomposed hydrous particles. The results confirmed that hydrous dust particles are much more abundant in the interplanetary space than in the micrometeorites recovered on the Earth.
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U2 - 10.1016/S0016-7037(01)00722-0
DO - 10.1016/S0016-7037(01)00722-0
M3 - Article
AN - SCOPUS:0035668191
SN - 0016-7037
VL - 65
SP - 4385
EP - 4397
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 23
ER -