Mineralogical evidence of heating events in Antarctic carbonaceous chondrites, Y-86720 and Y-82162

Antarctic carbonaceous chondrites, Y-82162 and Y-86720 were examined by transmission electron microscopy. Matrix phyllosilicates in both meteorites are probably serpentine and saponite. There is evidence that these phyllosilicates were affected by thermal metamorphism. The serpentine was almost completely transformed to olivine or an intermediate phase between serpentine and olivine. The degrees of thermal metamorphism in Y-86720,Y-82162 and Y-793321 were compared, and heating experiments of terrestrial saponite and Murchison CM chondrite were also carried out for detailed comparison. Based on these observations and experiments, estimated degrees are as follows : Y-86720>Y-82162>Y-793321

Void structures in olivine grains in thermally metamorphosed Antarctic carbonaceous chondrite B-7904

The Antarctic carbonaceous chondrite Belgica-7904 suffered thermal metamorphism. The main constituent phyllosilicates of this meteorite are serpentine and/or saponite, which display transformation from serpentine to olivine or from saponite to enstatite in various degrees (J. AKAI : Geochim. Cosmochim. Acta, 52,1593,1988; Proc. NIPR Symp. Antarct. Meteorites, 3,55,1990). Detailed Transmission Electron Microscopy (TEM) observation of this meteorite reveals characteristic void structures, which are found mainly in olivine grains. The voids or bubble-like structures are very frequently found in some grains. The shapes of the voids are varied. Some are irregular and some are crystallographically controlled. Five possible mechanisms for their origin are : 1) thermal metamorphism, 2) irradiation by high energy particles in the solar nebula, 3) shock effects, 4) sample preparation artifacts and 5) alteration process. Among five or more possible mechanisms, the voids mostly likely formed during thermal metamorphism or as a result of irradiation damages by high energy particles in the early solar system

T-T-T diagram of serpentine and saponite, and estimation ofmetamorphic heating degree of Antarctic carbonaceous chondrites

For the estimation of peak thermal metamorphic degree of phyllosilicates in Antarctic carbonaceous chondrites, heating experiments of serpentine and saponite were carried out. A T-T-T diagram of serpentine for elevated temperature was obtained by heating the serpentine in the Murchison CM2 chondrite in vacuo at different temperatures (250∿1100℃) and for different durations (1 hour to 350 hours). Serpentine decomposed into olivine and enstatite through characteristic transitional structures. First decomposition started at about ∿300℃, the transitional structure appears at ca. 400∿500℃, and well-crystallized olivine and enstatite appeared above ∿750℃. In the T-T-T diagram, a minor time dependence was found. A T-T-T diagram of saponite was obtained in a similar was using terrestrial Fe-saponite. Saponite maintained its structure till about 800∿900℃, then decomposition started and finally enstatite was formed. New data on thermally affected phyllosilicates in Yamato (Y)-793321 indicate coexistence of 7Å unaltered serpentine and a transitional structure. Thermally affected saponite in Belgica (B)-7904 was also documented. Based on the T-T-T diagrams, the peak metamorphic degrees and peak temperatures were estimated in the order of B-7904≥Y-86720>Y-82162>Y-793321

TEM observation of void-like defect structures in matrix-and isolated-olivine grains in Allende (CV3)

Olivine grains in the Allende CV3 carbonaceous chondrite matrix were examined mainly by TEM. Compositional data of olivine by EPMA are also given. It was revealed that void-like defects were observed occurring in iron enriched portions of both matrix olivine and rims of isolated olivine grains. Detailed observations of these void-like defects suggest that the void structure was formed during thermal metamorphism and that the thermal metamorphism was also related to some chemical reaction to form fayalitic olivine either in the pre-solar nebula or on a parent body

Shock effects experiments on serpentine and thermal metamorphic conditions in Antarctic carbonaceous chondrite

The unique Antarctic carbonaceous chondrites, Belgica (B)-7904,Yamato (Y)-86720,Y-82162,Y-793321 are thermally metamorphosed. However, the heat source of the thermal metamorphism is not known. Two strong possibilities are shock-induced heating and heating on the parent body. The explosive impact method was used to check the possibility of heating of phyllosilicates by shock compression. Examining the shocked specimens from the Murchison meteorite and terrestrial lizardite, the following were found : (1) Phyllosilicates in the shocked (>32.1GPa) specimens changed to nearly amorphous substances; (2) the phyllosilicates in specimens shocked at lower pressures were still crystalline and undamaged; (3) some void-like (bubble) textures were widely observed in the amorphous substances; (4) the other minerals such as pyroxenes and olivines which did not change to glass phases seem to be little affected by shock. These facts do not suggest that the unique Antarctic chondrites experienced significant shock

Ⅳ. Remediation

新潟大学富山大学日本歯科大学新潟生命歯学部Editor : Tazaki, Kazue |田崎, 和

Direct Observation of Stacking Faults in Wollastonite

Wollastonite is well known to have diffuse streaks on k odd level in its X-ray diffraction photographs which have been said to be due to stacking fault with displacement of b/2 in stacking along the a direction (JEFFERY, 1953). The stacking fault is directly observed by high-resolution electron microscopy. In the electron micrograph frequent stacking faults are observed in some places (faulting density α=0.4~0.6), but the density varies very much from portion to portion. A dislocation is observed in two dimensional lattice image. Electron diffraction of wollastonite indicates some features which were not found in X-ray photographs; faint streaks along a * on k even levels of (hkO) reciprocal lattice plane and faint streaks along c * on k odd levels of (Okl) reciprocal lattice plane and fine structure of the streaks along a * on k odd levels of (hkO) reciprocal lattice plane