I have been studying mineralogy and petrology of various kinds of extraterrestrial samples: meteorites, Antarctic micrometeorites (AMMs), interplanetary dust particles (IDPs), and fine-grained samples returned from Moon, asteroid (25143) Itokawa, comet 81P/Wild 2, and international space station (ISS). Here I introduce some of my mineralogical studies of extraterrestrial materials by using transmission electron microscope (TEM). I began my meteorite studies during a graduate student, which led me to electron petrography of the extraterrestrial materials by TEM. Next, I describe how I introduced ultramicrotomy to the studies of extraterrestrial materials in Japan. Then, I show some of my studies of AMMs; they are (1) the mineralogy of the vast majority of hydrated AMMs, which is not similar to that of CM chondrites but that of chondritic smooth (CS) IDPs and that of Tagish Lake ungrouped C chondrite, (2) GEMS and enstatite whisker and platelet-bearing highly porous AMMs, which give us another opportunity to investigate samples originated from comets and comet-like icy bodies, and (3) GEMS-bearing AMMs experienced various degrees of aqueous alteration, which shed light on the relationships between CP IDPs and CS IDPs. Then, I introduce micrometeoroids captured at the ISS. A large micrometeoroid revealed similarity to chondrule-like objects recovered from comet 81P/Wild 2 on the standpoint of mineralogy and oxygen isotopic ratios of ferromagnesian silicates. Finally, I show TEM studies of surface modification (space weathering) of Itokawa grains. TEM observation revealed that solar wind is the major cause of the space weathering of the asteorid Itokawa. Some Itokawa grains show blistering on their surfaces, which resulted from the segregation of gaseous species such as H and He near the surface in the space weathering rims.
In this paper, I introduce three topics of my researches including the technical development of high pressure generation in the Kawai-type multianvil apparatus and rheological studies for the lower mantle and the inner core, as follows: (i) We extended the attainable pressure of the Kawai-type multianvil apparatus to 120.3 GPa at room temperature by equipping sintered diamond cubic anvils. In the experiments, however pressure dropped to 105 GPa from 120 GPa at 1673 K and we did not observe the phase transition from bridgmanite to post-perovskite. (ii) The electrical conductivity of mantle rocks during phase transformation from ringwoodite to silicate perovskite and ferro-periclase was measured at 25 GPa and 1300-1900 K. The electrical conductivity was high at the early stage of annealing, suggesting that ferro-periclase forms interconnected layers in aggregates of bridgmanite and ferro-periclase. At the later stage, the electrical conductivity decreased and reached to that of bridgmanite, suggesting the cut-off of the interconnected ferro-periclase because of rounding of crystals. The interconnection of ferro-periclase, which has a lower viscosity than bridgmanite, can be maintained in a cold descending slab over geological time scales (~ 1 My), indicating that a colder slab is less viscous than the warmer mantle surrounding it. The low-viscosity slab can be prevented from penetrating into the deeper part of the lower mantle by the high viscosities encountered at a depth of ~ 1000 km, that cause stagnation at this depth as observed by seismic tomography. (iii) A formation age of the inner core is a key to understanding Earth's evolution and its thermal history. Knowledge of grain size of the inner core material can provide a constraint of formation age of the inner core. We determined grain growth rate of ε-iron at ~ 55 GPa and 1200-1500 K by means of in-situ X-ray diffraction observation. Extrapolation of the grain growth law of ε-iron to the inner core conditions suggests that at least ~ 2.9 Gy is required to reach the equivalent size of the inner core inferred from seismology. Based on the translation model of the inner core, the present result indicates that the age of the inner core is older than 2.9 Gy.
The deeper part of the Earth's mantle is considered to be reduced state where CH4, H2, and hydrocarbons would exist. Thus, the reduced species of carbon-hydrogen compounds are likely to play an important role in the deep cycle of carbon and hydrogen at the upper mantle. Effects of H2 fluid on stability and phase relation of silicate minerals were investigated on MgO-SiO2-H2 systems using laser-heated diamond-anvil cells. It was revealed that SiO2 components dissolve into H2 fluid, with formation of Si-H compounds and H2O molecules at 2-15 GPa, >1500 K. Aromatic hydrocarbons are considered as the most abundant organic materials in nature and the existence of aromatic compounds in interior of the Earth and icy planets has been reported. In this study, pressure-induced chemical reactions of aromatic hydrocarbons, such as benzene and naphthalene were reported. Mass spectrometry of the reaction products revealed that oligomerization occurred after compression >15 GPa at room temperature. The dimerization products were classified into three groups, simple dimerization, naphthylation, and condensation. It was indicated that release of hydrogen by the pressure-induced oligomerization was lesser as compared to the temperature-induced pyrolysis. The results provide a basic information on the chemical reaction of organics in the deep interior of the Earth.