Carbon materials from eight Antarctic carbonaceous chondrites (CM2, CO3, and CV3 types) and two nonAntarctic carbonaceous chondrites (CM2 and CV3 types) were studied using a high-resolution transmission electron microscope (HRTEM). Nanodiamonds and “carbonaceous globules” were found in seven of the Antarctic chondrites and one nonAntarctic CM2 chondrite. These results suggest that nanodiamonds and carbonaceous globules are common in carbonaceous chondrites. The finding of carbonaceous globules in the meteorites that experienced post-hydration high-temperature thermal metamorphism suggests that they are resistant to thermal metamorphism. Well-crystallized graphite was mainly found in the CV3- and CO3-type chondrites, whereas poorly crystallized graphite was mainly found in the CM2-type chondrites. The CM2-type chondrites also contain characteristic carbon structures that are similar to tetrahedral carbon onions and multilayered fullerene. In this study, we discuss the origin of these types of carbon material.
We determined the K-Ar ages for the high-magnesian andesites from Miocene-Pliocene period from Hikosan and Nagasaki areas in northern Kyushu, Japan. The obtained K-Ar ages of Hikosan and Nagasaki samples are 4.0-3.6 Ma and 5.8-3.7 Ma, respectively. These K-Ar ages suggest that the magmatisms of the high-magnesian andesites coincided with the development of the Okinawa Trough. The upwelling of the hot asthenospheric mantle associated with the backarc rifting probably has a close relationship with the origin of the northern-Kyushu high-magnesian andesite magma.
A new experimental facility has been developed to investigate the fragmentation of vesicular magma undergoing rapid decompression. The facility based on a vertical shock tube was designed and constructed to produce starch syrup foams of high void fraction but low permeability, high liquid viscosity, and at high pressure as alternative to foamy magma exposed to expansion waves. The highly viscous material (starch syrup) of dynamic shear viscosities ranging from 5 to 1012 Pa s were intermixed with nitrogen gas bubbles under 1.6 to 2.6 MPa and were subsequently loaded into an acrylic test section. The foamy starch syrup was pressurized up to 2.6 MPa and then rapidly decompressed to 0.1 MPa, which caused its fragmentation and simultaneous ejection of the fragmentation products into a large-volume chamber. In ejecting high-pressure foam into a low-pressure chamber, the foam's fragmentation process was characterized with the help of pressure measurements and high-speed video recording. Prior to decompression experiments, we examined the viscoelastic properties of the foam specimens by using a rheometer. The rapid decompression generated wide varieties of fragment shapes similar to those observable in volcanic explosions, such as elongated to blocky pumice and flat, pointed shards with smooth or conchoidal fracture. The results prove that a shock dynamic approach is effective to model magma fragmentations.
Using a gas-jet levitator, Mg-silicate glasses containing forsterite (Mg2SiO4) or enstatite (MgSiO3) are obtained, with a particle diameter of 2 mm. The levitator aids in floating the melt droplets using a gas-jet. It can transform the levitated droplets into crystals or glass during cooling. The critical cooling rate for glass formation (Rc) for levitated Mg-silicate melts («10-1-102 K/s) is considerably lower than that for non-levitated melts (>103 K/s) because of the inhibition of crystallization by the sample holder in the former. Furthermore, the crystallization temperature of the levitated Mg-silicate melts is very low (∼ 900 °C). Chondrules in chondrites are assumed to have been formed from the melt droplets in such a levitation environment. Several researchers have concluded that Rc for a chondrule melt held by a platinum wire is equal to the upper limit of the cooling rate for the chondrule melt, estimated to be ∼ 1000 K/h. However, in a levitation environment, SiO2-rich melts do not crystallize at this cooling rate. We conclude that the previously proposed maximum cooling rate for chondrule melts has been greatly overestimated. We infer that spontaneous crystallization of a completely melted chondrule precursor is extremely difficult, and almost all chondrules turn into glass, despite a low cooling rate. For crystallization of chondrule melts, it is necessary that the precursor of the chondrules should be partially melted or the completely melted precursor should be in contact with nebular dust.
The following are errata for the original article entitled “Crystal orientation analyses of biominerals using Kikuchi patterns in TEM” by Kazuko SARUWATARI, Junji AKAI, Yoshihiro FUKUMORI, Noriaki OZAKI, Hiromichi NAGASAWA and Toshihiro KOGURE (Vol. 103, no. 1, 16-22, 2008).
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