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21.
Kazushige Tomeoka 《Geochimica et cosmochimica acta》2011,75(20):6064-6079
We report the discovery of a highly hydrated clast in the Mokoia CV3 carbonaceous chondrite that contains eight chondrules and one amoeboid olivine inclusion (AOI), embedded in a matrix dominated by hydrous phyllosilicates. Anhydrous silicates in the chondrules and AOI were extensively replaced by phyllosilicates. The matrix has a composition intermediate between saponite and serpentine and contains abundant framboidal magnetite, being similar to the matrices of the CI chondrites. The mineralogy and texture indicate that the clast resulted from intensive aqueous alteration of a precursor equivalent to the host Mokoia meteorite, and it can be regarded as the CV2-type lithology. The results indicate that there was in fact an extremely wet region within the Mokoia parent body and that this clast escaped subsequent thermal metamorphism.This clast reveals abundant evidence of progressive and dynamic aqueous alteration processes. The chondrules were replaced inward from their margins, and the alteration products were disaggregated and mixed into the matrix; as a result, the chondrules increasingly became smaller in size and irregular in shape. Some of the chondrules were separated into pieces. The results indicate that a major proportion of the matrix materials were produced by the alteration of chondrules and inclusions and that there was a stage in which the alteration products and the original matrix material were extensively and uniformly mixed together.The presence of the clast supports a previously proposed model, which suggests that hydrated chondrules, inclusions, and the rims surrounding them in the host Mokoia chondrite are clasts produced by brecciation within a wet region of the parent body and that they were subsequently transported and incorporated into the dry matrix. 相似文献
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Manchuk Nuramkhaan Toshiyuki Kurihara Kazuhiro Tsukada Yoshikazu Kochi Hokuto Obara Tatsuya Fujimoto Yuji Orihashi Koshi Yamamoto 《Island Arc》2013,22(4):494-507
The dating of radiolarian biostratigraphic zones from the Silurian to Devonian is only partially understood. Dating the zircons in radiolarian‐bearing tuffaceous rocks has enabled us to ascribe practical ages to the radiolarian zones. To extend knowledge in this area, radiometric dating of magmatic zircons within the radiolarian‐bearing Hitoegane Formation, Japan, was undertaken. The Hitoegane Formation is mainly composed of alternating beds of tuffaceous sandstones, tuffaceous mudstones and felsic tuff. The felsic tuff and tuffaceous mudstone yield well‐preserved radiolarian fossils. Zircon grains showing a U–Pb laser ablation–inductively coupled plasma–mass spectrometry age of 426.6 ± 3.7 Ma were collected from four horizons of the Hitoegane Formation, which is the boundary between the Pseudospongoprunum tauversi to Futobari solidus–Zadrappolus tenuis radiolarian assemblage zones. This fact strongly suggests that the boundary of these assemblage zones is around the Ludlowian to Pridolian. The last occurrence of F. solidus is considered to be Pragian based on the reinterpretation of a U–Pb sensitive high mass‐resolution ion microprobe (SHRIMP) zircon age of 408.9 ± 7.6 Ma for a felsic tuff of the Kurosegawa belt, Southwest Japan. Thus the F. solidus–Z. tenuis assemblage can be assigned to the Ludlowian or Pridolian to Pragian. The present data also contribute to establishing overall stratigraphy of the Paleozoic rocks of the Fukuji–Hitoegane area. According to the Ordovician to Carboniferous stratigraphy in this area, Ordovician to Silurian volcanism was gradually reduced to change the sedimentary environment into a tropical lagoon in the early Devonian. And the quiet Carboniferous environment was subsequently interrupted, throwing it once more into the volcanic conditions in the Middle Permian. 相似文献
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The spatial distribution of deep slow earthquake activity along the strike of the subducting Philippine Sea Plate in southwest Japan is investigated. These events usually occur simultaneously between the megathrust seismogenic zone and the deeper free-slip zone on the plate interface at depths of about 30 km. Deep low-frequency tremors are weak prolonged vibrations with dominant frequencies of 1.5–5 Hz, whereas low-frequency earthquakes correspond to isolated pulses included within the tremors. Deep very-low-frequency earthquakes have long-period (20 s) seismic signals, and short-term slow-slip events are crustal deformations lasting for several days. Slow earthquake activity is not spatially homogeneous but is separated into segments some of which are bounded by gaps in activity. The spatial distribution of each phase of slow earthquake activity is usually coincident, although there are some inconsistencies. Very-low-frequency earthquakes occur mainly at edges of segments. Low-frequency earthquakes corresponding to tremors of relatively large amplitude are concentrated at spots where tremors are densely distributed within segments. The separation of segments by gaps suggests large differences in stick-slip and stable sliding caused by frictional properties of the plate interface. Within each segment, variations in the spatial distribution of slow earthquakes reflected inhomogeneities corresponding to the characteristic scales of events. 相似文献
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我们对日本西南部南海俯冲板块界面过渡带发生的超低频地震进行报道。由矩震级为3.1～3.5的超低频地震引起的地震波显示其卓越长周期约为20s。超低频地震活动伴随深源低频颤动和慢滑事件同时发生,并随之移动。慢地震一直被认为会增加向上倾斜的大型逆冲地震断裂带上的应力,而这3种现象的同时发生提高了人们对慢地震的探测和特性鉴定水平。 相似文献
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Abstract— Mokoia is a CV3 chondrite that contains abundant phyllosilicate mineralization. We present a detailed petrographic and scanning electron microscopic study of 24 dark inclusions (DIs) that we found in Mokoia. The overall texture and constituent minerals of the DIs resemble those in the host meteorite. Fe‐bearing saponite and Na‐rich phlogopite, the same phyllosilicates as in the host meteorite, occur in the DIs, which strongly suggests that the DIs have a similar alteration history to the host meteorite. However, the DIs show several distinct differences from the host meteorite. Olivine grains in the DI matrices are more homogeneous in Fe/(Fe + Mg) ratio than those in the host meteorite matrix. Phyllosilicates in the DIs are less abundant than in the host meteorite, and they have been dehydrated to various extents. These characteristics suggest that the DIs have experienced higher degree of thermal metamorphism than the host meteorite. In addition, the matrices in the DIs are more compacted than those in the host meteorite. Most olivine grains in the DIs show undulatory extinction in transmitted crossed‐polarized light and some show planar fractures, while such olivine grains are rare in the host meteorite. Two of the DIs contain Si‐, Mg‐, Fe‐ and O‐rich melt veins. These characteristics indicate that most DIs have been shocked to shock stage S3‐S4, while the host meteorite is shock stage S1 (virtually unshocked). Thermal metamorphism of the DIs was likely caused by shock heating. These results are consistent with the contention previously proposed for the DIs in CV3 chondrites (i.e., the DIs have experienced aqueous alteration and subsequent dehydration on the CV parent body). We suggest that thermal and shock metamorphism occurred locally to various extents after pervasive aqueous alteration in the Mokoia parent body. 相似文献
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Abstract— We have studied the mineralogy and petrology of mesostases of 783 type I chondrules in seven CO3 chondrites that range in petrologic subtype from 3.0 to 3.7. Chondrule mesostases in the CO chondrite of subtype 3.0 consist mainly of primary glass and plagioclase, while chondrule mesostases in the CO chondrites of higher subtypes (3.2–3.7) contain various amounts of nepheline in addition to glass and plagioclase. Nepheline has replaced glass and plagioclase, forming finegrained aggregates and thin parallel lamellar intergrowths with plagioclase. The nephelinization has proceeded preferentially from the outer margins of chondrules toward the inside. Although the degree of nephelinization differs widely among chondrules in each of the metamorphosed chondrites, our modal analyses and bulk chemical analyses of individual mesostases indicate that the amounts of nepheline in chondrules systematically increase with the increasing petrologic subtype of the host chondrites. Nepheline also has a tendency to increase in grain size with increasing petrologic subtype. We conclude that nepheline in chondrules in the CO3 chondrites has formed largely as a result of effects related to heating on the meteorite parent body. We suggest that nepheline initially formed as hydrous nepheline under the presence of aqueous fluids and subsequently was dehydrated after exhaustion of aqueous fluids. The degree of hydrothermal activity must have increased with increasing degree of heating, and thus, chondrules in more thermally metamorphosed chondrites produced larger amounts of nepheline. The results imply that CO3 chondrites have gone through low‐grade aqueous alteration and subsequent dehydration at the early stage of heating on the meteorite parent body. 相似文献
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Godi Fischer Sangmok Lee Michael Obara Prasan Kasturi H. Thomas Rossby Conrad W. Recksiek 《Oceanic Engineering, IEEE Journal of》2006,31(4):975-985
This paper presents the design of an ultralow-power acoustical receiver circuit intended to track small aquatic animals. The receiver forms a crucial component of a versatile data logger capable of sensing and storing a variety of biologically important data such as geographic position, ambient temperature, pressure, etc. The size of the final tag will be determined by the size of the cylindrical hydrophone, which will serve as housing for all electronic components, the external sensors, the timing crystal, and the battery. The receiver circuit was prototyped on a 0.5-mum complementary metal-oxide-semiconductor (CMOS) chip. Extensive lab tests proved all system component functional. The fully operational receiver consumes 21 muW at 3 V. Results from a preliminary field test, conducted with a source generating a sound pressure level of 180 dB re 1 muPa, predict a tracking range of 100-120 km 相似文献
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Andreas Morlok Chiyoe Koike Kazushige Tomeoka Andrew Mason Carey Lisse Mahesh Anand Monica Grady 《Icarus》2012,219(1):48-56
Mid-infrared (5–25 μm) transmission/absorption spectra of differentiated meteorites (achondrites) were measured to permit comparison with astronomical observations of dust in different stages of evolution of young stellar objects. In contrast to primitive chondrites, achondrites underwent heavy metamorphism and/or extensive melting and represent more advanced stages of planetesimal evolution. Spectra were obtained from primitive achondrites (acapulcoite, winonaite, ureilite, and brachinite) and differentiated achondrites (eucrite, diogenite, aubrite, and mesosiderite silicates). The ureilite and brachinite show spectra dominated by olivine features, and the diogenite and aubrite by pyroxene features. The acapulcoite, winonaite, eucrite, and mesosiderite silicates exhibit more complex spectra, reflecting their multi-phase bulk mineralogy.Mixtures of spectra of the primitive achondrites and differentiated achondrites in various proportions show good similarities to the spectra of the few Myr old protoplanetary disks HD104237A and V410 Anon 13. A spectrum of the differentiated mesosiderite silicates is similar to the spectra of the mature debris disks HD172555 and HD165014. A mixture of spectra of the primitive ureilite and brachinite is similar to the spectrum of the debris disk HD113766. The results raise the possibility that materials produced in the early stage of planetesimal differentiation occur in the protoplanetary and debris disks. 相似文献