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Shift and Rotation of Composition Trends by Magma Mixing: 1983 Eruption at Miyake-jima Volcano, Japan 总被引：1，自引：0，他引：1
KURITANI TAKESHI; YOKOYAMA TETSUYA; KOBAYASHI KATSURA; NAKAMURA EIZO 《Journal of Petrology》2003,44(10):1895-1916
Pre-eruption processes are investigated for magmas erupted in1983 from Miyake-jima volcano, which is one of the most activevolcanoes in Japan. The whole-rock compositional trends of theeruptive products are principally smooth and linear. Magmaserupted from some fissures have compositions that deviate fromthe main linear trend. Phenocryst contents of samples displacedfrom the linear compositional trends are significantly lowerthan those of samples on the main trends. Anorthite-rich plagioclasephenocrysts, present throughout the 1983 products, are too calcicto have crystallized from the erupted magma composition, andwere derived from a basaltic magma through magma mixing. Althoughthe linear whole-rock composition trends favor simple two-componentmagma mixing, this cannot explain the presence of samples thatdeviate from the main trend. Instead, the observed compositiontrends were formed by mixing of a homogeneous basaltic magmawith andesitic magmas exhibiting compositional diversity. Theoriginal linear composition trends of the andesitic end-membermagma were rotated and shifted to the direction of the basalticend-member magma by magma mixing. The samples out of the maintrends represent magmas with less basaltic component than thoseon the trend. The density and viscosity of the basaltic end-membermagma were comparable with those of the andesitic end-membermagmas. The basaltic magma, discharged from one magma chamberat 2 kbar pressure, was injected into a magma chamber at lowerpressure occupied by the chemically zoned andesite magma (1kbar), and possibly as a fountain. To establish the characteristicmixing trend of the 1983 magma, the basaltic component musthave been distributed systematically in the zoned andesite magma.A requirement is that the basaltic magma spread laterally andmixed with the andesite magma at various levels of ascent ofthe fountain in the host andesite magma. Analysis of compositionalzoning in titanomagnetite crystals revealed that the eruptionof the 1983 magmas was initiated soon after the replenishmentof the basaltic magma in the 1 kbar magma chamber. KEY WORDS: compositional trend; liquid–liquid blending; magma chamber; magma mixing; Miyake-jima Volcano 相似文献
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Middle Eocene conglomerates which overlie the Sanbagawa metamorphic rocks contain clasts of metamorphic rock with isotope ages of 120-85 Ma, which fall within the age range reported from the Sanbagawa metamorphic rocks. They were derived from the chlorite to oligoclase zones of the Sanbagawa metamorphic belt. Clasts of garnet amphibolite and oligoclase-biotite schist show a mineral assemblage similar to the highest grade Sanbagawa schists. However, the metamorphic temperatures estimated by various mineralogical thermometers show that some of the clasts were formed at higher temperatures than the in situ Sanbagawa metamorphic rocks. Such higher grade rocks were at the surface by the Middle Eocene and for the most part they have been eroded away. Cretaceous and post-Cretaceous sediments overlie, or are in fault contact with, the Sanbagawa metamorphic rocks which suggests that rocks in the belt were uplifted and eroded from the latest Cretaceous to Middle Eocene time after strike-slip movement along the Median Tectonic Line. Since the Middle Eocene, the belt has experienced relatively slow uplift which was locally around 2 km in central Shikoku. 相似文献
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C. LANA C. R. SOUZA FILHO Y. R. MARANGONI E. YOKOYAMA R. I. F. TRINDADE E. TOHVER W. U. REIMOLD 《Meteoritics & planetary science》2008,43(4):701-716
Abstract— The 40 km wide Araguainha structure in central Brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation. The dominant structural features preserved at Araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity. Slumping of the transient cavity walls resulted in inward‐verging inclined folds and a km‐scale anticline in the outer ring of the structure. The folding stage was followed by radial and concentric faulting, with downward displacement of kilometer‐scale blocks around the crater rim. The central uplift records evidence for km‐scale upward movement of crystalline basement rocks from the transient cavity floor, and lateral moment of sedimentary target rocks detached from the cavity walls. Much of the structural grain in the central uplift relates to structural stacking of km‐scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks. Outward‐plunging radial folds indicate tangential oblate shortening of the strata during the imbrication of the thrust sheets. Each individual sheet records an early stage of folding and thickening due to non‐coaxial strains, shortly before sheet imbrication. We attribute this folding and thickening phase to the kilometer‐scale inward movement of the target strata from the transient cavity walls to the central uplift. The outer parts of the central uplift record additional outward movement of the target rocks, possibly related to the collapse of the central uplift. An inner ring structure at 10–12 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks. 相似文献
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Baffin Bay, Texas is the flooded Last Glacial Maximum incised valley of the Los Olmos, San Fernando and Petronila Creeks along the north‐western Gulf of Mexico. Cores up to 17 m in length and high‐resolution seismic profiles were used to study the history of Baffin Bay over the last 10 kyr and to document the unusual depositional environments within the valley fill. The deposits of the Baffin Bay incised valley record two major and two minor events. Around 8·0 ka, the estuarine environments backstepped more than 15 km in response to an increase in the rate of sea‐level rise. Around 5·5 ka, these estuarine environments changed from environments similar to other estuaries of the northern Gulf of Mexico to the unusual suite of environments found today. Another minor flooding event occurred around 4·8 ka in which several internal spits were flooded. Some time after 4·0 ka, the upper‐bay mud‐flats experienced a progradational event. Because of its semi‐arid climate and isolation from the Gulf of Mexico, five depositional environments not found in the other incised‐valley fills of the northern Gulf of Mexico are found today within Baffin Bay. These deposits include well‐laminated carbonate and siliciclastic open‐bay muds, ooid beaches, shelly internal spits and barrier islands, serpulid worm‐tube reefs and prograding upper‐bay mud‐flats. Based on these unusual deposits, and other characteristics of Baffin Bay, five criteria are suggested to help identify incised valleys that filled in arid and semi‐arid climates. These criteria include the presence of: (i) hypersaline‐tolerant fauna; (ii) aeolian deposits; and (iii) carbonate and/or evaporite deposits; and the absence of: (iv) peat or other organic‐rich deposits in the upper bay and bay‐margin areas; and (v) well‐developed fluvially dominated bayhead deltas. 相似文献
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Hirohisa YAMADA Kenji TAMURA Shingo YOKOYAMA Yujiro WATANABE 《《地质学报》英文版》2006,80(2):290-295
Lath-shaped and highly crystalline trioctahedral smectites were synthesized under hydrothermal conditions. The quenched glasses with stoichiometrically dehydrated Na-smectite compositions were treated at （a） 500℃ and 100 MPa for 1 and 11 days; and （b） 300℃ and 100 MPa for 7 days. The crystallinity and particle size of products were dependent on the chemical composition of the starting glass, synthetic temperature and duration of hydrothermal treatment. The high structural ordering and large dimensions of the products were confirmed from the sharpness of XRD peaks; and hydration behavior under controlled relative humidity. Transmission electron microscopy was also performed for the characterization of the particle size of product. Particle sizes vary from ca. 10 nm to a few pm by changing the chemical compositions of the starting materials. The product with the highest structural ordering and largest dimension was obtained from Nao.33（Mg1.83Al0.67）Si4O11 glass treated at 500℃ and 100 MPa for 1 day. The obtained results also confirmed the metastability and compositional dependency in the formation of highly crystalline trioctahedral smectite at hydrothermal conditions. 相似文献
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M. A. R. VASCONCELOS K. WÜNNEMANN A. P. CRÓSTA E. C. MOLINA W. U. REIMOLD E. YOKOYAMA 《Meteoritics & planetary science》2012,47(10):1659-1670
Forward modeling is commonly applied to gravity field data of impact structures to determine the main gravity anomaly sources. In this context, we have developed 2.5‐D gravity models of the Serra da Cangalha impact structure for the purpose of investigating geological bodies/structures underneath the crater. Interpretation of the models was supported by ground magnetic data acquired along profiles, as well as by high resolution aeromagnetic data. Ground magnetic data reveal the presence of short‐wavelength anomalies probably related to shallow magnetic sources that could have been emplaced during the cratering process. Aeromagnetic data show that the basement underneath the crater occurs at an average depth of about 1.9 km, whereas in the region beneath the central uplift it is raised to 0.5–1 km below the current surface. These depths are also supported by 2.5‐D gravity models showing a gentle relief for the basement beneath the central uplift area. Geophysical data were used to provide further constraints for numeral modeling of crater formation that provided important information on the structural modification that affected the rocks underneath the crater, as well as on shock‐induced modifications of target rocks. The results showed that the morphology is consistent with the current observations of the crater and that Serra da Cangalha was formed by a meteorite of approximately 1.4 km diameter striking at 12 km s?1. 相似文献
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在热液条件下合成了板状的和高度结晶的三八面体蒙脱石。用淬冷玻璃与脱水的钠基蒙脱石分别在(a)500°C,100MPa下1天和11天;(b)300°C,100MPa下7天的条件下进行反应。产物的结晶度和颗粒大小取决于初始玻璃的化学组成,合成温度和热液改性的时间。XRD尖锐的峰和一定相对湿度下水合作用证明了产物结构的高度有序和大尺寸。透射电镜也被用来表征产物颗粒尺寸。通过改变初始材料的化学成分,产物的颗粒大小可以在10nm到几个微米之间变化。有序度最高和尺寸最大的产物是在500°C,100MPa的条件下用Na0.33(Mg1.83Al0.67)Si4O11成分的玻璃反应… 相似文献
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Abstract: The gold content in siliceous deposits formed from geothermal waters at the Hatchobaru geothermal power station in central Kyushu, Japan, was determined by inductively coupled plasma quadrupole mass spectrometry (ICP–MS). It ranges from 1.3 to 4.6 mg/kg. The highest gold content was obtained from the siliceous deposit formed from acidic waters. In this siliceous deposit, hydrous iron(III) oxide of amorphous state is a major constituent, suggesting that hydrous iron(III) oxide might play an important role when gold is concentrated into siliceous deposits formed from acidic and iron-rich geothermal waters. 相似文献