排序方式： 共有65条查询结果，搜索用时 156 毫秒
31.
the Neapolitan Yellow Tuff (NYT) (12 ka BP) is considered to be the product of a single eruption. Two different members (A and B) have been identified and can be correlated around the whole of Campi Flegrei. Member A is made up of at least 6 fall units including both ash and lapilli horizons. The basal stratified ash unit (A1) is interpreted to be a phreatoplinian fall deposit, since it shows a widespread dispersal (>1000 km2) and a constant thickness over considerable topography. The absence of many lapilli fall units in proximal and medial areas testifies to the erosive power of the intervening pyroclastic surges. The overlying member B was formed by many pyroclastic flows, radially distributed around Campi Flegrei, that varied widely in their eruptive and emplacement mechanisms. In some of the most proximal exposures coarse scoria and lithic-rich deposits, sometimes welded, have been identified at the base of member B. Isopach and isopleth maps of fall-units, combined with the distribution of the coarse proximal facies, indicate that the eruptive vent was located in the NE area of Campi Flegrei. It is considered that the NYT eruption produced collapse of a caldera approximately 10 km diameter within Campi Flegrei. The caldera rim, located by geological and borehole evidence, is now largely buried by the products of more recent eruptions. Initiation of caldera collapse may have been contemporaneous with the start of the second phase (member B). It is suggested that there was a single vent throughout the eruption rather than the development of multiple or ring vents. Chemical data indicate that different levels of a zoned trachyte-phonolite magma chamber were tapped during the eruption. The minimum volume of the NYT is calculated to be about 50 km3 (DRE), of which 35 km3 (70%) occurs within the caldera. 相似文献
32.
《International Geology Review》2012,54(7):905-914
Twenty-first century advances in both the analytical procedures and instrumentation used in geochronology promise age accuracy better than ±1‰, but realizing this potential requires knowledge of decay constants (λ) that exceed this level. Given the paucity of improved recent measurements of λ, the community has experimented with hybrid methodologies utilizing data largely generated during the 1970s. In this article, we perform a systematic review of laboratory decay constant determinations relevant to geochronology (i.e. 87Rb, 147Sm, 176Lu, 230Th, 232Th, 235U, and 238U), focusing on methodological consistency. For radioisotopes for which multiple studies are available, results are combined through a random effects model to yield the best available values and associated uncertainties. Unfortunately, despite its vital role in modern geochronology, only one experimental determination of 238U decay met our criteria for consideration, significantly limiting the ability to assess its reliability. Thus, utilizing λ238 as an anchor for establishing other decay constants (e.g. 40K, 176Lu, and spontaneous 238U fission) places an unverified result at the core of geochronology. For geochronology to attain its greatest potential, more and better laboratory determinations of decay constants are required, along with a community methodology that permits us to continuously take advantage of new data. 相似文献
33.
The Dayizishan scoria cone and Jinlongdingzi scoria cone in the Longgang volcanic swarm, northeastern China are comprised of basaltic fallout tephra that derived from sub - Plinian eruptions. The airborn tephra formed a scoria cone and tephra sheets in geomorphological character. The fallout tephra deposits consist of basaltic scoria, bomb, ultramafic xenolith and lithic fragments. The tephra deposits in the tephra sheets developed plane parallel bedding and graded bedding. The size parameters of tephra changed with distance away from source and the lapse of time regularly.The Dalongwan tuff ring and Longquanlongwan tuff ring in Longgang volcanic swarm are comprised of base surge deposits, formed by phreatomagmatic explosions. The base surge deposits are composed of basaltic lapilli, pisolites and tuff.The sedimentary structure, for instance, dunelike structure, chute and pool structure, U - shaped channel and cross bedding, massive beds and sandwave beds etc. are recognized. 相似文献
34.
贵州盘县峨眉山玄武岩系顶部凝灰岩LA-ICP-MS 锆石U-Pb年龄:对峨眉山大火成岩省与生物大规模灭绝关系的约束 总被引：7，自引：3，他引：4
在20世纪90年代,有学者认为峨眉山大火成岩省(Emeishan Large Igneous Province, ELIP)大规模火山活动与二叠-三叠系之交(Permian-Triassic Boundary, P-TB)的生物大灭绝事件在时间上有耦合关系,随后的40Ar/39Ar同位素测年结果也显示峨眉山大火成岩省是晚二叠世形成的。但是,近些年大量的SHRIMP U-Pb测年结果表明,ELIP大规模火山喷发约在~260Ma;因此有研究认为,ELIP火山活动与中二叠世瓜德卢普期末(end-Guadalupian)的生物灭绝事件在时间上联系更加紧密。至于P-T界线生物大灭绝,现在多数学者认为是,由于西伯利亚大火成岩省火山强烈活动释放大量气体和火山灰所造成环境变化引起的。最近,我们在ELIP东部的贵州盘县峨眉山玄武岩系剖面中发现顶部发育厚度达近百米的凝灰岩层,其LA-ICP-MS U-Pb法测年结果为251.0±1.0Ma,与浙江煤山剖面中二叠系-三叠系边界处黏土层或火山灰层的锆石U-Pb年龄接近。因此,峨眉山玄武岩喷发结束的时间应该在P-T边界,与西伯利亚大火成岩省的主体喷发时间一致。新的测年结果暗示了ELIP火山活动与地球历史上最大的一次生物灭绝事件(P-T边界)可能存在着成因联系。 相似文献
35.
36.
Abstract. Sulfur isotope ratios are analyzed for a series of mudstones in a successive section in the Taiheizan area in the central Green Tuff region, northeast Japan. The δ34 S values for the Uyashinai Mudstone Member (Nishikurosawa stage), the Onnagawa Formation and the Funakawa Formation range from -45 to -14%, -20 to +4% and -30 to -10%, respectively. There is a marked positive shift in δ34 S values at the Nishikurosawa/Onnagawa boundary and an upward δ34 S-decreasing trend in the Onnagawa and Funakawa Formations. The present data provides evidence for environmental change in the form of stratification of the seawater, initiated at the Nishikurosawa/Onnagawa boundary and persisting during deposition of the Onnagawa Formation, followed by gradual oxygenation during deposition of the Funakawa Formation. 相似文献
37.
Deterioration mechanisms of tuffs in Midas monument 总被引：3，自引：0，他引：3
Slightly weathered white and pink tuffs of the Midas monument have deterioration problems. In this study, depths and characteristics of the weathering zones developed within the tuffs are investigated through optical microscopy, X-ray diffractometry (XRD), chemical analyses, scanning electron microscopy (SEM) and some index parameters. Accelerated weathering tests including wetting–drying, freezing–thawing and salt crystallization are performed, and durability assessment methods are used to predict the durabilities of the tuffs. The findings are compared with field observations. By examining quantitative weathering indices and comparing them with thin section studies, it is found that thin section analyses of the crystals, LoI, and WPI are good indicators to quantify the depth of weathering for the tuffs. However, thin section studies have limited value for fine-grained tuffaceous matrix. The chemical weathering of the tuffs produces weathered zones that are 4.5-cm thick within the white tuff and 2.5-cm thick within the pink tuff. Physical weathering causes scaling of outer layers of the tuffs and fracturing of feldspars along their cleavage planes. However, variations of the index properties of the tuffs due to weathering are not so significant to quantify the weathering depths in the tuffs. Among the accelerated weathering tests, salt crystallization is found to be the most destructive environmental condition. Pigeon droppings rich in NO31− are found to be the main source of soluble salt at the Midas monument. The salt transported up by capillary rise due to surface water causes spalling of the tuffs in the capillary zone. Surface water and salt of any kind in the close vicinity of the monument should be totally eliminated for the purpose of conservation. Field observations and the durability equations reveal that the white tuff is less durable than the pink tuff. Wet-to-dry strength ratio yields a better stone durability assessment among various durability methods used in this study. 相似文献
38.
Ranking welding intensity in pyroclastic deposits 总被引：1，自引：1，他引：1
Welding of pyroclastic deposits involves flattening of glassy pyroclasts under a compactional load at temperatures above the glass transition temperature. Progressive welding is recorded by changes in the petrographic (e.g., fabric) and physical (e.g., density) properties of the deposits. Mapping the intensity of welding can be integral to studies of pyroclastic deposits, but making systematic comparisons between deposits can be problematical. Here we develop a scheme for ranking welding intensity in pyroclastic deposits on the basis of petrographic textural observations (e.g., oblateness of pumice lapilli and micro-fabric orientation) and measurements of physical properties, including density, porosity, point load strength and uniaxial compressive strength. Our dataset comprises measurements on 100 samples collected from a single cooling unit of the Bandelier Tuff and parallel measurements on 8 samples of more densely welded deposits. The proposed classification comprises six ranks of welding intensity ranging from unconsolidated (Rank I) to obsidian-like vitrophyre (Rank VI) and should allow for reproducible mapping of subtle variations in welding intensity between different deposits. The application of the ranking scheme is demonstrated by using published physical property data on welded pyroclastic deposits to map the total accumulated strain and to reconstruct their pre-welding thicknesses.Editorial Responsibility: T. Druitt 相似文献
39.
Recently, a tuffaceous tight oil reservoir with considerable reserves has been found in the Santanghu Basin, northwestern China. The physical properties of the tuff are characterised by high porosity (5–25%) and low permeability (0.01–0.50 mD). This reservoir has the peculiar property of being a sedimentary organic-matter-bearing tuff formed by air-fall volcanic ash in a lake. Its discovery offered an excellent opportunity to further our knowledge of the formation and evolution of tuffaceous tight-reservoir micropores. This study integrates analyses of organic geochemistry, thin-sections, scanning electron microscopy, CT scanning, mineral and element compositions, mercury injections and measurements of porosity and permeability to determine the characteristics of the formation and evolution of the tuff micropores. (1) The tuff reservoir comprises vitric, crystal-vitric, pelitic and silicified tuffs, which are characterised by high porosity and big throat, moderate porosity and thin throat, low porosity and thin throat, and very low porosity and moderate throat pore structures, respectively. (2) Of the four types, vitric tuff has the properties best suited to a reservoir, and devitrification of volcanic glasses is the principal mechanism of reservoir micropore formation. (3) The tuff of the Permian Tiaohu Formation contains sedimentary organic matter, and organic acids produced during kerogen maturation facilitate the process of devitrification. (4) Evolution of tuff porosity depends mainly on the original composition and burial depth. The porosity of vitric tuff is higher than crystal-vitric tuff at the same depth, but both their porosities initially decrease and then increase with depth. At depths >3000 m, vitric tuff retains the trend towards higher porosity, whereas the porosity of crystal-vitric tuff remains largely unchanged or even decreases. The results of this investigation could be considered a reference regarding the distribution and prediction of tuffaceous reservoirs in similar situations in other parts of the world. 相似文献
40.
