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1.
Analogue study of particle segregation in pyroclastic density currents, with implications for the emplacement mechanisms of large ignimbrites 总被引：3，自引：0，他引：3
Abstract Analogue flume experiments were conducted to investigate the transport and sedimentation behaviour of turbulent pyroclastic density currents. The experimental currents were scaled approximately to the natural environment in three ways: (1) they were fully turbulent; (2) they had a very wide range of particle sizes and associated Rouse numbers (the ratio of particle settling velocity to effective turbulent eddy velocity in the current); and (3) they contained particles of two different densities. Two sets of surge‐type experiments were conducted in a 5 m long, water‐filled lock‐exchange flume at five different volumetric particle concentrations from 0·6% to 23%. In one set (one‐component experiments), the currents contained just dense particles; in the other set (two‐component experiments), they contained both light and dense particles in equal volume proportions. In both sets of experiments, the population of each component had a log‐normal size distribution. In the two‐component experiments, the size range of the light particle population was selected in order to be in hydrodynamic equivalence with that of the dense particles. Dense particles were normally graded, both vertically and downstream, in the deposits from both sets of experiments. The mass loading (normalized to the initial mass of the suspension) and grain size of the dense component in the deposits decreased with distance from the reservoir and were insensitive to initial total particle concentration in the currents. On the other hand, in the two‐component experiments, the light particles were extremely sensitive to concentration. They were deposited in hydrodynamic equivalence with the dense particles from dilute currents, but were segregated efficiently at concentrations higher than a few per cent. With increasing particle concentration, the large, light particles were carried progressively further down the flume because of buoyancy effects. Deposits from the high‐concentration currents exhibited reverse vertical grading of the large, light particles. Efficient segregation of the light component was observed even if the bulk density of the current was less than that of the light particles. In both sets of experiments, marked inflexions in the rate of downstream decline in mass loading and maximum grain size of the dense component can be attributed to the presence of two different particle settling regimes in the flow: (1) particles with Rouse numbers >2·5, which did not respond to the turbulence and settled rapidly; and (2) particles with Rouse numbers <2·5, which followed the turbulent eddies and settled slowly. The results are applied to the transport and sedimentation dynamics of pyroclastic density currents that generate large, widespread ignimbrites. Field data fail to reveal significant departures from aerodynamic equivalence between pumice and lithic clasts in three such ignimbrites: the particulate loads of some large ignimbrites are transported principally in turbulent suspensions of low concentration. In some ignimbrites, the well‐developed inflexions in curves of maximum lithic (ML) size vs. distance can be attributed to the existence of distinct high and low Rouse number particle settling regimes that mark the transition from an overcharged state to one in which the residual particulate load is transported more effectively by turbulence. 相似文献
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Axel K Schmitt Jan M Lindsay Shan de Silva Robert B Trumbull 《Journal of Volcanology and Geothermal Research》2003,120(1-2):43-53
High spatial resolution U–Pb dates of zircons from two consanguineous ignimbrites of contrasting composition, the high-silica rhyolitic Toconao and the overlying dacitic Atana ignimbrites, erupted from La Pacana caldera, north Chile, are presented in this study. Zircons from Atana and Toconao pumice clasts yield apparent 238U/206Pb ages of 4.11±0.20 Ma and 4.65±0.13 Ma (2σ), respectively. These data combined with previously published geochemical and stratigraphic data, reveal that the two ignimbrites were erupted from a stratified magma chamber. The Atana zircon U–Pb ages closely agree with the eruption age of Atana previously determined by K–Ar dating (4.0±0.1 Ma) and do not support long (>1 Ma) residence times. Xenocrystic zircons were found only in the Toconao bulk ignimbrite, which were probably entrained during eruption and transport. Apparent 238U/206Pb zircon ages of 13 Ma in these xenocrysts provide the first evidence that the onset of felsic magmatism within the Altiplano–Puna ignimbrite province occurred approximately 3 Myr earlier than previously documented. 相似文献
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在流纹质岩浆中,存在两种不混溶的熔体,一种熔体富SiO_2贫FeO,另一种则相反。从熔体结构的角度看,前一种熔体富桥氧,后者则富自由氧。水的解聚作用可以改变流纹质岩浆的结构,即降低它的聚合程度,因此,含水流纹岩浆的粘度低于玄武岩浆的粘度,从而使流纹岩中流纹构造发育。 实际工作中应避免混淆流纹岩与酸性熔结凝灰岩。 相似文献
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Compositional Zoning of the Bishop Tuff 总被引：14，自引：0，他引：14
Compositional data for >400 pumice clasts, organized accordingto eruptive sequence, crystal content, and texture, providenew perspectives on eruption and pre-eruptive evolution of the>600 km3 of zoned rhyolitic magma ejected as the Bishop Tuffduring formation of Long Valley caldera. Proportions and compositionsof different pumice types are given for each ignimbrite packageand for the intercalated plinian pumice-fall layers that eruptedsynchronously. Although withdrawal of the zoned magma was lesssystematic than previously realized, the overall sequence displaystrends toward greater proportions of less evolved pumice, morecrystals (0·5–24 wt %), and higher FeTi-oxide temperatures(714–818°C). No significant hiatus took place duringthe 6 day eruption of the Bishop Tuff, nearly all of which issuedfrom an integrated, zoned, unitary reservoir. Shortly beforeeruption, however, the zoned melt-dominant portion of the chamberwas invaded by batches of disparate lower-silica rhyolite magma,poorer in crystals than most of the resident magma but slightlyhotter and richer in Ba, Sr, and Ti. Interaction with residentmagma at the deepest levels tapped promoted growth of Ti-richrims on quartz, Ba-rich rims on sanidine, and entrapment ofnear-rim melt inclusions relatively enriched in Ba and CO2.Varied amounts of mingling, even in higher parts of the chamber,led to the dark gray and swirly crystal-poor pumices sparselypresent in all ash-flow packages. As shown by FeTi-oxide geothermometry,the zoned rhyolitic chamber was hottest where crystal-richest,rendering any model of solidification fronts at the walls orroof unlikely. The main compositional gradient (75–195ppm Rb; 0·8–2·2 ppm Ta; 71–154 ppmZr; 0·40–1·73% FeO*) existed in the melt,prior to crystallization of the phenocryst suite observed, whichincluded zircon as much as 100 kyr older than the eruption.The compositions of crystals, though themselves largely unzoned,generally reflect magma temperature and the bulk compositionalgradient, implying both that few crystals settled or were transportedfar and that the observed crystals contributed little to establishingthat gradient. Upward increases in aqueous gas and dissolvedwater, combined with the adiabatic gradient (for the 5 km depthrange tapped) and the roofward decline in liquidus temperatureof the zoned melt, prevented significant crystallization againstthe roof, consistent with dominance of crystal-poor magma earlyin the eruption and lack of any roof-rind fragments among theBishop ejecta, before or after onset of caldera collapse. Amodel of secular incremental zoning is advanced wherein numerousbatches of crystal-poor melt were released from a mush zone(many kilometers thick) that floored the accumulating rhyoliticmelt-rich body. Each batch rose to its own appropriate levelin the melt-buoyancy gradient, which was self-sustaining againstwholesale convective re-homogenization, while the thick mushzone below buffered it against disruption by the deeper (non-rhyolitic)recharge that augmented the mush zone and thermally sustainedthe whole magma chamber. Crystal–melt fractionation wasthe dominant zoning process, but it took place not principallyin the shallow melt-rich body but mostly in the pluton-scalemush zone before and during batchwise melt extraction. KEY WORDS: Bishop Tuff; ignimbrite; magma zonation; mush model; rhyolite 相似文献
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Emplacement of small‐volume (<0·1 km3) pyroclastic flows is significantly influenced by topography. The Arico ignimbrite on Tenerife (Canary Islands) is a characteristic small‐volume pyroclastic flow deposit emplaced on high relief topography. The pyroclastic flow flowed down pre‐existing valleys on the southern slopes of the island. In proximal areas deep (up to 100 m) valleys acted as efficient conduits for the pyroclastic flow, which was mostly channelled; in this particular area the ignimbrite corresponds to a homogeneous, moderately welded deposit, consisting of flattened pumices in an abundant ashy matrix with a relatively low lithic fragment content. In intermediate zones significant changes occur in the steepness of the slope and, although still channelled, here the pyroclastic flow was influenced by hydraulic jumps. In this area, two different units can be clearly distinguished in the ignimbrite: the lower unit is composed of a lithic‐rich ground‐layer deposit that formed at the turbulent, highly concentrated head of the flow; the upper unit consists of a well welded pumice‐rich deposit that occasionally reveals a basal layer formed by shearing with the lower part. This division into two units is maintained as far as distal areas near the present‐day coastline, where the slope is very gentle or null and the ignimbrite is not channelled. The ground layer is not found in distal areas. The ignimbrite here only consists of the upper unit, which is occasionally repeated due to a surging process provoked by the lower flow speed, as the pyroclastic flow spread out of the channelled zone. A theoretical model on how topography controlled the deposition of the Arico ignimbrite is derived by interpreting the observed lithological and sedimentological variations in terms of changes in topography and bedrock morphology. This new model is of general applicability and will help to explain other deposits of similar characteristics. 相似文献
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位于塔里木-卡拉库姆板块与伊犁-哈萨克斯坦板块之间的南天山造山带是最具代表性的造山带之一,其中发育一条不完整的蛇绿岩带——南天山南缘库勒湖-科克铁克达坂蛇绿岩带,暗示古缝合线的存在。该蛇绿岩带北面的开都河水电站泥盆纪地层中发育一套中酸性火山岩(包括熔结凝灰岩、含晶屑酸性玻璃质熔岩和安山岩等),属于碱性-钙碱性系列,A/CNK值在0.82~1.31之间;该套岩石整体REE含量较高,轻重稀土元素分异显著,轻稀土元素相对富集,重稀土元素亏损,并具强烈的Eu负异常。对熔结凝灰岩中岩浆锆石进行LA-ICP-MS U-Pb定年,获得其结晶年龄为393.1±3.4 Ma,与中泥盆统(D2)萨阿尔明组一致。岩石地球化学研究表明,安山岩可能来自于玄武岩分离结晶;熔结凝灰岩、含晶屑酸性玻璃质熔岩来自于上地壳部分熔融。推测其形成机制是:库勒洋盆在中泥盆世向北俯冲消减过程中释放出流体,导致地幔楔发生部分熔融并形成玄武质岩浆,部分玄武质岩浆结晶分异形成安山岩,随着俯冲持续和地壳不断加厚,玄武质岩浆对上部地壳加热使得其发生部分熔融形成酸性母岩浆,并最终喷发至地表形成熔结凝灰岩和含晶屑酸性玻璃质熔岩。 相似文献
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扬子克拉通北缘神农架群火山岩锆石Hf同位素特征 总被引：3，自引：2，他引：1
报道神农架群安山质熔结凝灰岩中锆石的Hf同位素数据,以此对其岩浆来源与岩石成因进行约束.结果显示,Nd-Hf同位素之间存在脱耦现象,虽然各样品之间显示出较为一致的元素地球化学和Nd同位素组成,但同一样品中具有高度变化的锆石εHf(t)值,对应锆石Hf同位素模式年龄为2587～1608 Ma,表明岩浆在形成和上升过程中可能受到了陆源碎屑物质组分的混染.该组分改变了岩浆结晶时部分锆石中的Hf同位素组成,然而未改变全岩地球化学及Nd同位素组成,可能表明锆石原位Hf同位素在反映岩浆形成和演化过程中对于碎屑沉积岩混染的识别比全岩Nd同位素具有更为灵敏的指示意义.结合该岩石的全岩地球化学特征,认为神农架安山质熔结凝灰岩来自深部幔源岩浆上涌引起的元古宙基性下地壳岩石部分熔融,且在上升过程中受到了俯冲碎屑沉积岩的混染. 相似文献
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Larissa L. Maughan Eric H. Christiansen Myron G. Best C. Sherman Gromm Alan L. Deino David G. Tingey 《Journal of Volcanology and Geothermal Research》2002,113(1-2)
Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km3) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates.The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km3. It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO2) is poorly correlated with phenocryst abundance.These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit.The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere. 相似文献