Bubble growth in highly viscous melts: theory, experiments, and autoexplosivity of dome lavas

We examine the physics of growth of water bubbles in highly viscous melts. During the initial stages, diffusive mass transfer of water into the bubble keeps the internal pressure in the bubbles close to the initial pressure at nucleation. Growth is controlled by melt viscosity and supersaturation pressure and radial growth under constant pressure is approximately exponential. At later stages, internal pressure falls, radial growth decelerates and follows the square-root of time. At this stage it is controlled by diffusion. The time of transition between the two stages is controlled by the decompression, melt viscosity and the Peclet number of the system. The model closely fit experimental data of bubble growth in viscous melts with low water content. Close fit is also obtained for new experiments at high supersaturation, high Peclet numbers, and high, variable viscosity. Near surface, degassed, silicic melts are viscous enough, so that viscosity-controlled growth may last for very long times. Using the model, we demonstrate that bubbles which nucleate shortly before fragmentation cannot grow fast enough to be important during fragmentation. We suggest that tiny bubbles observed in melt pockets between large bubbles in pumice represent a second nucleation event shortly before or after fragmentation. The presence of such bubbles is an indicator of the conditions at fragmentation. The water content of lavas extruded at lava domes is a key factor in their evolution. Melts of low water content (<0.2 wt%) are too viscid and bubbles nucleated in them will not grow to an appreciable size. Bubbles may grow in melts with 0.4 wt% water. The internal pressure in such bubbles may be preserved for days and the energy stored in the bubbles may be important during the disintegration of dome rocks and the formation of pyroclastic flows.     

Evolution of the seismic activity from the 1982 eruption of El Chichon Volcano, Chiapas, Mexico

 We analyzed more than 1700 earthquakes related to the 1982 eruption of El Chichon volcano in southern Mexico. The data were recorded at specific periods throughout the whole eruptive interval of March to April 1982, by three different networks. The seismic activity began several months before the first eruption on 28 March. During this period the seismicity consisted of hybrid and long-period shallow earthquakes most likely related to processes of faulting, fracturing, and fluid movement underneath the volcano. The foci of events occurring before the eruption circumscribe an aseismic zone from approximately 7 to 13 km below the volcano. After the eruption, the seismic activity consisted of tectonic-type earthquakes that peaked at 1200 events/h. This later activity occurred over a wide range of depths, mostly between 5 and 20 km, that includes the former aseismic zone and is roughly limited by the major tectonic faults in the area. Received: 19 May 1998 / Accepted: 13 June 1999     

Fragmentation, transport and deposition of a low-grade ignimbrite: The Citlaltépetl Ignimbrite, Eastern México

 The Citlaltépetl Ignimbrite records one of the largest explosive events during the Holocene activity of Citlaltépetl Volcano (Pico de Orizaba). Multiple pyroclastic flow units, a fall deposit, and some lahar units were emplaced between 8500–9000 y B.P. as a result of repetitive but discrete explosive events. The whole ignimbrite resulted from discrete fluctuations in eruptive intensity that decreased with time. The initial pyroclastic flow pulse was by far the most violent and widespread event, and its deposits show conspicuous variations in structure and texture that could be associated with different mechanisms of transport and emplacement. Subpopulation Sequential Fragmentation Transport (SFT) analyses were carried out in order to determine the physical mechanisms that selectively concentrate or remove particles in the moving flows. We suggest that lateral and temporal changes in the flow rheology, in which fluidization, yield strength, entrainment of atmospheric air, and sedimentation played a dominant role in flow propagation and emplacement, may imprint a unique signature in the grain-size spectra. The lowermost unit of the Citlaltépetl Ignimbrite can be envisaged by a model in which progressive aggradation near the vent became replaced by en masse emplacement farther outward. Received: 24 March 1998 / Accepted: 9 October 1998     

内蒙古贺斯格乌拉牧场白音高老组火山岩U-Pb年龄及地球化学特征

白音高老组火山岩在大兴安岭地区广泛分布,本文对大兴安岭中南部贺斯格乌拉牧场白音高老组酸性火山岩进行了锆石U-Pb年代学、岩石地球化学和Sr-Nd同位素分析,探讨其岩石成因及构造背景。位于大兴安岭中南段东乌旗地区的白音高老组火山岩,主要岩性为流纹岩、流纹质凝灰岩等,LA-ICP-MS锆石U-Pb年龄分别为129. 8±0. 8 Ma、128. 2±2. 8 Ma,属早白垩世早期;从岩石地球化学特征上看,其主量元素SiO_2含量68. 76%～76. 18%、Al_2O_3含量11. 84%～13. 38%,属高钾钙碱性岩石;稀土总量较高,(La/Yb)N 6,(La/Sm)_N 3,显示为轻稀土富集型,δEu显示负铕异常(0. 4～0. 6),微量元素以富集大离子亲石元素K、Rb、Ba,贫高场强元素Nb、P、Ti为特征; Sr-Nd同位素组成上具有低I_(Sr)、高ε_(Nd)(t)和低t_(DM)的特征(I_(Sr)=0. 7039～0. 7045,ε_(Nd)(t)=2. 65～4. 39)。火山岩浆来源于下地壳基性物质的部分熔融,综合研究并结合前人资料认为,白音高老组流纹岩形成于滨太平洋板块俯冲后伸展及蒙古-鄂霍次克洋闭合后的综合效应下。     

Mathematical modelling of volcanic tsunamis

The height of widespread tsunami runup about the Bay of Plenty from various volcanic scenarios involving eruptions at White Island is shown to be insignificant in all cases considered, except for Krakatoa‐type explosions. The problem of localised inundation remains unanswered. Some definitions describing volcanic tsunami risk, introduced recently by J. Latter (Bulletin volcanologique 1982), are briefly mentioned. All numerical calculations assumed cylindrical symmetry and used a flux‐corrected Lax‐Wendroff algorithm to solve the modified shallow water equations of Peregrine.     

长白山及邻区地壳、上地幔顶部三维速度结构

根据沿长白山布设的宽频带流动地震台站及吉林省地震台网所记录的近震P波走时数据,利用层析成像方法对长白山及邻区（39°N-45°N、122°E-130°E）深至40 km的地壳和上地幔顶部三维速度结构进行了研究。结果表明：地震的发生和分布多集中于断裂等复杂地质构造。利用较高分辨率的地壳、上地幔顶部三维速度结构证实了长白山火山区岩浆囊存在,并推测岩浆囊的位置位于火山口的西南方向,深度为10~40 km。壳内岩浆囊分布对进一步解释、认识火山灾害提供了重要的深部信息。     

Petrology of two contrasting Mexican volcanoes,the Chiapanecan (El Chichón) and Central American (Tacaná) volcanic belts: the result of rift- versus subduction-related volcanism

The alkaline El Chichón and calc-alkaline Tacaná volcanoes, located in southern Mexico, form parts of the Chiapanecan Volcanic Belt and Central American Volcanic Arc, respectively. El Chichón has emitted potassium-, sulphur-, and phosphorus-rich trachyandesites and trachybasalts (as mafic enclaves), whereas Tacaná has erupted basalts to dacites with moderate potassium contents, and minor high-Ti magmas (1.5–1.8 wt.% TiO₂). The magmatic evolution in the two volcanoes has involved similar fractionating assemblages: Fe-Ti oxides, olivine, plagioclase, pyroxenes, amphibole, and apatite. K₂O/P₂O₅ ratios and isotopic signatures indicate that magmas from both El Chichón and Tacaná have undergone significant crustal contamination. The volcanism at both Tacaná and El Chichón was previously related to northeastward subduction of the Cocos Plate, representing the main arc and the backarc, respectively. Although such an origin is in accord with Tacaná occurring 100 km above the Cocos Benioff Zone, it is inconsistent with: (a) the absence of a calc-alkaline belt between El Chichón and the Middle America Trench; and (b) truncation of the subducted Cocos Plate by the southwesterly dipping Yucatan slab near the Middle America Trench (i.e. the Cocos Plate does not presently underlie El Chichón). On the other hand, El Chichón and the Chiapanecan Volcanic Belt are located on the sinistral Veracruz fault zone that forms the northern boundary of the Southern Mexico block, which has been migrating relatively to the east since ca. 5 Ma. In this context, the anomalous high potassium, sulphur, and phosphorus levels in the El Chichón magmas are explicable in terms of rifting in a pull-apart system with the weak subduction fingerprint inherited from the Yucatan slab.     

大兴安岭中段莫克河地区新生代火山活动

通过野外地质观测、岩石地球化学分析及高精度加速器质谱(AMS)14 C测年等工作,对大兴安岭中段莫克河地区新生代火山活动进行了详细研究。结果表明:莫克河地区新生代火山活动活跃,覆盖面积超过80km2,喷发方式为斯特朗博利型喷发。火山活动最早始于晚更新世,火山活动经历了4个火山喷发旋回,并在第一、二个火山旋回之间有短暂的间歇。火山岩为以低硅、高镁、高钾、高钛为主要特征的高钾钙碱性橄榄玄武岩。综合周边地区资料,研究区新生代火山岩是在拉张的构造环境下、以复活的深大断裂为通道产生的板内OIB型玄武岩,岩浆在上升过程中没有或很少发生壳源物质混染,也没有发生斜长石结晶分异,但有橄榄石、单斜辉石的分离结晶作用。     

板内橄榄玄粗岩(shoshonite)地幔流体交代作用及成因的元素地球化学证据:以赣南会昌橄榄玄粗岩为例

根据稀土元素和微量元素ICP-MS分析结果,通过对流体作用敏感的元素对比值(U/Th、Pb/Ce、Ba/La、Cs/Rb和Ce/Y等)的研究对比,为赣南会昌橄榄玄粗岩的形成有地幔流体交代作用参与提供了重要的地球化学证据.应用主元素氧化物(K2O、Na2O、FeOT、CaO、MgO和TiO2)与SiO2的变异分析,判明会...     

Geochemisty and tectonic setting of igneous rocks in the Glenrock Station area,N.S.W.

In the Upper Murray Valley, Victoria, Late Silurian, high‐Si igneous rocks, which are closely associated with alkalic, basaltic dykes, were emplaced at high crustal levels following the peak of the Benambran Orogeny, which deformed and metamorphosed the Wagga Zone in Late Ordovician‐Early Silurian times. These rocks, which are informally termed ‘the Upper Murray high‐Si magmatic suite’, include leucogranites, rhyolite dykes and flows, and ash‐flow tuffs characterised by the following features. They are transitional from mildly peraluminous to mildly metaluminous; they represent relatively anhydrous magmas, in which halides were important volatile constituents; they have high Si, total alkalies, Rb, Th, U, Nb, Sn and heavy rare earth elements; and they are relatively repleted in Mg, Ca, Sr, Eu, V, Cr and Ni. In these respects and in their post‐orogenic setting and close association with alkalic basalts, they resemble many post‐orogenic granitoids from elsewhere. Such granitoids appear to have formed as partial melts during crustal extension following major episodes of deformation and high‐Si magmatism. A residual granulitic crust, from which an earlier generation of granitoid magmas had been extracted, is argued to be the source rock‐type for these post‐orogenic magmas. Tectonic extension, affecting such a crust, was accompanied by deep fracturing and basaltic vol‐canism. Mantle‐derived, CO₂‐ and halide‐rich fluids moved into the residual crust, causing widespread metasomatism, and emplacement of basaltic magma caused temperatures to rise until melting took place and a second group of magmas was produced. This model explains most aspects of the trace and major element chemistry of post‐orogenic, high‐Si igneous rocks and, for the Upper Murray high‐Si suite it also provides an explanation for variations in trace elements and isotopic characteristics. Other processes, such as crystal fractionation, magma mixing, thermogravi‐tational diffusion, and separation and loss of a volatile phase, provide explanations for variations within individual units of the suite, but they do not explain overall variations or the highly fractionated nature of the suite.